The Brain and Body Movement: Level II – Rex M Heyworth PhD

This Level II article is a continuation of the topic “The Brain and Body Movement” begun in the Level I article, but is treated at a deeper level. There are even deeper levels that could be discussed but these are not part of the current project. As should be evident to the reader of the Level I article, the topic is indeed complicated, made more difficult as the 'experts' in the field do not always agree and sometimes even give conflicting information. Further, few of these 'experts' are good teachers and do not always explain things in a way that is readily comprehensible to readers. My aim, as well as providing information of the topic, is also to present it in a way that most readers can understand. Whether or not I have been successful, the reader will have to be the judge. Level I attempted to give an overall picture of the brain and how it is involved in voluntary (not involuntary) movements. This Level II article is more fragmented and does not attempt to combine everything into an overall account of how voluntary movements occur. For the reader who, after studying this Level II article, may be interested in pursuing his or her own studies, the Internet has a very large number of websites that can be accessed. Some these websites, primarily those I have referred to in preparing the two articles, are listed at the end of this article. Below are some pictures from this Level II article.

Contents Topic The Human Nervous System: An Overview

Page 1

- The central nervous system

1

- The peripheral nervous system

2

- Nerve cells of the nervous system

2

- How do neurons communicate?

5

Parts of the Brain

9

- Forebrain

10

- Diencephalon

15

- Brainstem

16

- Cerebellum

18

Basal Ganglia

22

- Parts of the basal ganglia

22

- Direct pathway

24

- Indirect pathway

26

Spinal Cord

30

- Vertebral column

30

- Structure of the spinal cord

31

- Spinal nerves

32

- Dermatomes

34

- Spinal tracts

35

Sensations and Movements involving the Body and Face

36

Sensations and Movements involving the Body: Spinal Cord Tracts

39

- Ascending tracts

40

- Transmission up the conscious tracts

42

- DCML pathways

44

- Spinothalamic pathways

46

- Transmission up the unconscious tracts to the cerebellum

47

- Ascending unconscious tract from the cerebellum to the cerebral cortex

51

- Descending Tracts

52

- Tracts working together

55

- Posture and anti-gravity muscles (an introduction)

65

- Sensations and Movements involving the Face: Brainstem Tracts

71

- Sensory pathways

71

- Motor pathway: the corticobulbar tract

77

The Human Nervous System: An Overview The (human) nervous system The human nervous system is a complex network of nerve cells (called neurons) that carry messages to and from the brain. It is the most complex system of any organism and infinitely more complex that any computer system. The nervous system has three main functions: gathering sensory information from external stimuli, processing that information, and responding to those stimuli, usually as movements. The human nervous system is divided into two major parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system is made up of the brain and spinal cord. The peripheral nervous system is made up of the somatic and the autonomic nervous systems. Refer to the chart below and to the diagram on the right as you read the next page or two.

The central nervous system The brain and spinal cord comprise the CNS. Brain: The brain lies within the skull and is shaped like a mushroom. The ancient Egyptians were among the first neuroscientists, and were the first known civilisation with a written word for 'brain' which could have been as far back as 3000 BC. (The picture above right is the Egyptian hieroglyph for 'Brain'.) Despite its relatively small size, the human brain contains over 100 billion nerve cells. Scientists estimate that there are more neurons in the human brain than there are stars in the Milky Way Galaxy. The adult human brain has a volume of about 1 400 cm3 and weighs between 1 200 to 1 500 g which is about 2 % of the body weight. The brain contains about one trillion cells (nerve cells and glia cells) and receives 20% of the blood, oxygen and calories supplied to the body. Spinal cord: The adult spinal cord is approximately 40 to 50 cm long and occupies about 150 cm3. The spinal cord is a vital communication link between the brain and the peripheral nervous system. –1–

Website link: Ancient Egyptians and the brain https://faculty.washington.edu/chudler/papy.html The peripheral nervous system The term 'peripheral' in anatomy and medicine (as elsewhere) is the opposite of 'central'. It means situated away from the centre. The word 'peripheral' comes from the Greek 'peripheria' ('peri-', around or about + 'pherein', to bear, to carry). Early researchers made the distinction between the central nervous system and the peripheral nervous system based on where nervous tissue was located in the body – centrally or away from the centre (peripherally). Thus the peripheral nervous system is outside the brain and spinal cord (see the diagram on the previous page). As shown in the chart, the PNS consist of the somatic nervous system and the autonomic nervous system (also called the visceral system). The main function of these two systems is to communicate between the CNS and the rest of the body. Somatic nervous system: The term 'somatic' refers to the 'body' (as distinct from the mind). The primary function of the somatic nervous system is to control the muscles of the body involved in voluntary movements, namely, in the the head, trunk and limbs. (Because body muscles that control voluntary movement are always attached to the skeleton/bones, an alternative name is the skeletal muscle system.) The somatic nervous system and the brain communicate with each other. This is done through two sets of nerves: 1. Sensory nerves, which transmit signals from the sense receptors (such as touch in the skin and proprioception in the muscles) first to the spinal cord and from there to the brain. 2. Motor nerves, which carry instructions from the brain first down the the spinal cord and from there to the skeletal muscles to the muscles of the head and body telling them how to move (motor = movement). In the diagram (top, previous page), these sensory and motor nerves are shown by the lines that spread outwards from the spinal cord. Autonomic nervous system: This system influences the action of organs in the body over which we have no (or little) voluntary control, such as the digestive system and the heart. Muscles in this system, for example heart muscles, unlike those in the somatic system, are not connected to bones. As we are concerned with voluntary movements, we will ignore the autonomic system.

Nerve cells of the nervous system Now that we have introduced the idea of sensory and motor nerves, we will look more deeply into the structure and function of nerve cells. Types of cells: The nervous system is composed of only two main types of cells: neurons and cells that support the neurons, which are called glial cells. (The word 'glial' comes from a Greek word that means glue'.) Neurons are the basic units of the nervous system. The activity of neurons is supported by the glial

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cells. Glial cells outnumber neurons by about 10 to 1, and account for about half of the volume of the nervous system. Collectively, glial cells nourish the neurons, remove their wastes, defend against infection and provide physical support for all the neurons (so that they don't flop about). Glial cells also speed up neural conduction by acting as insulating sheaths around certain axons. They also dispose of the waste materials generated when neurons die. In the remainder of the discussion, we are only concerned with neurons.

Neurons

Symbols:

Symbol:

A neuron is a nerve cell. The diagrams above show three common types of neurons. All neurons share four features: they have dendrites, a cell body (soma), an axon (also known as a nerve fibre), and branching ends called axon terminals. [Ignore the colours in the diagrams; neurons are not actually coloured like this.] The message sent along a neuron is known as an impulse (though other common words such as messages and signals are also used; I will tend to use all three.) Dendrites: These are short, tree-like branching terminals that receive/collect nerve impulses from other neurons or from sensory receptors, and relay the impulses to the cell body. The dendrites are numerous and highly branched, which increases the surface area available to receive information. Cell body (or soma): This contains the nucleus. The cell body processes input from the dendrites and relays it along the axon. Axon (or nerve fibre): Axons range in length from 1 mm to 1 m. Axons in the brain are very short. Neurons that pass down the spinal cord all the way down to muscle cells in the feet are long. For example, a nerve to the leg contains axons that extend from the spinal cord all the way to the muscles in the foot, a distance of over 1 m. Axon terminal: This is at the end of an axon. It branches into many fibres which transmit the impulse to the next neuron. The ends of the axons are bulb-shaped and each axon has many of these bulb-like structures. –3–

Myelin sheaf: The neuron at the top left, unlike that the other two, does not have a myelin sheath around its axon. Myelin is a substance made of fats and proteins that increases the speed at which impulses are conducted along an axon. Neurons without this myelin (known as unmyelinated neurons) sheaf look pinkish-grey and make up the 'grey matter' in the brain. Myelin has a whitish appearance and myelinated neurons neurons make up the 'white matter' in the brain. (More about grey matter and white matter later.) Symbols: The diagrams also include the symbols for the neurons. The black dot ( ) represents the cell body while the axon terminal is represented by

. The straight line represents the axon. Note that the

symbol does not include anything special for the dendrites. Sensory and motor neurons Sensory neurons, receive signals from – logically enough – sensory receptors. These are are located in all parts of the body, but especially in the skin and the muscles. The different kinds of sensory receptors were discussed in Level I. They are: mechanoreceptors which detect touch and pressure, thermoreceptors which detect heat and cold, nociceptors which detect painful stimuli, and proprioceptors in muscles and muscle tendons which detect proprioception, the sense of knowing where the body and limbs are located in space (e.g. if the hand is raised of not, if the knees are bent, and if the head is turned). Motor neurons carry signals from the brain or spinal cord to muscles instructing them on how to move. The diagrams below show the structure of sensory and motor neurons. The sensory receptors in this example are those for for touch and pressure located in the skin. Compare these two neurons with the kinds of neurons shown earlier. Compare also the position of the cell bodies in the neurons.

Sensory neuron: Because the sensory neuron differs from the 'typical' neurons (in that its cell body is not located at the end), it is called a 'pseudo' neuron (pseudo- = false). It has two axons, one from the cell body to the sensory receptors (in the skin, muscles) and the other from the cell body to the axon terminals The signals from the receptor travel from the receptor endings to the cell body and then continue to the axon terminals (which as we will see later, are located in the spinal cord). Motor neurons: The motor neuron is a 'normal' neuron, with dendrites at one end, a cell body (soma) at the other end and an axon from the cell body to the axon terminals. The impulses travel from the –4–

dendrites, through the cell body and along the axon to the axon terminals. These terminals are dispersed throughout the muscle fibres. When the muscle fibres receive a signal, they contract and move.

How do neurons communicate? Synapses [Pronounced sin-aps] At the end of an axon is a small gap called a synapse. This is the point where the axon of one neuron connects to a dendrite of another neuron. This word comes from the Greek syn (together) and haptein (join). Neurons communicate with another neuron across a synapses. But because it is a gap, neurons must communicate without touching one another. Neurons transmit impulses from one neuron to the next. This is achieved using electrochemical signals, that is a combination of electricity and chemicals. This happens in two different ways: 1. The impulse travels along an axon from the dendrites at the receiving end to the axon terminals at the other end as a small electric current (using ions and not electrons). The speed of the impulse varies depending on the type of neuron, but the fastest travel at about 400 km/h (250 mph). The fastest are only in neurons that need to transfer information urgently. For example, if you burn your fingers it is important that your brain gets the message to withdraw the finger very quickly. 2. When an electrical impulse reaches the synapse at the end of a neuron, it cannot pass directly to the next one. Instead, it triggers the neuron to release special chemical molecules called neurotransmitters (chemical messengers). The axon terminals release the chemicals which pass across the synapse to the dendrites of the next neuron. This chemical transmission is much slower that the electrical transmission.

The chemicals drift across the gap between the two neurons. On reaching the other side, they bind (i.e. fit into) with tailor-made receptors on the surface of the target neuron, like a key in a lock. This docking process converts the chemical signal back into an electrical impulse which travels through the second neuron and so on. The diagram on the right shows the transmission across a synapse. The red dots in the diagram represent the neurotransmitters. A neuron has many axon terminals, each with a bulb-like structure at the ends. Each of these connects with the dendrites of other neurons. So one neuron can connect with many other neurons (often thousands).

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Neurotransmitters Many neurotransmitters belong to the family of chemical compounds called amino acids (which also happen to be the building blocks of the proteins in our bodies). There are many different neurotransmitters (perhaps as many as 60), each with a different effect. Dopamine is a neurotransmitter involved in controlling movement and posture (which is what we are discussing). The loss of dopamine in certain parts of the brain causes the muscle rigidity typical of Parkinson’s disease. Each type of neurotransmitter has a molecular form that lets it bind to the receptors on the dendrites of the second neuron to produce its particular effect. Excitatory and inhibitory neurotransmitters Neurotransmitters are divided into two groups according to the effect that they have on the second neuron once they are released across the synaptic gap. Neurotransmitters that help this neuron to propagate/send the nerve impulse along this second neuron are called excitatory neurotransmitters. Neurotransmitters that reduce (or even stop) the propagating/sending of an impulse along the second neuron are called inhibitory neurotransmitters. Two neurotransmitters that are used by more than 80% of the neurons in the brain are glutamate and GABA (gamma-aminobutyric acid). Glutamate is an excitatory neurotransmitter whereas GABA is an inhibitory neurotransmitter. Some neurons release glutamate across the synapse between neurons, while others release GABA. Another important neurotransmitter is dopamine: this is special because it sometimes acts as as an excitatory neurotransmitter and sometimes as an inhibitory neurotransmitter. We will use the diagrams below to shown the effects of glutamate and GABA. For clarity, neurons that release glutamate are drawn in green ( (

), while those that release GABA are drawn in red

). (The colours are appropriate as green signifies 'Go' while red signifies 'Stop'.)

Example 1: Three glutamate-releasing neurons are connected. That is:

When the impulse through the first neuron reaches the synapse, it releases glutamate molecules (also shown in green). These pass across the synapse and bind with the second neutron. Because they are excitatory neurotransmitters, they cause the second neuron to become – well – excited, and generate an impulse (represented by the arrow

) that passes along this second neuron. And the same for the

third neuron. Example 2: A glutamate-releasing neurons is connected to a GABA-releasing neuron, which in turn is connected to another glutamate-releasing neuron. That is:

X As in Example 1, when the impulse through the first neuron reaches the synapse, it releases glutamate molecules (again shown in green). These pass across the synapse to the second neutron. Because they are excitatory neurotransmitters, they cause the second neuron to again become excited and generate an impulse that passes along this second neuron. But when the impulse arrives at the synapse, because it is a

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GABA-releasing neuron, it releases GABA molecules (shown in red). Now, GABA molecules are inhibitory neurotransmitters and so on arrival at the third neuron, they inhibit (i.e. stop) all (or most) of the action and so no impulse (or perhaps just a very weak one) passes along the third neuron (represented by the 'X'). So if there is another neuron in this chain (whether 'green' or 'red'), it does not fire as it does not receive an excitatory neurotransmitter. Example 3: Similar to Example 2, except that the third neuron is a GABA-releasing neuron:

X The result is the same as the second neuron release a strong dose of inhibitory neurotransmitters thus preventing an impulse from being produced in this third neuron. Example 4: In the brain, most neurons are connected to perhaps thousands of other neurons. So let us modify Example 2 by adding an additional side neuron as shown below:

Suppose the additional neuron (the green one) is active. It releases glutamate neurotransmitters and so allows the final neuron to fire, shown by the brown arrow to indicate that the impulse has come from this additional neuron and not those in the original chain. Chains of neurons that release excitatory and inhibitory neurotransmitters are important in the action of the basal ganglia as are discussed later in this article. More on neurotransmitters We will not discuss the details about neurotransmitters but if you are interested, here is a suitable link: http://thebrain.mcgill.ca/flash/d/d_01/d_01_m/d_01_m_ana/d_01_m_ana.html There are three levels of difficulty – beginner, intermediate and advanced. So take it as far as you like. Video animation How transmission across a synapse occurs. Here are two of many video animations available on the web: https://www.youtube.com/watch?v=p5zFgT4aofA https://www.youtube.com/watch?v=WhowH0kb7n0 Note the use of terms not refereed to in this text: electrical impulse = action potential, vesicle = container/sac that holds neurotransmitter molecules) See also diagram at right. Neural networks Each neuron can communicate with many other neurons – up to 15 000 (or perhaps more) other neurons!! All these connections form what are called neural circuits/networks. The diagram (right) is an illustration of a

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network of neurons in the brain. Terminology The meanings of terms used in neuroscience are sometimes confusing. Here are some terms we have used so far that you need to be clear about. neuron = a single nerve cell, the basic unit for transmitting signals/impulses in the body – sensory or motor. nerve fibre = just the axon of a neuron (often quite long as mentioned above). The axon may carry information either to the brain (sensory) or from the brain (motor) or sometimes mixed. nerve = a bundle of nerve fibres (axons) in the peripheral nervous system (PNS, i.e. outside the spinal cord) that carries impulses to or away from the central nervous system (CNS, i.e. brain and spinal cord). This confused me for a long time. I thought that a 'nerve' was the same as a 'neuron' but not so; a nerve is actually just the axons – and a whole lot of them! See the diagram on the right. Note: A nerve can have one or more bundles of nerve fibres; in this diagram, it has four. tract = a bundle of nerve fibres (axons) in the CNS. So, a nerve and a tract are the same but in different places – a nerve is in the PNS whereas a tract is in the CNS. It is also called a neural tract or a neural pathway. Think of a tract as like a 'pipe'. [We have not yet discussed tracts but keep it in mind for later.] Note for advanced learners: 1.There is a slight difference in meaning between a neuron and a nerve cell which we will not go into – just keep it in mind. I will tend to use the terms interchangeably. 2. Sometimes a neuron is also called a nerve but technically this is wrong. Another description of nerves The words afferent and efferent are often used to describe the direction in which nerve impulses move. 1. Afferent nerves (from the Latin meaning 'to carry towards') carry signals from outside something to inside something, for example, from muscle cells towards the spinal cord or from the spinal cord towards the brain. The term is also used for the transmission of signals towards one part of the brain from another, for example, into/towards the PMA from the cerebellum (where the emphasis in on 'towards the PMA'). So, in general, when you see the word afferent, think of from something towards something else. 2. Efferent nerves (from the Latin meaning 'to carry away'). For example, to carry/transmit motor impulses/signals away from the spinal cord to the muscles. The term is also used for the transmission of signals from one part of the brain towards another, for example, from the cerebellum to the PMA (where the emphasis is on 'out of the cerebellum'). So, in general, when you see the word efferent, think of from something coming out and going elsewhere). Exercise: Go back and sort out the different kinds of nerves: sensory, motor, afferent, efferent.

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Parts of the Brain The discussion here overlaps and extends that in the Level I article. Not every part of the brain will be mentioned – just those important for movement. There are different ways to group the major structures of the brain. Here is one way:

Brain = Forebrain + Brainstem + Cerebellum

For our purposes, that is for a discussion of voluntary movements, the major parts of the brain are as follows. Compare this with the diagram above, which shows the location of these parts. parts of the brain Forebrain cerebrum

Brainstem

Cerebellum

diencephalon

cerebral cortex

thalamus

midbrain

hippocampus

subthalamic nucleus pons

basal ganglia

hypothalamus

medulla (oblongata)

amygdala The parts shaded are not involved in movements and so are ignored in this article. The medulla oblongata is often just called the medulla, hence the brackets.

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Forebrain The forebrain consists of the cerebrum (also called the telencephalon) and the diencephalon. (Strictly speaking, the term 'telencephalon' is the name of the structure in the human embryo from which the cerebrum develops.) We now look at each of these parts.

Cerebrum The cerebrum is the largest part of the brain and accounts for more than 80% of its total weight. It contain the centres for so-called 'high' levels of processing, which include reasoning, memory, consciousness, language and of course voluntary movement. The cerebrum includes the cerebral cortex and sub-cortical structures (sub- = below; -cortico = cortex), such as the basal ganglia (an area at the inner core of the brain). For voluntary movements, sensory information ascends from sensory receptors to the cerebral cortex. After processing the information, motor impulses leave the cerebral cortex and descend to the muscles.

Cerebral cortex The cerebral cortex is the outer layer of the cerebrum. It is a very this being just 2 to 4 millimetres thick. The diagrams below show the cerebral cortex. It is divided into two halves, called cerebral hemispheres – the left cerebral hemisphere and the right cerebral hemisphere. The two hemispheres are linked by the corpus callosum, a large bundle of nerve fibres that allows for transmission of information from one hemisphere to the other. The corpus callosum is about 10 cm is length and is C-shaped. (a)

(b)

corpus callosum

left hemisphere

right hemisphere

Diagram (a) shows the two cerebral hemispheres viewed from above; the corpus callosum cannot be seen in this view. Diagram (b) is a sagittal view of the brain (i.e. a view that divides the brain into left and right – so imagine the brain in Diagram (a) split vertically down the middle); it shows the C-shaped corpus callosum (in green). Looking at Diagram (a), the corpus callosum is right down the middle between the two hemispheres. Note: The left and right sides of the brain are as we look forwards (see the nose in Diagram (a) at the front and the ears at the sides). The left and right cerebral hemispheres control opposite sides of the body (most of the time). So if you move your right hand, a signal came from the motor area in the left hemisphere. (Later, when we discuss ascending and descending tracts, we will see how this happens.) – 10 –

The cerebral cortex is highly convoluted (i.e. folded). This allows it to increase its surface area without increasing its volume so that it is able to contain many more neurons than if it was not convoluted. If unfolded, each cerebral hemisphere has a total surface area of about 1200 cm2 (1.3 square feet) (i.e. about the size of two sheets of A4 paper). The cerebral cortex layer is composed of grey matter (i.e. unmyelinated neurons). Under the grey matter is white matter (i.e. myelinated neurons). Ascending fibres from the spinal cord carry information into the cerebral cortex; descending fibres carry information out of the cerebral cortex. Cerebral lobes The cerebral cortex consists of four pairs of lobes (i.e. four in each hemisphere). Look back at the Level I article for information on these lobes. Here is some additional information, a lot of which was mentioned in the Level I article: Frontal lobe: The frontal lobe is the largest of the brain's lobes. As the name suggests, it is located at the front of the brain. It is the main area for the so-called 'higher' abilities, that is, those involved with consciousness, planning of movements, thinking and working memory. It has several general areas involved with voluntary movement. They are (i) the primary motor cortex (M1), (ii) the premotor area (PMA), and (iii) the supplementary motor areas (SMA). The diagram shows their approximate locations. M1 is also called Area 4, while the SMA and PMA are Area 6. SMA: Plays a preparatory role in planning complex sequences of movements (based on remembered general sequences of movements stored in memory). PMC: Not well understood. Probably involved in selecting appropriate movements based on visual stimuli, that is, specific movements and their sequence for the real-world movement that is about to be undertaken. M1: Involved in the execution of the planned voluntary movements; it sends impulses to specific muscles or groups of muscles. Note: The number of neurons for the muscles in each part of the body is proportional to the precision and complexity of movement in those muscles. Parietal lobe: This lobe is associated with sensory skills and plays an extremely important role in voluntary movements. Opposite M1 is the primary somatosensory cortex (S1, also called Areas 1. 2 & 3)

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which receives somatosensory (somato- = body, -sensory = senses) information from some sensory receptors, especially touch, pain, temperature and proprioception. Behind S1 is the posterior parietal lobe/cortex which integrates and processes all sensory information from multiple senses such as touch and proprioception (from S1) as well as auditory and visual information to create a mental model of the environment, which is then passed to PMA and SMA so that suitable voluntary movements can be planned. Temporal lobe: The temporal lobe houses our ability to receive and interpret auditory information from the ear and does the initial processing of the auditory information. It also collects and interprets information from the nose. An important area within the temporal lobe (Wernicke’s area) gives us the ability to recognise speech and interpret the meaning of words. The temporal lobe also stores different kinds of memories. This includes memory of the general steps involved in movements, that is, the steps you might list if asked how an action is carried out. (But it does not store the knowledge of how to perform the actions; this is the so-called procedural knowledge, which is stored in the cerebellum and which we are unable to describe as it is hidden from our consciousness.) When we learn a new procedure, whether as a child or as an adult, the memories of the general steps of the movement are stored in the temporal lobe. For example, the general steps for reaching for a cup might be: stretch out one arm, lower the forearm, open the hand when it is near the cup, enclose the hand around the cup, squeeze to hold the cup, lift the cup, etc. (See the extension note below on the role of the temporal lobe in perception of moving objects.) Occipital lobe: The occipital lobe is the visual processing centre of the brain. It is divided into several different areas. The region right at the rear edge of the lobe is the primary visual cortex (V1), a region that receives input from the retina of the eye and does the initial processing of this information. V1 does not create a picture of what we are looking at. Instead, it breaks up the image into simple features such as colour, depth and lines/edges of the image. Think of a jig-saw puzzle. The primary visual cortex breaks it into its pieces then sends these to other parts of the brain to be re-assembled. Seems to be a lot of trouble! Another area of the occipital lobe is where visual images of written words are received (the visual receiving area) and another is where they are interpreted (visual association area) which is critically important for reading and reading comprehension. For example, you may see the words of another language, but unless you understand that language, you will only use the visual receiving area. Perception of motion: There is another visual area in the occipital lobe known as visual area MT (middle temporal or V5), that plays a major role in the perception of motion. From this area, information about moving objects is passed to the posterior parietal lobe where a model of a scene with moving objects is constructed.

Extension: Perception of movement Note: This section is adapted from another article dealing with the brain and memory but is included here as it relates to movements. The occipital, temporal and parietal lobes are involved.

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The primary visual cortex (V1) sends a large proportion of the information it processes to the secondary visual cortex (V2) (too complicated to explain what happens here!). From V2, information flows along two main pathways. (See above diagrams for locations of V1 and V2.) 1. The dorsal stream/pathway, to the parietal lobe, which detects (i) spatial locations of objects (2D or 3D), that is, where objects are located, and (ii) movements, for example, the location/position of a person and any movement of that person, or of objects on a table you may be looking at. For this reason it is called the "where" pathway. 2. The ventral stream/pathway, to the inferior (front) part of the temporal lobe, where recognition of what the objects are occurs, for example, a person's face or a cup on the table. This is called the “what" pathway. parietal lobe MT (about here) (the 'where' area') V2 (V1) V2 inferior (lower) temporal lobe (the 'what' area) Thus, in the picture on the right, when you recognise the cup and the pencil (and so can probably name them), your brain is working in the inferior temporal lobe. When use use your eyes to move your hand to pick up the objects, areas in the parietal lobe are being used. So if, for example, an fMRI scan of your brain was being done for this exercise, those two areas of your brain would light up. 'Where' and 'what' movement As mentioned, the cerebral cortex has two major regions that specialise in analysing visual movement, for example, a person walking, or cars on the road: 1. the 'where' area, and 2. the 'what' area. 1. Where area: One of the most prominent is the MT (middle temporal motion area), a part of the dorsal pathway (the “where” area in the post parietal lobe). This area is good at detecting the movement of objects. However, it sees only in black and white, not in colour, and images are blurred. Thus while MT is excellent at determining the movement of an object and its direction of movement, it cannot identify the object. (See the black and white blurred image of a moving car on the right.)

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2. What area: The other major area is the 'what' area in the temporal lobe. Here identification/recognition occurs, and in colour. But this area has a poor sense of motion. So a car on the road is seen clearly in colour and can be identified as a car. However, the car is not in continuous motion; instead the visual motion is just a series of stills, that is, frame by frame movement. (see the picture on the right for what the temporal lobe sees.) Both streams are needed in order to perceive what an object is and where it is moving to.

Basal Ganglia The basal ganglia are part of the brain's sub-cortical structures (i.e. below the cerebral cortex; sub- = below, -cortical = cerebral cortex). They are not a single structure but are clusters of tightly interconnected nerve cells (neurons) located in the central regions of each cerebral hemisphere on both sides of the thalamus. The key parts of the basal ganglia are: the caudate nucleus (with a 'head' and a 'tail') the putamen the globus pallidus (two parts – external and internal) the subthalamic nucleus the substantia nigra The diagrams show two different views of the location of these parts. The basal ganglia are not a single structure but consist of various parts and actually overlap with other parts. The subthalamic nucleus is part of the diencephalon (see below). As its name implies, it is located just below the thalamus (sub- = under, -thalamic = thalamus). The substantia nigra are located in the midbrain. The name means 'black substance' (substantia = substance; nigra = black) because they contain a blackish substance which gives them that colour. And the thalamus, while a separate structure, is sometimes included in the basal ganglia. They are included as they are involved in certain basal ganglia functions. The caudate and putamen are often together referred to as the striatum (or neostriatum) as they seem to do the same things. The basal ganglia play an important role in starting and controlling movements but exactly how is not understood in detail. Current theories suggest they are involved in a loop that receives information from different regions of the cerebral cortex. Once this information is processed by the basal ganglia, it is

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returned it to the SMA and PMA in the cerebral cortex where it (a) allows appropriate movements for an action to occur, and (b) inhibits inappropriate movements. For example, to reach for a cup, moving the arm forwards is appropriate while shaking of the hand is not. The control exerted by the basal ganglia thus ensures that voluntary movements are able to be performed smoothly and accurately. Although the basal ganglia influence the movements that can and cannot occur, they do not directly cause motor output – this is done by M1. Proper functioning of the basal ganglia appears to be necessary in order for M1 to relay the appropriate motor commands to the muscles. In patients with Parkinson's disease, the basal ganglia have been damaged. These patients display difficulty in starting the movements they have planned, as well as trembling and slowness once they do begin the movements. (See picture for other symptoms of Parkinson's disease.) Later in this article, we will look in greater detail as to how the basal ganglia actually work. More terminology The basal ganglia technically should be called the basal nuclei, but they were named prior to the term being accurately defined and the name has stuck. This is because of the difference in the meanings of the terms nucleus and ganglion. Nucleus (pl. nuclei): This term has two meanings. The first meaning is the control centre of a cell (see the earlier diagrams of cells). But the term is also used to mean a cluster/collection of nerve cell bodies within the central nervous system (CNS), that is, the brain and spinal cord. As these clusters involve many nuclei, the plural is always used. Ganglion (pl. ganglia): Refers to a collection/cluster of cell bodies in the peripheral nervous system (PNS), that is, outside the central nervous system. Thus basal nuclei is the correct term as all the cells that make it up are in the CNS. But we will still use the traditional term basal ganglia, which is what most people seem to do.

Diencephalon [Diencephalon: from dia- = 'across or between' + Greek enkephalos '= brain'. The diencephalon is the part of the brain between the cerebrum and the midbrain.]

Thalamus The key part of the diencephalon (for our purposes!) is the thalamus. It is an oval structure in the middle of the brain located under the cerebral cortex and between the two – 15 –

halves of the basal ganglia. It is involved in many important mental functions, including movement. The thalamus is the principal relay station for most information passing to (but not from) the cerebral cortex. This includes: All sensory information, such as touch from the skin, and proprioceptive sensory information from muscles and joints, pass through the thalamus on their way up the spinal cord to S1. All the signals that pass from the cerebellum to the cerebral cortex, such as feedback signals to the motor areas to correct any voluntary movement errors, and All signals from the basal ganglia (to the secondary motor areas, that is, SMA and PMA). Subthalamic nuclei As noted above under Basal Ganglia, while the subthalamic nuclei are part of the diencephalon, they are often also included as part of the basal ganglia due to the important role they play in the control of voluntary movements (discussed later). Exercise: Go back to the Level I article. 1. Revise the functions of the frontal and parietal lobes in voluntary movement (pages 9 – 10). 2. Look at the diagram on page 12. In this simplified diagram, the thalamus is not shown. Where do you think it should be placed? Note: Because it is a flow diagram rather than a diagram of the structure of the brain, you may draw it is more than one place.

Brainstem Spelling: The word can be spelt as either brainstem or brain stem. Advance note: The brainstem is a very complicated part of the brain. At this point, rather than going into the many specific details of the parts of the brainstem, it will be less confusing if we leave these until these they are required in later parts of the article. The brainstem consists of the midbrain, pons, and medulla oblongata. These three parts are located between the thalamus and the spinal cord as shown in the diagram below.

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The length of the human brainstem is about 7.5 cm, with the midbrain about 2 cm (smallest of the three parts), the pons is about 2.5 cm and the medulla about 3 cm. The brainstem is a very important part of the brain. All information from the body passing up to the cerebral cortex and to the cerebellum and vice versa must pass through the brainstem. For example, sensory signals from (a) the face and neck and (b) the lower body (via the spinal cord) enter the brainstem. Similarly, motor signals from the cerebral cortex (M1) pass down to the brainstem then leave it to go to muscles in the neck and face*. Other motor signals pass right through the brainstem then down the spinal cord and on to the muscles in the rest of the body. (All this upward movement of sensory information to the cerebral cortex and downward movement of motor signals from the cerebral cortex to muscles involves what we referred to earlier as ascending tracts and descending tracts. There will be more – a lot more – on these tracts later. *Note: Inside the frame of the diagram (previous page, bottom left) you can see several small things that stick out of the brainstem. These are cranial nerves and this is where neurons leave the brainstem to go to the muscles of the face and neck. But again, more of this later when we discuss senses and movement of the facial area.

Midbrain The midbrain, which is the upper part of the brainstem, allows motor and sensory information to pass through it to and from the cerebral cortex, the cerebellum and the body as mentioned above. It also contains parts closely associated with voluntary movements such as the substantia nigra (involved in the basal ganglia) and even the inner ear. The midbrain has many other functions and is associated with vision, hearing, sleep/wake, arousal (alertness), and temperature regulation.

Pons The word comes from the Latin pons for 'bridge' which is appropriate as it is the 'bridge' between the midbrain and the medulla oblongata. It has an important role in conducting motor signals to and from the cerebellum as well as up to the cerebral cortex and down to the muscles. The pons contains five of the 12 cranial nerves. The pons is also involved in the control of breathing, communication between different parts of the brain, and sensations such as hearing, taste, and balance.

Medulla (Oblongata) The term medulla is of uncertain origin. The term oblongata of course, means oblong-shaped. It is located between the pons and the spinal cord. In addition to motor (movement) functions, the medulla also helps regulate respiration as well as cardiovascular and digestive functioning.

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Cerebellum The word 'cerebellum' comes from the Latin word for 'little brain'. The cerebellum is located posterior (i.e., to the rear/back) to the medulla and pons (see the position of the cerebellum in the above diagrams for the brainstem). The cerebellum is concerned with movements and coordination of movements, especially skilled movements, such as those required for walking and to play a sport, and also with balance and posture. It is especially involved with unconscious movements – look back at procedural knowledge on page 12. The cerebellum is made up of three main parts (refer to the diagram above): 1. Vestibulocerebellum – balance and control of eye movement and upright posture with respect to one’s position in space. 2. Spinocerebellum – involved in skilled voluntary movement – synchronisation/coordination and timing of movements (i.e. to make two or more movements happen together or in the right sequence so that they occur smoothly and accurately, such as those needed to hit a tennis ball, ride a bicycle, or write). To complete even the simplest movements, such as walking or lifting a fork to one’s mouth are indeed complex acts – the cerebellum integrates and organises the necessary sequence of events. The diagram mentions muscles tone – this refers to the maintenance of the correct tension or firmness (i.e. tone) of a muscle. 3. Cerebrocerebellum – planning and initiation of voluntary activity by providing input to the motor areas of the cerebral cortex (e.g. PMA, SMA) involved in planning movements. Cerebellar peduncles [From the Latin meaning 'little foot' or 'stalk-like/thick'] Cerebellar peduncles are thick bundles of myelinated neurons (and so are part of the white matter) that connect the cerebellum to the brainstem. There are six cerebellar peduncles in total, three on the left and three on the right. They are: Superior (i.e. upper) cerebellar peduncle – primary output of the cerebellum to the midbrain. Middle cerebellar peduncle – connects the cerebellum to the pons. Inferior (i.e. lower) cerebellar peduncle – connects the medulla oblongata with the cerebellum. It is the path for proprioceptive information that travels from body (not face) muscles and joints to the cerebellum.

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The cerebellum controls and corrects movements As has been mentioned several times, an important function of the cerebellum is to monitor voluntary movements to ensure they are happening correctly as planned. This is especially true for the control of the fine details and timing of movements. If there are errors, the cerebellum causes corrections to be made so that the resulting movements are so fluid (smooth), harmonious (all working together) and accurate and more or less automatic that we are not even aware of them. But how does the cerebellum control movements? There are two current hypotheses: One involves feedback control whereas the other involves feedforward control. The feedback control hypothesis is the method that has been traditionally accepted. However, the modern view is that the cerebellum guides movements in both a feedback and feedforward control manner. Let us look at these two hypotheses. The cerebellum and feedback control To understand the idea of feedback, take a daily life example. Suppose you are learning how to use a tennis racquet to hit a ball correctly. After you have hit a ball, the coach, who knows how the task should be performed, comments on your performance and perhaps tells you how to improve it. The coach's comments are feedback. On receiving the feedback, you do the task again in an improved way. Feedback in the control of voluntary movements by the brain is similar. The basic idea is that planned movements are compared continuously with the actual output, and adjustments are made during the execution of the movement until the actual movement matches the desired movement. This feedback happens as follows: 1. The motor cortex (M1), when sending signals to muscles, also sends a copy of the outgoing motor commands to the cerebellum, informing the cerebellum of what the intended motion is. For any intended movement, the cerebellum integrates this input with with its knowledge of learned sequences of movements that are stored in the cerebellum. 2. As the movement is being performed, the cerebellum receives proprioceptive input from the muscles and tendons in the moving parts, such as the head and limbs, informing it of what the actual motion is. 3. The cerebellum then compares the intended movements with the actual movements. If there are any differences, it determines what adjustments need to be made to correct these differences, especially in the fine-tuning of the small details and in the timing of the steps in a sequence of movements. 4. The cerebellum sends this corrective feedback to the motor cortex (M1), which then sends corrective commands to the muscles. (In addition, the cerebellum also sends signals to the brainstem [to a part called the the red nucleus] and thence down to the muscles to alter their activity.) In summary, with feedback control of movements, a desired movement is compared continuously with the actual movement, and adjustments are made just after the execution of the movement until the actual movement matches the desired movement. Problems with feedback control: Feedback control can produce very accurate outputs. However, in general it is slow as it can only occur after some movement has occurred. Consider a person running and then stumbling. By the time the feedback has occurred, the person will have fallen over as the commands to correct the stumble arrive too late. Thus, feedback control is useful for slow movements, such as

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adjustments in posture and perhaps even for slow walking. Feedback control is not effective for most of the fast movements we make routinely (such as an eye movement or reaching for a cup). For these movements, neuroscientists suggested that feedforward control is used. Question: In Step 4, are the corrective signals that are sent to M1 and directly to the muscles via the brainstem the same? If so, why are two pathways needed? If the signals are different, won't the muscles be confused? Perhaps the signals are not the same and both are needed to fully adjust the movements. The cerebellum and feedfordward control Again, take an example of learning how to use a tennis racquet to hit a ball correctly. Before you hit the ball, the coach takes all (hopefully all) factors into consideration for a perfect execution; this may include, for example, your height and strength, your existing skill level and even whether or not it is a windy day. Based on these factors, the coach predicts what is needed to execute the action. The coach then tells you how to carry out the movement. The predicting and telling you how to do the movement is the idea of feedforward control. However, the predictions may not be fully correct; there may be something the coach didn't consider, such as you not having had any food to eat and so have little energy to hit the ball. (Can you see any disadvantages with this feedforward method?) Now, we will take take the example of reaching for a cup and see how the cerebellum uses feedforward control. M1 first sends a copy of the intended movements to the cerebellum before it sends instructions to to the arm and hand muscles to reach for the cup. In addition, sensory information such as the visual details about the location of the person and the cup on the table, are also sent to the cerebellum. The cerebellum uses all this information, together with its stored knowledge of skilled movements to predict the best (note – the best not necessarily the correct!) sequence of movements and timing to reach for the cup. The cerebellum then sends its predictions to MI which then sends commands for all these movements to occur. (I assume that the process of making the predictions is very fast; a person does not want to to wait while the cerebellum decides what to do! But I have not been able to find any comment on this in the texts I have read!) Advantages and disadvantages: The advantage of a feedforward system is that it can produce a precise set of commands for the muscles without needing to constantly check the output and make corrections during the movement itself (as happens with feedback control). One disadvantage with a pure feedforward system is that once the commands are sent from M1, there is no way to alter them because there is no feedback loop. But the main disadvantage is that feedforward control requires a period of trialand-error learning before it can function properly. It is hard (perhaps impossible) to know all the possible factors necessary to make an accurate prediction. Through this trial-and error learning, the cerebellum picks up knowledge to predict future movements more exactly. Furthermore, even environmental conditions, such as weather, under which actions are made, are constantly changing, so feedforward control must be able to adapt its output commands to account for these changes. Example: The coordination of movements requires that muscle groups be activated in a precise order and for the different muscles and joints to work together. For example, one muscle needs to be activated to

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start a reaching movement, while another muscle needs to be activated at the end of the reaching movement to stop the movement at the correct time. The precise timing of muscle movements and the force necessary for each movement will vary, for example, with the amount of load placed on a muscle. These variables are constantly changing throughout life, as a person grows, gains/loses weight, and ages. Moreover, a similar movement will require slightly different actions if the cup is empty or full. The cerebellum needs to have stored all this knowledge necessary for predicting smooth and accurate actions. Again, this is likely through a trial-and-error learning. When a child acts clumsily in reaching for a cup, this signals to the cerebellum that the prediction is not correct and to make adjustments so that the next time the prediction will be better. This trial and error sequence will be repeated until the movement is perfectly predictable. Note: In a way, we could say that an incorrect prediction provides the cerebellum with feedback! Any error signal about the action is used by the feedforward system only to change its knowledge and predictability of future movements. Thus the cerebellum may act as a feedback control system for slow movements and a feedforward controller for fast movements.

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Basal Ganglia The function of the basal ganglia in motor control is not understood in detail. But it does appear that they are involved in the enabling of appropriate voluntary movements and in preventing/inhibiting inappropriate voluntary movements. Voluntary movements are not initiated in the basal ganglia (they are initiated in the motor cortex); however, proper functioning of the basal ganglia appears to be necessary in order for the motor cortex to relay the appropriate motor commands to the muscles. Functions of the basal ganglia Voluntary movements are planned in Area 6 of the frontal lobe (PMA and SMA) and executed from the primary motor cortex (Area 4 or M1). However, without the action of the basal ganglia, no movements occur. It seems as if the motor cortex needs to get 'permission' from the basal ganglia, before any voluntary movement can be initiated. So, without the basal ganglia there is no voluntary movement! In the motor cortex, there are many potential voluntary movements that could occur. The function of the basal ganglia are then to: (a) give 'permission' to the movements that are appropriate at a given time, and (b) to block any movements that are inappropriate to the situation. For example, to reach for a cup, movement of the arm, hand and fingers would be appropriate, whereas jumping up and down or inverting the cup would be inappropriate. Motor loop As was pointed out earlier, the basal ganglia are part of a loop (sometimes called a 'motor loop') from the motor cortex to the basal ganglia and back to the SMA in the motor cortex. The basal ganglia actually receive information from many regions of the cerebral cortex but to simplify things so that we can gain at least some understanding, we will just take input from the motor cortex. Output however is always back to the SMA via the thalamus. Parts of the basal ganglia These have already been introduced. Revise the earlier discussion. The diagram below is a coronal/frontal section of the cerebral cortex to show a different view of the parts. Match the parts in this diagram with those shown in earlier. Note that the globus pallidus consists of two parts called the internal segment of the globus pallidus (GPi) and the external segment of the globus pallidus (GPe).

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Two pathways There are two distinct pathways that process signals through the basal ganglia: the direct pathway and the indirect pathway. The normal functioning of the basal ganglia apparently involves a proper balance between the activity of these two pathways. The basal ganglia allow the planned voluntary movements that are appropriate for the present task via the direct pathway while simultaneously inhibiting competing or inappropriate voluntary movements via the indirect pathway. The two pathways involve chains of neurons through the basal ganglia, some of which are excitatory and some inhibitory. Unfortunately, these connections can make an understanding of the pathways very complicated and non-intuitive. Before we explain how these pathways operate, glance at the diagrams for the two pathways below. You will notice that some of neurons involve excitatory connections (the green ones) while others are inhibitory (the red ones). Now go back at the discussion of excitatory and inhibitory neurotransmitters earlier in this article to remind yourself how they allow impulses to move (or not move) along neurons. This must be understood clearly; if not, the discussion of how the two pathways work will not make sense. Prior to operation When the brain does not need to make any voluntary movements, the basal ganglia are said to be at rest. In this condition, the neurons of the globus pallidus (internal) (GPi) are spontaneously active (but see my Q1 in the box below). So an impulse moves along this neuron. As the neuron from GPi to the thalamus is red in colour, and as it is active, it releases a lot of inhibitory GABA neurotransmitter (red dots) which thus prevents/inhibits the neurons from the thalamus to the motor cortex from working (as indicated by he 'X'). That is:

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GPi

Thalamus Motor cortex

When the motor cortex wants to produce a voluntary movement, this rest-state inhibition must be removed so that the neurons from the thalamus to the motor cortex can fire. This is what happens in the direct pathway through the basal ganglia. Q1: Can a neuron really 'spontaneously' fire? Surely something must activate/stimulate this neuron to cause it to fire. The 'experts' do not say anything about this, but my guess is that as all neurons in the brain are connected to thousands of other neurons (which of course are not shown in this very simplified diagram), one of these other neurons may connect with the GPi neuron and fire it. I cannot find any such comment anywhere in my searches!

Direct pathway The direct pathway involves the motor cortex getting 'permission' from the basal ganglia to execute the appropriate movements. The diagram below shows the direct pathway from the motor cortex to the basal ganglia and back again. It consists of a main loop and and an additional loop. The main loop has four steps (1 to 4). The additional loop has two steps (steps 6 and 7). Note: The diagram only shows the direct pathway on one side of the brain. Of course, there is another on the other side. Two such pathways are needed as each side of the brain controls the muscles on the opposite side of the body (mostly).

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Here are the steps involved in the direct pathway (corresponding to the numbers shown on the diagram). 1. Motor cortex to striatum: The motor cortex sends impulses for the planned voluntary movements to the striatum. Excitatory glutamate neurotransmitters move across the synapse and activate/fire the second neuron in the loop (the one heading for GPi). 2. Striatum to GPi: This second neuron (red) is now activated. Therefore an impulse travels along it to neurons in the internal segment of the globus pallidus (GPi). But this time, inhibitory GABA neurotransmitters are released across the synapse to the neuron heading to the thalamus. 3. GPi to thalamus: Because the neuron to the thalamus has received inhibitory neurotransmitters, no impulse (or perhaps only a very weak one) travels along this neuron to the thalamus. 4. Thalamus to motor cortex: As no impulse is sent to in the thalamus, it does not release any neurotransmitters. Thus the inhibition of the (green) neuron from the thalamus to the motor cortex that existed in the 'rest' state is now removed, allowing this neuron to fire (again spontaneously?) and excite the motor cortex. Now that the motor cortex has received the 'permission' it desired, it can send signals to the muscles for the appropriate voluntary movements to take place. The diagram below shows the same four steps with the neurotransmitters included: Striatum

GPi

Thalamus

Motor cortex

Motor cortex 1

2

3

4

Additional loop (called the nigrostriatal pathway – nigro- = SNc, -striatial = striatum): The direct pathway involves the subthalamic nucleus (STN) and one part of the substantia nigra called the substantia nigra pars compacta (SNc). The neurons from the SNc release the neurotransmitter dopamine, which can act in an excitatory or inhibitory manner. In this loop, it is excitatory. Note: It is incorrect to describe dopamine as either excitatory or inhibitory. Its effect on a target neuron depends on which types of receptors it binds to after crossing the synapse. There are five kinds of dopamine receptors, denoted by D1 to D5. (The 'D' means 'dopamine'.) D1 receptors are excitatory whereas D2 receptors are inhibitory. In the additional loop for the direct pathway, D1 receptors are involved. This is what happens (again using the numbered steps in the diagram): 5. STN to SNc: The STN (?again spontaneously) sends an impulse along the neuron to the SNc. Glutamate excitatory neurotransmitters cross the synapse and activate/fire the neuron from the STN to the striatum. 6. SNc to striatum: The neuron from the SNc releases dopamine. For the direct pathway, the receiving neuron in the striatum (i.e. shown in red) has D1 dopamine receptors, which are excitatory. So when the dopamine passes across the synapse to these D1 receptors, it excites/activates the neuron. You will notice from the diagram that this receiving neuron in the striatum (i.e. the red one passing to GPi) has two excitatory connections, one from the motor cortex neuron and one from the SNc. The

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dopamine excitatory neurotransmitters are shown in purple. This is for clarity only. They, like the green glutamate neurotransmitters, are excitatory neurotransmitters. The neuron in step 2 from the striatum to GPi thus receives a 'double dose' of excitatory neurotransmitters which on arrival in the GPi releases a 'double dose' of inhibitory GABA neurotransmitters. As this is so strong, no impulse will travel along the neuron from GPi to the thalamus (as indicated by the large 'X' in step 3). Therefore in the thalamus there is nothing to inhibit the neuron in step 4 and so a strong impulse passes from the thalamus to the motor cortex. Dopamine therefore has the effect of increasing the excitatory effect of the direct pathway, that is, the pathway which allows the motor cortex to send appropriate signals to the muscles to produce movements. The diagram below shows the complete direct pathway with the six steps showing how the neurons in steps 2 and 3 receive a 'double dose' of excitatory glutamate neurotransmitters and the effects of this on the motor cortex. Striatum

GPi

Thalamus

X

Motor cortex 1 SNc

2

6

Motor cortex

X

3

4

5 STN Thus the overall effect of the direct pathway is to send very strong signals to the motor cortex (especially the SMA) to carry out the planned voluntary movements for the current task.

Indirect pathway The indirect pathway involves the motor cortex receiving signals from the basal ganglia to prevent inappropriate voluntary movements from occurring. This pathway occurs simultaneously with the direct pathway but uses a different set of neurons. (It would be somewhat ridiculous if both pathways, which do opposite things, tried to use the same neurons.) The diagram below shows the indirect pathway from the motor cortex to the basal ganglia and back again. As with the direct pathway, it consists of a main loop and and an additional loop. The main loop involves steps (i) to (vi), with the additional loop adding in steps (vii) and (viii). Again, neurons are coloured red and green, but again remember, they are different neurons from those in the direct pathway. Note: Again this diagram only shows the pathway on one side of the brain. Another identical pathway is on the other side. Here are the steps involved in the indirect pathway: (i) Motor cortex to striatum: The motor cortex sends impulses for inappropriate voluntary movements to the striatum. Excitatory glutamate neurotransmitters move across the synapse and activate/fire the second neuron in the loop (just as happened in the direct pathway).

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(ii) Striatum to GPe: This second neuron (red) is activated. An impulse travels along it to the neuron in the external segment of the globus pallidus (GPe). Inhibitory GABA neurotransmitters are released across the synapse in the GPe to the neuron heading to the subthamamic nucleus (STN). (iii) GPe to STN: Because this neuron from the GPe to the STN has been inhibited, no impulse (or perhaps only a very weak one) travels along the neuron to the STN (as indicated by the 'X'). (iv) STN to GPi: The (green) neuron from the STN connects to the internal segment of the globus pallidus (GPi). As there are no neurotransmitters to block it, it fires (again spontaneously?) thus sending an impulse along the neuron to the GPi and releasing excitatory glutamate neurotransmitters. (v) GPi to thalamus: In the GPi, the released excitatory neurons from step (iv) activate/fire the neuron heading for the thalamus allowing an impulse to travel along its axon to the thalamus. As it is a red neuron, it releases inhibitory GABA neurotransmitters into the synapse in the thalamus. (vi) Thalamus to motor cortex: The inhibitory neurotransmitters in the thalamus then inhibit/stop/block an impulse from travelling along the neuron to the motor cortex (again as indicated by the 'X'). Thus the motor cortex is blocked from acting, so no signals for inappropriate voluntary movements are sent to the muscles. The diagram below shows the same six steps with the neurotransmitters included: Striatum

GPe

GPi

Thalamus

Motor cortex

Motor cortex (i)

(ii)

(v) (iii)

(iv) STN – 27 –

(vi)

Additional loop (steps vii and viii): The indirect pathway also involves the release of the neurotransmitter dopamine, which in this loop is inhibitory. This is what happens: (vii) The STN (?again spontaneously as there are no neurotransmitters to fire it) sends an excitatory impulse to the SNc. Glutamate excitatory neurotransmitters are released in the SNc and activate/fire the (red) neuron to the striatum. (viii) The neuron from the SNc releases dopamine. For the indirect pathway, the receiving neuron in the striatum (shown in red and heading to GPe) has D2 dopamine receptors, which are inhibitory. So, dopamine passes across to these D2 receptors thus reducing the overall excitatory effect from the glutamate neurotransmitters released from the neuron from the motor cortex (shown by fewer red dots released into GPe. The neuron arriving in GPe thus releases a smaller number of inhibitory neurotransmitters (shown by fewer red dots in the GPe box). This only partially inhibits the neuron going to the STN so some inhibitory neurotransmitters are released into the STN. So the (green) excitatory neuron from the STN to the thalamus must fire a little (shown by the smaller arrow) and release a few glutamate neurotransmitters into GPi (shown by fewer green dots). This causes a weaker impulse to pass to the thalamus (again shown by a smaller arrow). So although the motor cortex will be inhibited, the existence of the dopamine/additional loop does not fully inhibit it as would happen without the additional loop. This suggests that perhaps some inappropriate movements are possible. [See Q2 in the box below]. The diagram below shows the eight steps with the neurotransmitters included: Striatum

GPe

GPi

Thalamus

Motor cortex

Motor cortex (i)

(ii) (viii)

(v) (iii)

SNc

(vi)

(iv) STN

(vii) Thus the overall effect of the full indirect pathway is to send very weak signals (or no signals) to the motor cortex thus inhibiting the execution of the inappropriate movements (or at least most of them). Net effect of the direct and indirect pathways The net effect is to: 1. excite the direct pathway which excites the motor cortex (positive feedback loop) allowing the desired/planned voluntary movements to occur. 2. excite the indirect pathway which inhibits the motor cortex (negative feedback loop) thus preventing inappropriate movements to occur. Presumably, the function of the basal ganglia is to provide a proper balance between these two pathways. Q2: The research shows that dopamine does slightly reduce the inhibitory effect of the indirect pathway on the motor cortex, which would seem to allow for some inappropriate movements to occur. But why? If

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the movements are inappropriate, then surely they should be completely blocked. As the function of the basal ganglia in motor control is not understood in detail, perhaps an answer to this question is not available yet. Perhaps some 'inappropriate' movements are necessary for voluntary movements to occur properly. Of course, I might have got it all wrong!! Parkinson's disease and dopamine loss This disease was introduced in the Level I article. But brief comments here as well. The disease is characterised by slowness or absence of movement, rigidity, and a resting tremor (especially in the hands and fingers). Patients also have difficulty initiating movements, and once initiated the movements are abnormally slow. (Refer also back to the comments about Parkinson's disease on page 15.) Parkinson's disease occurs when neurons of the substantia nigra die. When this happens, the excitatory input from dopamine in the striatum is lost. This affects the balance of the two pathways in favour of activity in the indirect pathway. The direct pathway receives fewer excitatory neurotransmitters in the striatum and so the motor cortex becomes less active (which is the same as the digram which shows just the four steps of this pathway). Thus it is less able to function and so no planned movements are initiated/started or are initiated but are performed slowly or not very well. With a dopamine loss, the indirect pathway becomes overactive and blocks everything going to the motor cortex (corresponding to the diagram for this pathway showing steps i to iv). This seems to be a bad thing (though see my Q2 again). In Parkinson's patients, up to 75-80% of cells in substantia nigra can be lost! As more of the dopamine neurons die, difficulty in moving progresses which shows as greater trembling and slowness.

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Spinal Cord The spinal cord is the most important structure between the body and the brain. It is a vital link between the brain and the body, and also the reverse, from the body to the brain. It conveys sensory information from sensory sites such as the skin to the brain and conveys motor signal for voluntary movements from the motor cortex to the skeletal muscles. The spinal cord is a continuation of the brainstem (medulla oblongata) and is 40 to 50 cm long and 1 cm to 1.5 cm in diameter. The spinal cord is located within the vertebral column. Vertebral column The vertebral column is also known as the backbone or the spine. It has several important functions, including: Protecting the spinal cord and nerves Providing structural support for the body, allowing us to stand upright. The spine supports about more than half the weight of the body. Most people are born with a vertebral column consisting of 33 small

(C1—C7)

bones called vertebrae stacked on top of each other from the bottom of the skull to the pelvis. (By the time a person becomes an adult, most have only 24 vertebrae because some vertebrae at the bottom end of

(T1—T12)

the spine fuse together during normal growth and development. But in our discussion, we will focus on the 33 vertebrae.) The spinal cord is divided into four regions according to their region in the spine. These regions and the number of vertebrae they contain

(L1—L5)

are: cervical (neck area) (C), thoracic (chest and mid-back area) (T),

(S1—S5)

lumbar (lower part of the back) (L), sacrum (base of spine) (S), and coccyx (or tailbone) Each of these regions is comprised of several vertebrae. There are 7 cervical vertebrae (C1–C7), 12 thoracic vertebrae (T1–T12), 5 lumbar vertebrae (L1–L5), 5 sacrum vertebrae (S1–S5) and 4 coccyx vertebrae (sometimes 3, 4 or 5). In an adult, the vertebrae of the sacrum and coccyx are usually fused and unable to move independently. The coccyx vertebrae are not numbered. (Note: In the diagram above, I can only count 6 cervical vertebrae!) Structure of a vertebra Each vertebra is composed of several parts that act as a whole to surround and protect the spinal cord and to provide structure to the body. The diagram (right) shows one vertebra. The main parts are: The ventral body (the solid part shown in pink). – 30 –

The vertebral arch (the greyish spiky part); the spiky parts are for muscle attachment. The vertebral foramen or just foramen (the approximately circular white part in the middle with the greyish H-shape visible). This is where the spinal cord is housed and protected. The vertebral arch is at the back of the body and is the part we feel when we touch our spines. The diagram (right) shows several vertebrae and the location of the spinal cord in the foramen. Structure of the spinal cord The spinal cord is a long cylindrical cable of nervous tissue inside the vertebral column. Like the vertebral column, the spinal cord also consists of many layers, called segments. But unlike the vertebral column, the spinal cord is divided into just four regions: cervical (C), thoracic (T), lumbar (L) and sacral (S). Further, there are just 31 segments compared with 33 vertebra in the vertebral column. A spinal segment is not always in line with the corresponding vertebra having the same number. This is because there are only 31 segments but also because the spinal cord segments become smaller than vertebrae going down the spine. Thus spinal cord segments are located more and more above their corresponding vertebra especially in the lower regions of the spinal cord. The diagram on the right shows this, with labels for three spinal segments and vertebra. Note that for T2, the spinal segment and vertebra are almost next to each other. But the L1, the spinal segment is well above the L1 vertebra. (Note also the little string-like bits shown between the vertebrae. These are spinal nerves, which we will come to shortly.) Cross section of the spinal cord segments (introduction): The diagram of a vertebra shown above is repeated here. Note the part in the foramen. This is actually a cross section of the spinal cord. dorsal horn white matter grey matter lateral horn ventral horn The spinal cord is composed of grey and white matter. The butterfly or H-shaped structure is the grey matter while the white part surrounding it is the white matter. Grey matter is composed of unmyelinated neurons whereas the white matter consists of myelinated neurons. The ends of the H shape are called horns (I suppose because that it what they look like!) – the dorsal (rear) horn and ventral (front) horn;

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there is also a lateral (side) horn. The white matter consists of sensory nerve fibres that move up (ascend) the spinal cord to the brain. The grey matter consists of motor nerves that move down (descend) the spinal cord to the muscles. (We will learn a lot more about these later in the article.) In the spinal cord, the cervical segments are the largest segments. The thoracic and sacral segments are relatively small. The diagram below show three spinal segments (drawn to scale) for comparative purposes.

Spinal nerves The mention of 'nerve fibres' in the last section leads naturally to the idea of spinal nerves. As mentioned right at the beginning of the discussion on the spinal cord, the spinal cord is the essential link between the peripheral nervous system (PNS) and the brain; it conveys sensory information originating from the skin and other sensory sites to the brain for interpretation and at the same time relays motor signals from the brain down to the muscles. The signals from the sensory receptors (e.g. skin) are carried by nerves to the spinal cord. Motor signals are carried from the spinal cord to the muscles. The pair of sensory nerves and motor nerves that arrive at or leave the same spinal cord segment is called a spinal nerve. (The spinal nerves are the little string-like bits coming out from the spinal cord segments between the vertebrae referred to in the diagram just above for the structure of the spinal cord.) Definition of spinal nerve: A spinal nerve is a mixed nerve, which carries motor and sensory signals between the spinal cord and the body. The diagram below shows one spinal nerve (made up of a sensory neuron plus a motor neuron). The sensory neuron enters the spinal cord segment in the dorsal horn, whereas the motor neuron leaves from the dorsal horn. Because there are receptors and muscles on both sides of the body (e.g. two legs or two arms), there is another spinal nerve on the other side of the spinal cord segment. These two spinal nerves (one on each side of the spinal cord segment) make up a pair of spinal nerves. Note: For clarity, the diagram shows just one spinal nerve entering and leaving; actually a whole bundle/cluster of nerves enter/leave.

other sensory neuron enters here

other motor neuron exits here

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Pairs of spinal nerves: In the human, there are 31 pairs of spinal nerves, one on each side of the vertebral column, corresponding to the 31 spinal cord segments. So this is two fewer than the number of vertebra. Compare: pairs of spinal nerves

Total:

name

number of vertebrae

8

cervical

7

(C1 – C7)

12

thoracic

12

(T1 – T12)

5

lumbar

5

(L1 – L5)

5

sacral

5

(S1 – S5 fused in adult)

1

coccygeal/coccyx

4

(fused in adult)

(31)

(33)

So, although there are 7 cervical vertebrae (C1-C7), there are 8 cervical nerves (C1–C8). All cervical nerves except C8 emerge above their corresponding vertebra, while the C8 nerve emerges below the C7 vertebra (so this vertebra has two spinal nerves, C7 emerging above it and C8 below it). See the diagram on the right (which labels 'spinal nerve' as just 'nerve'). Elsewhere in the spine, all the spinal nerves emerge below the vertebra with the same name. So in the diagram, the T1 spinal nerve is shown below the T1 vertebra. Note: The spinal nerves for each segment (e.g. L1) still enter or leave the spinal cord below the corresponding vertebra but have to travel further down the cord to do so. Compare T2 and L1 in the diagram shown on page 31 above for the structure of the spinal cord. The T2 spinal nerves emerge just slightly below the corresponding T2 vertebra. However, the spinal nerves from spinal cord segment L1 have to travel much further down the spine in order to exit/enter below the L1 vertebra. Reminder: terminology This may be a good point just to add a reminder about terminology. nerve = a bundle of nerve fibres (axons) in the peripheral nervous system (PNS, i.e. outside the spinal cord) that carries impulses to or away from the central nervous system (CNS, i.e. brain and spinal cord). spinal nerve = a bundle containing a mix of sensory and motor nerves that carries signal towards the spinal cord (sensory nerves) and from the spinal cord to the muscles (motor nerves). Note: Spinal nerves only go to or from the spinal cord. They do not go up or down the spinal cord (see 'Spinal tracts' below). More on spinal segments and spinal nerves There is some more detail about spinal nerves and terminology the ready need to be be acquainted with. Look first at the diagrams below. The diagram on the left is another cross-sectional view of a spinal

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segment whereas the other shows a '3-D' view of a spinal segment. Note the terms dorsal root and ventral root. These are where each spinal nerve attaches to a segment of the spinal cord. (They are called 'roots' because – as the 3-D diagram suggests – they look like roots of a tree. Also, for reference, the dorsal root is absent in the first cervical and coccygeal nerves.) Note: Do not confuse dorsal and ventral roots with dorsal and ventral horns; they are not the same, the horns being used to describe the shape of the H-shaped grey matter region in the spinal segment.. Grey matter

White matter

Ventral root

The dorsal root contains sensory nerve fibres from the sensory receptors into the spinal cord. The ventral root contains motor neuron axons and conducts impulses from the spinal cord to the periphery. Now look back at the diagram of the pair of spinal serves on page 32. Note the cell bodies of the two neurons. For sensory nerves, that carry information to the spinal cord, all the cell bodies are clustered together just outside the spinal cord in a kind of 'bump/swelling' on the dorsal root called the dorsal root ganglion. (A ganglion = 'knot'. Recall that a ganglion is a collection/cluster of cell bodies in the peripheral nervous system (PNS), that is, outside the central nervous system, that is, brain and spinal cord.) There is no corresponding ventral nerve ganglion. This is because the cell bodies of motor neurons are located within the spinal cord. Thus there is no 'swelling'. Dermatomes Sensory neurons begin in sensory areas of the skin. An area of skin that supplies a sensory neuron to a dorsal root of a spinal segment is called a dermatome. Refer to the diagram on the right. C (= cervical) The pairs of cervical nerves supply the skin covering the back of the head, the neck, shoulders, arms and hands. T (= thoracic) The pairs of thoracic nerves supply the skin on the chest, back, and under arms. L (= lumbar) The pairs of lumbar nerves supply the skin on the lower abdomen, thighs and fronts of the legs. The pairs of sacral nerves supply the skin on the rear of the legs and the feet.

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Spinal tracts [An introduction to spinal tracts is given here. It is dealt with in much greater detail in the sections that follow.] Millions of sensory neurons deliver information from sensory receptors to the dorsal horns of spinal cord segments all the time. Once in the spinal cord, this information then ascends upwards to the brain in large bundles of nerve fibres called, appropriately, ascending tracts. In contrast, millions of neurons descend from the brain down the spinal cord to connect with the motor neurons that pass from ventral horns in the spinal cord segments to muscles to allow the brain to control voluntary movement of the muscles of the body. These pathways, again appropriately enough, are called descending tracts. The sensory and motor neurons travel by different tracts within the spinal cord to or from the brain. So, all ascending tracts are sensory; they deliver information to the CNS. All descending tracts are motor; they deliver information to the PNS (muscles). The diagrams below show where the ascending and descending tracts are located. dorsal horn

sensory neuron

The greenish parts show the white matter in the spinal segment. Ascending tracts ascend in these areas. So, when a sensory nerve enters the dorsal horn, it immediately crosses to the white matter area (in the green area) and connects with a nerve fibre in one of the ascending tracts and ascends to the brain.

motor neuron ventral horn

The bluish parts show the grey matter in the spinal segment. Descending tracts move down the spinal cord in these areas. A motor nerve from the brain passes down in a descending tract (in the bluish area), crosses into the ventral horn and from there connects with a motor neuron to the muscles. Ascending and descending tracts is a very large and complex topic which is discussed in the remainder of this article.

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Sensations and Movements involving the Body and Face If we put a finger into a flame, the heat can burn the flesh. To prevent this, sensory signals in the skin detect heat and send signals to the spinal cord where a reflex action causes us to remove the finger without even thinking. At the same time, signals are sent to the brain so that we become aware of the pain caused by the heat. Similarly, if we see a car coming towards us, we need to move out of the way quickly. Visual signals are sent to the brain. Based on this information, the motor regions of the brain send signals to muscles in our legs which cause us to move. Without sensory information and motor actions, it should be clear that we would not survive for very long. Sensations and movements involve both the body and the face. [Note: In neuroscience, the body usually refers to everything below the neck, whereas the face area is the head, face and neck areas.] For the body: Sensations (i.e. sensory signals) from the body enter the spinal cord and travel up to the cerebral cortex (and the cerebellum). Movement: Signals from the primary motor cortex (M1) travel down pathways (also called tracts) in the spinal cord. They then leave the spinal cord and pass to the muscles of the body, such as our finger and our legs (in the above two examples). Body tracts always pass down the spinal cord. For the face: Sensations from the face area enter the brainstem and not the spinal cord. From there, most then ascend up to the primary somatosensory region (S1) of the cerebral cortex. Movement: Signals from M1 pass to the brainstem then to facial muscles. Tracts from the cerebral cortex for the face (nearly) always pass through the brainstem. Before we discuss sensations and movements involving the body and face, we need to to introduce (and revise!) some terminology. Terminology The meanings of the terms used in neuroscience are sometimes confusing. Here are some terms that you need to be clear about.

– 36 –

neuron = a single nerve cell, the basic unit for transmitting signals/impulses in the body – sensory or motor (see the diagram on the right). Note: The axon in the diagram has a thick layer around it. This is a substance called myelin and such neurons are said to be myelinated. Myelinated neurons make up the white matter (socalled as it looks whitish). Unmyelinated neurons make up the grey matter (so-called as it looks greyish). Myelin helps to increase the speed of transmission of nerve signals through the axons. nerve fibre = just the axon of a neuron (often quite long). The axon may carry information either to the brain (sensory) or from the brain (motor) or sometimes mixed. nerve = a bundle of nerve fibres (axons) in the peripheral nervous system (PNS, i.e. outside the spinal cord) that carries impulses to or away from the central nervous system (CNS, i.e. brain and spinal cord). This confused me for a long time. I thought that a 'nerve' was the same as a neuron but not so; a nerve is actually just the axons – and a whole lot of them! tract = a bundle of nerve fibres (axons) in the CNS. So, a nerve and a tract are the same but in different places – a nerve in the PNS and a tract in the CNS. Also called a neural tract or a neural pathway. Think of a tract as a like a 'pipe'. Note for advanced learners: 1.There is a slight difference in meaning between a neuron and a nerve cell which we will not go into – just keep it in mind. I will tend to use the terms interchangeably. 2. Also, sometimes a neuron is called a nerve but technically this is wrong. nucleus (pl. nuclei): This term has two meanings. The first meaning is the control centre of a cell (see the diagram above). But the term is also used to mean a collection or cluster of nerve cell bodies within the central nervous system, that is, the brain, brainstem and spinal cord, for example, the trigeminal nuclei. As these involve a cluster (i.e. many) nuclei, the plural is always used. Actually, why they cannot use the term 'cell bodies' such as 'trigeminal cell bodies' I don't know as the cluster is actually of these cell bodies. ganglion (pl. ganglia): Refers to a collection/cluster of cell bodies in the PNS, that is, outside the central nervous system. A good example of the use of the terms 'nuclei' and 'ganglion' is in the nerve fibres that carry information into or out of the spinal cord. Refer to the diagram below. For sensory nerves, that carry information to the spinal cord, all the cell bodies are clustered together just outside the spinal cord in the 'dorsal root ganglion'. For motor nerves, that carry signals out of the spinal cord to muscles, their cell bodies are

– 37 –

clustered just inside the spinal cord in the ventral (or anterior) horn. (See later under spinal cord tracts.) Exercise The diagram shows one segment of the spinal cord. Sensory nerves enter it while motor nerves leave it. Try to (a) match terms in the diagram with those described above, and (b) understand them. If you can't, don't worry as we will return to them later.

– 38 –

Sensations and Movements involving the Body: Spinal Cord Tracts Two kinds of spinal tracts Spinal tracts/are pathways in the spinal cord. They are are either ascending or descending. These tracts are clustered together in the white matter of the spinal cord, that is, the outer part, not the H part which contains grey matter. So, in the diagrams below, the H- shaped part in the middle contains grey matter while the (coloured) parts on the outside contain white matter. Ascending pathways carry sensory information from the body to the cerebral cortex (and also the cerebellum, which is part of the brain). They go up the spinal cord, hence the term 'ascending'. In some texts, ascending tracts are also known as somatosensory pathways or systems. In contrast, descending pathways are motor nerve pathways that go down the spinal cord from the brain and control movement of the muscles of the body. The diagrams below show where the ascending and descending tracts are located. (Remember the inner 'butterfly' shaped part is grey matter while the surrounding area is the white matter.) The 'green' areas of white matter are where the ascending tracts are located and are more towards the outer sides of the spinal cord, whereas the 'blue' areas are for descending tracts are are more internal. Note: The colours are irrelevant; they are only used to compare the different locations.

As we will see, there are many ascending and descending tracts. Each tract contains thousands, perhaps millions, of individual nerves, (i.e. nerve fibres). Sensory nerve fibres deliver information up to the CNS, whereas motor nerves/nerve fibres deliver commands from the CNS to the muscles causing them to move. Some of axons of these motor neurons are very long such as those that descend from the brain to the bottom of the spinal cord which may be 1 metre in length. We will now discuss specific ascending (sensory) and descending (motor) tracts.

– 39 –

Ascending Tracts Ascending tracts carry sensory information from all parts of the body to the brain (i.e. to the cerebral cortex or the cerebellum). The information in the tracts may be conscious or unconscious. 'Conscious' means we are aware of something, while 'unconscious' – obviously – means we know nothing of what is going on. Conscious tracts deliver sensory information to the cerebral cortex. Unconscious tracts deliver information to the cerebellum. For our purposes in the study of movements, the kinds of sensory information information transmitted include touch, vibration, proprioception, temperature (hot and cold) and pain.

Conscious tracts The act of being conscious or aware of something happening occurs in the (frontal lobe) of the cerebral cortex. This is why conscious tracts deliver information from the spinal cord to the cerebral cortex. There are four conscious/sensory tracts (four that we will consider anyway!). All carry sensations up the tracts to the cerebral cortex. They are: 1. Fasciculus cuneatus tract 2. Fasciculus gracilis tract

DCML pathways

3. Lateral spinothalamic tract 4. Anterior/Ventral spinothalamic tract The first two are often grouped together as the posterior column tracts (you will see why soon) or because posterior = dorsal, the dorsal column tracts. Furthermore, as both tracts pass very close together in a part of the lower medulla called the medial lemniscus (lemniscus means 'ribbon'), they are also referred to as the dorsal column-medial lemniscal pathways (DCML). The other two are often grouped together as the anterolateral system/pathways (antero- for anterior; -lateral for, well, lateral). Note that sometimes articles just refer to spinothalamic tracts, so you often need to ask yourself if they are referring to just one of these tracts or to both. All very complicated!

Unconscious tracts These tracts carry information up the spinal cord to the cerebellum. They are collectively known as the spinocerebellar tracts, which is why 'spinocerebellar' occurs in all their names. All carry (unconscious) proprioceptive sensations from muscles and tendons in lower body, such as the legs, up the spinal cord to the cerebellum. There are four of these unconscious sensory tracts but we will consider just three. They are (continuing the numbering from the list of conscious tracts above): 5. Posterior/Dorsal spinocerebellar tract 6. Anterior/Ventral spinocerebellar tract

– 40 –

7. Cuneocerebellar tract The diagram below shows the location of these seven ascending tracts in the spinal cord. Note again that all the tracts (ascending and descending) are in the white matter part of the spinal cord (the outer parts shown in green). (The grey matter in the H- or butterfly-shaped area serves other functions.)

Tracts are on two sides Also note that although the labels in the diagram are only shown on one side of the diagram, identical tracts occur on both sides. Thus, for example, there are two lateral spinothalamic tracts, one on the left of the spinal cord and one on the right. This is because each delivers sensory information from opposite sides of the body such as from the right leg and the left leg. Naming the tracts If the tract name begins with 'spino-' (as in spinocerebellar, for example), the tract begins in the spinal cord and so must be a sensory tract delivering information up the spinal cord to the cerebellum (in this case). The end of the name indicates where a tract goes to. Thus for '-cerebellar' (as in spinocerebellar), the tract delivers sensory information up the spinal cord and to the cerebellum. As another example, consider the spinothalamic tracts. These deliver information from the spinal cord (spino-) to the thalamus (and as we will see, from there to the cerebral cortex). More on naming Some of the names in the text and diagram above are listed with alternative names. However, the meanings are the same and these alternatives are widely used. Alternative terms: Dorsal = rear, back, posterior. Ventral = front, anterior. Lateral – at or towards the sides, outer side. For example, the names ventral spinothalamic tract and anterior spinothalamic tract are the same thing. Similarly, dorsal spinocerebellar tract and posterior spinocerebellar tract are the same thing. (There does not seem to be an alternative name for 'lateral'.) – 41 –

The two dorsal column tracts, fasciculus gracilis and fasciculus cuneatus are strange names. They are entirely Latin. 'Fasciculus' (plural 'funiculi') is Latin for 'little bundle'. (All nerve fibres move up tracts in bundles so why we need another name I don't know, unless there are historical reasons for its use.) 'Gracilis' means 'slender' while 'cuneatus' means 'wedge'. Look again at the diagram. The fasciculus gracilis tract does look kind-of slender, while the 'fasciculus cuneatus' definitely has a wedge shape.

Transmission up the conscious tracts The DCML and spinothalamic pathways carry conscious sensory information/signals from the skin and muscles up the spinal cord to the primary somatosensory cortex/region (S1) in the cerebral cortex. The diagrams below shows how this happens for: (a) the two DCML pathways (diagram [a]), and (b) the two spinothalamic tracts (diagram [b]). Some notes on understanding the diagrams: 1. The diagram of the brain shows what is called a 'frontal (or coronal) plane' section of the brain (Diagram or right shows a frontal plane.). 2. Get oriented! The diagrams show the brain and spinal cord as we look at a person (see diagram of the head). The right and left sides of the brain/body are, by convention, as they appear when we think of our own bodies. Similarly, in the spinal cord is the label 'Dorsal columns'; 'dorsal' means 'back/rear', so again this shows that we are viewing the diagram from the front of the body. 3. To simplify things in order to make it clearer to the viewer, only one segment of the spinal cord and only the medulla in the brainstem are shown. (Remember, the spinal cord has 31 segments and the brainstem also has a midbrain and pons.) You will have to imagine all the other 30 segments of the spinal cord below the medulla. 4. Similarly, the diagram shows the path of just one neuron on just one side of one segment. There will be another neuron on the other side that ascends from the other side of the body. Also, this is repeated for all the other segments in the spinal cord (which, for clarity, are not shown). Further, only one neuron is shown; there will actually be hundreds (if not thousands or even millions!) of neurons going up each tract – far to complicated, and unnecessary, to show in such diagrams. 5. A quick look at the diagrams shows the presence of the thalamus. All ascending pathways to the cerebral cortex pass though the thalamus (but not those going to to the cerebellum). 6. As discussed in the Level I article, specialised sensory receptors are located in the skin, muscles and joints. Proprioceptors (in the muscles and joints) detect proprioception; mechanoreceptors in the skin detect touch and pressure; thermoreceptors in the skin detect heat and cold; nociceptors in the skin detect painful stimuli. Three-neuron pathway and decussation The pathways begin in the skin (for touch, temperature and pain) or the muscles and joints (for proprioception) where senses are detected. A signal travels along a sensory neuron (shown in blue) into the spinal segment then up the tract to the thalamus and from there to the primary somatosensory cortex (S1) in the cerebral cortex where touch, pain or whatever the sense is is consciously detected – such as the

– 42 –

heat (or pain!) from putting a finger in a flame. right side of body

left side of body

Three-neuron pathway and decussation The pathways begin in the skin (for touch, temperature and pain) or the muscles and joints (for proprioception) where senses are detected. A signal travels along a sensory neuron (shown in blue) into the spinal segment then up the tract to the thalamus and from there to the primary somatosensory cortex (S1) in the cerebral cortex where touch, pain or whatever the sense is is consciously detected – such as the heat (or pain!) from putting a finger in a flame.

– 43 –

Number of neurons: If you look closely at the diagram, you will see that each pathway consists of a chain of three neurons. These are called first-order, second-order and third-order neurons. The first-order neuron travels from the sensory receptor to the spinal cord. The second-order neuron travels up the spinal cord to the thalamus. The third-order neuron travels from the thalamus to the cerebral cortex. (You may ask why there can't be just one neuron? I don't know the answer.) Reminder: Note that the cell body of the first-order neuron is in the dorsal root ganglion, which, like all ganglia, is outside the spinal cord. It is called the 'dorsal root ganglion' (abbreviated as DRG) as it is located in the dorsal or back part of the spinal cord (i.e. the part we can feel when we touch our spine). As we will see below, the cerebellar sensory tracts have only two neurons. Decussation: The pathways make a number of twists and turns along the way. In diagram (b), the neuron enters the spinal segment then immediately crosses to the other side (through the grey matter) then ascends on this opposite side. This crossing over is called decussation (noun; verb = to decussate). Decussation also occurs in diagram (a). However, it does so in the medulla. Because of decussation, sensation detected on one side of the body ends up in the opposite side of the brain. Note the label 'medial lemniscus' in diagram (a). Remember that a lemniscus is just a thick bundle of nerve fibres in the medulla. Decussation occurs there for DCML pathways. As the DCML and spinothalamic tracts differ slightly in the routes they take, we will need to consider each in turn.

DCML pathways There are two DCML pathways: 1. Fasciculus cuneatus tract 2. Fasciculus gracilis tract Diagram (a) above shows the common features of these two dorsal tracts ('dorsal' because they are at the back of the spinal cord). The two tracts are essentially the same. Only the parts of the body served differ – upper body or lower body – and the kinds of sensory information carried differs. 1. Fasciculus cuneatus: This tract is the outer wedge-shaped tract of the two dorsal columns. It carries conscious fine/discriminative touch, vibration and proprioception (but not pain) from the upper body levels, that is, upper trunk, arms, neck (but not the face, which has it own special tract). It is a threeneuron tract. The sensory neurons enter at the T6 spinal cord segment and above (some texts say above T6 but let's stick with the former for our discussion). In the picture , the lady's fingers of the right hand touch the bar. The first neuron from the receptors in the finger enter the spinal cord at T6 or above; the second neuron travels up to the thalamus and the third neuron from the thalamus to S1 (left side). Look back at the diagram of Pathway (a) above and match the information in the two illustrations. Note: See the box below on fine touch (and crude touch).

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2. Fasciculus gracilis (or gracile fasciculus): This tract is in the inner, narrower of the two dorsal columns. It tract carries the same sensory information but from the middle thorax and the lower limbs of the body, legs, feet. The sensory neurons enter the spinal cord at the T7 segment and below (some texts say T6 but again stick with the former for now). In the picture, the lady's toes of her left foot touch the floor. The first neuron from a receptor in the toes enters the spinal cord at T7 or below; the second neuron is from the spinal cord to the thalamus and the third to S1 (right side). Note: In the pictures, because there are many points of contact of the toes and fingers, hundreds (thousands?) of touch receptors will be activated but only one is shown in the diagrams. In these two pathways, the first-order neuron carries the sensory signal from the sensory receptors in the skin/muscle receptors to the spinal cord. These too can be very long. Consider a neuron from a muscle/skin in your toe up to the spinal cord; it could be more than 1 metre in length. The second neuron from the spinal cord to the thalamus could also be up to 1 metre in length. So, just two (long) axons from the toes to the thalamus then a third very short one from the thalamus to S1. Note: Technically, the medial lemniscus is just the large bundle of axons of the fasciculus gracilis and cuneatus in the medulla that decussate (the 'V' shaped loop in the medulla in Diagram (a) above). After decussation, they pass to the cerebral cortex. Damage to the DCML tracts: A person is unable to perceive sensations, such as touch or pressure, in either the upper or lower parts of the body and his or her movements are poorly coordinated and clumsy because of the loss of conscious proprioception. that is, knowing the position of his or her body parts, such as the arms and legs. (Look back at the Level I text for loss of proprioception.) Fine touch and crude touch Imagine you are blindfolded (so cannot see things) and something touches you. Fine touch (or discriminative touch) allows you to say exactly where it touched you and to describe accurately details about the object such as what it is made of (e.g. wood, metal), its texture (e.g. rough, smooth) its shape (e.g. round, flat, pointed, rectangular, flat, bumpy), to identify small raised letters with the fingertips (as blind people do when reading Braille dots) and even to identify an object and distinguish between very similar objects (e.g. a pin and a nail). Crude touch (or non-discriminative touch) by contrast, is a sense of touch that allows us to sense that something has touched us, without being able say exactly where it touched us nor to give much, if any, information about the object. The difference is due to the density of touch receptors in different parts of our skin. Sensitive areas, such as the fingers, have many touch receptors close together which allows for fine touch. The legs and back, for example, have fewer touch receptors that are not as close together. So these areas can sense crude touch but not fine touch. That is why blind people read Braille with their fingertips and not with their toes or other parts of the body. – 45 –

Exercise Put on a blindfold. 1. Get someone to touch different parts of your body (e.g. fingertips, fingers, toes, arm, leg, back) with a variety of objects. You are to (a) say exactly where you were touched, (b) give some description of the objects, and maybe to (c) identify the objects. Which parts of the body were best for fine touch? 2. With eyes closed (or blindfolded) could you identify by feel/touch alone the object in the picture (or some other object) if it is placed in your hand? (The ability to do this is called stereognosis.)

Spinothalamic pathways There are two spinothalamic tracts (the numbers below are those from the original list): 3. Lateral spinothalamic tract 4. Ventral spinothalamic tract First, go back to the diagram of the spinal cord segment showing the seven ascending tracts. Identify the locations of the lateral and anterior (ventral) spinothalamic tracts. Both tracts carry conscious sensory information from all parts of the body to the cerebral cortex: The lateral spinothalamic tract carries pain, temperature sensory information to the brain. The anterior spinothalamic tract carries crude touch and pressure. Apart from this, the two tracts are essentially identical. Hence they are often collectively called the anterolateral system or the ventrolateral system (remember: anterior = ventral = front.) The two spinothalamic tracts, like the dorsal column-medial lemniscus tracts, use three neurons to convey sensory information from the periphery to the cerebral cortex. Decussation also occurs, but for these two tracts, it occurs in the same spinal cord segment where the first-order neuron enters. Look at Diagram (b) on page 43. In the grey matter (the H or butterfly-shaped area), the second-order neuron crosses to the opposite side, then proceeds upwards. The digram on the right shows another view of just the lateral spinothalamic tract (which carries pain signals to the brain – hence the hammer hitting the thumb!). Compare it with Diagram (b) above to see that they are identical. Because this pathway goes to the cerebral cortex, it is responsible for the immediate awareness of a painful sensation.

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3. Lateral spinothalamic tract: This tract carries the sensory of pain and temperature from all parts of the body up to S1. Damage to this pathway results in a loss of pain and temperature sensation in the upper or lower limb body depending on whether the upper or lower parts of the tract/spine are damaged. 4. Anterior spinothalamic tract: This tract is also called the ventral spinothalamic tract ('anterior' and 'ventral' both mean 'at or towards the front'.) It carries the sensory modalities of crude touch and (some sources include pressure, others don't!) from all parts of the body to S1. To repeat, the diagram for this tract is the same as for the lateral spinothalamic tract; only the kinds of sensations carried differs. New research on transmission of pain The discussion above on the spinothalamic tracts may be over-simplified. Newer work suggests that there are three spinothalamic tracts that carry pain (but not temperature): the neospinothalamic tract, the paleospinothalamic tract and the archispinothalamic tract, each pathway ending in different areas of the cerebral cortex. The neospinothalamic tract corresponds to the lateral spinothalamic tract described above. The other tracts send signals to parts of the brain that deal with the emotional response to pain, such as the hypothalamus, the frontal cortex and the limbic system. Because detection of pain is critical to survival, these multiple pathways are believed to have evolved to provide 'redundant' (extra) pathways, ensuring to inform the person to "Get out of this situation immediately”.

Transmission up the unconscious tracts to the cerebellum For unconscious sensations, there are three main tracts: 5. Dorsal spinocerebellar tract 6. Ventral spinocerebellar tract 7. Cuneocerebellar tract All the tracts carry proprioceptive information to the cerebellum. Numbers 5 and 6 carry the information from the lower body, tract number 7 from the upper body. Note: There is a fourth tract – perhaps – called the rostral spinocerebellar tract. But this is so unimportant that most sources ignore it. One source says it has not yet been identified in humans. Another even says “Nobody even cares about the rostral spinocerebellum tract!!” So, we too will ignore it. The general path for these tracts is: receptor → spinal cord (upper or lower)→ cerebellum. Although we cannot be aware of these signals (they are unconscious!), the proprioceptive information is used to help the brain coordinate and refine motor movements. The tracts transmit information from the skin/muscles/tendons to the cerebellum. These pathways have only two neurons. The first neuron is from the receptors to the spinal cord. The

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second ascends the spinal cord and ends in the cerebellum. 5. Dorsal spinocerebellar tract: This tract is also called the posterior spinocerebellar tract ('posterior' and 'dorsal' both mean 'at or towards the back/rear'). It carries proprioceptive sensory information mainly from muscles in the lower body and the legs. Only lower segments C8-L2 in the spinal cord receive this sensory information; information from the legs probably enters in L1 and L2), while other lower body information, not as low as the legs, probably enters between C8 and T12) to the ipsilateral (= same side of) cerebellum (see this in the above diagram and the one below.) [Remember there are 31 spinal segments: 8 cervical (C1-C8), 12 thoracic (T1-T12), 5 lumbar (L1-L5 and 6 others.]

The diagram (above) shows an example of this tract. Proprioceptive sensory information from leg muscle spindles (i.e. information about length and tension of muscle fibres), and to a lesser extent the Golgi tendon organs is detected. The sensory neurons from the legs pass into the spinal segment (probably just L1 or L2 – not L3-L5 as these are for the feet and toes which are lower than the legs), synapse (i.e. pass a signal from one neuron to another) in an area called Clarke's nucleus (or Clarke's column, or nucleus dorsalis of Clarke). The second neuron does a loop and ascends through the dorsal/spinocerebellar tract to the medulla then enters the cerebellum. In the diagram above, it is difficult to see the synapse between the two neurons in Clarke's nucleus so I have drawn a circle around this area. [Note: 'synapse' is both a verb and a noun.] The cerebellum uses this proprioceptive information to makes corrections, if necessary, to the movement of the lower limbs. As we will see later, it also uses this information to correct posture. Damage to the dorsal spinocerebellar tract results in the loss of unconscious proprioception and coordination on the same side of as the lesion. Notes: 1. The receptors that send sensory information to the dorsal spinocerebellar tract are primarily in – 48 –

muscles. Those that sends information to the ventral spinocerebellar tract (discussed next) are in the Golgi tendon organs (located at the junctions of muscles and the tendons that join the muscles to bones). 2. The dorsal spinocerebellar tract needs Clarke's nucleus. But Clarke's nucleus is only present for C8L2/L3. For sensations below this, for example, for the feet or toes (which enter the spinal cord lower than this), the sensory neuron has to first 'hitch a ride' up the fasciculus gracilis up to Clark's nucleus at L1/L2, where it connects with and continues up the dorsal spinothalamic tract. 3. As Clarke's nucleus does not extend above C8, the dorsal spinocerebellar tract does not convey information from the upper limb. 6. Anterior (or ventral) spinocerebellar tract: This tract also carries proprioceptive information from the lower limbs but from just the receptors in the Golgi tendon organs. But unlike its dorsal tract

counterpart, the neurons decussate twice (once on entry into the spinal cord and once in the pons before it exits the spinal cord) – and so still terminate in the ipsilateral cerebellum. Also, the ascending tract goes up to the pons then to the cerebellum (cf. its dorsal 'partner' which ascends only as far as the medulla.) Damage to this tract results in loss of unconscious proprioception and coordination in the lower limbs. 7. Cuneocerebellar tract: This tract is very similar to the dorsal spinocerebellar tract but carries proprioceptive information from the upper body (including the arms) which enters the spinal cord above spinal segment C8, again mainly from muscle spindles and, to a lesser extent, Golgi tendon organs) conveying unconscious proprioception. It doesn't seem to have a tract of its own but ascends via the fasciculus cuneatus – hence the 'cune' in its name. Look back at the diagram showing the fasciculus cuneatus and imaging it being used for two purposes – fine touch, vibration and proprioception from the upper body to the cerebral cortex, and just

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proprioceptive information from the upper body to the cerebellum as here 'hitching' a ride up the fasciculus gracilis but then leaving to to enter the cerebellum. (Cf. the dorsal spinocerebellar tract [number 5 above] which also involves some 'hitching' a ride.)

Damage to this tract results in the loss of unconscious proprioception and coordination on the same side as the lesion for the upper parts of the body. For example, it we move our arms about or try to pick up a cup from the table while our eyes are closed, our brain does not know where the arms arm are. However, if our eyes are open, we can visually see where they are and so would be able to manage though less effectively as we would have to continually look to see where our arms are, though in the dark this would be more difficult and we could find that they hit things because we don't know where they are. Summary for ascending tracts Tract

Part of body served

End

Sensations carried

1. Fasciculus cuneatus

upper (T6 and above)

cerebral cortex

fine touch, vibration, proprioception

2. Fasciculus gracilis

lower (below T6)

cerebral cortex

3. Lateral spinothalamic

all

cerebral cortex

pain, temperature

4. Anterior spinothalamic

all

cerebral cortex

crude touch

5. Dorsal spinocerebellar

lower (C8-L2)

cerebellum

proprioception

6. Ventral spinocerebellar

lower (C8-L2)

cerebellum

proprioception

7. Cuneocerebellar

upper (above C8)

cerebellum

proprioception

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– ditto –

Ascending unconscious tract from the cerebellum to the cerebral cortex 8. Cerebellothalamic tract This is another tract not listed above because it does not ascend the spinal cord as the other seven do. Instead, it originates in the cerebellum and ascends to the motor regions of the cerebral cortex. And again, because it goes to the cerebral cortex, it must pass though the thalamus. Thus the pathway is: cerebellum → thalamus → motor and pre-motor areas. Hence its name: cerebello- (cerebellum) and -thalamic (thalamus). Note that is a two-neuron tract and so differs from all the other ascending tracts which are three-neuron pathways. We discussed above that tracts numbered 5, 6, and 7 send information from the body up the spinal cord and into the cerebellum. Once there, the cerebellum compares the actions of the body with the planned actions. If they differ, corrections are needed. The corrective feedback is sent along this path to the motor areas in the cerebral cortex Then, as the diagram shows, the feedback to the motor system to correct any problems passes to the corticospinal tracts because, after feedback, these are the descending paths down which the revised/updated motor signals are sent from M1 to the muscles. Note: We are a little ahead of ourselves as the descending tracts have not yet been discussed. But the cerebellothalamic tract is discussed here as it is an ascending tract. Link it with the corticospinal tracts corticopontocerebellar motor tract (descending tracts 1 and 2 in the next section).

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Descending Tracts The descending tracts are the pathways along which motor signals are sent from the brain (cerebral cortex, brainstem and cerebellum) to the muscles to produce movement. These movements involve the muscles of the body and the face area. In this section, we discuss just those that descend to body muscles via the spinal cord. Tracts descending to parts of the face and head are dealt with in the section that follows. There are five major descending spinal cord tracts that we will discuss. All are two-neuron pathways. The first neuron goes from the brain (usually M1) to the spinal cord. When it exits the spinal cord, the second neuron goes to the muscles which are activated and produce its movement. (Cf. three neurons for ascending sensory tracts.) Upper and lower motor neurons: As these terms are frequently found in the literature, a brief comment here may be appropriate. The descending tracts are two-neuron tracts and as they are used to produce movement, are also called motor neurons. The first motor neuron goes from the brain and down to the brainstem or spinal cord. As this neuron starts in the brain which is at the 'top' of the body, it is called an upper motor neuron (abbreviation UMN). The second neuron passes out of the spinal cord and goes to the muscle that is to contract. As this is 'lower' in the body, it is called a lower motor neuron (LMN). The tracts are listed below according to their areas of the brain where they originate. Numbers 1 to 4 descend down through the spinal cord. Number 5 is the odd an out – it descends to the cerebellum. Tracts originating in the cerebral cortex and descending through the spinal cord: 1. Corticospinal tracts: (a) Lateral/dorsal corticospinal tract (b) Anterior/ventral corticospinal tract Tract (just one) originating in the cerebral cortex but descending to the cerebellum: 2. Corticopontocerebellar tract Tracts originating in the brainstem and also descending through the spinal cord: 3. Rubrospinal tract 4. Reticulospinal tracts 5. Vestibular tracts (There is another spinal tract, called the tectospinal tract which we will not discuss.) The diagram below shows the location of the descending tracts that pass down the spinal cord (i.e. numbers 1 and 3 - 5). Note again that descending tracts (like the ascending tracts) are in the white matter part of the spinal cord (the outer parts shown in green). Again note that while the diagram only shows tracts on one side of the spinal segment, there are identical tracts on the other side (because there are two sides to the brain and two sides to the body). – 52 –

Note: Sources in the literature do not agree on the precise locations of the reticulospinal and vestibulospinal tracts in the spinal cord.

3 4 5

1. Corticospinal tracts There are two tracts – the lateral (dorsal) corticospinal tract and the anterior (ventral) corticospinal tract. [Extra note: Corticospinal tracts are also called pyramidal tracts because the primary motor cortex (M1), from where the pathways begin, contains large neurons (called pyramidal neurons due to their shape).] These tracts supply motor signals from the brain to lower motor neurones in the spinal cord. The lower motor neurones then leave the spinal cord and innervate body muscles to produce voluntary movement. These tracts are the largest and perhaps the most important descending tracts in the human central nervous system. The nerve fibres making up the tracts originate mainly the motor areas (especially M1) (55%) but also the primary somatosensory cortex (especially S1) (35%). The tracts end, of course, in various segments of the spinal cord (depending on where they exit the spinal cord). The diagrams below show (a) the lateral corticospinal pathway, and (b) the anterior corticospinal pathway. Note the upper and lower neurons labelled in the diagrams. Take your time to study the two diagrams as they contain a large amount of information. This is what happens. Refer to the diagrams on the next page: 1. From the primary motor area M1 (mainly), a large bundle of motor nerve fibres (shown just as a single black line) descends to the brainstem at the juncture of the medulla and spinal cord. The fibres pass but don't enter the thalamus; the thalamus plays no part in any descending motor tracts. 2. In the medulla, this large bundle of nerve fibres divides. A majority (about 90%) of the fibres decussate/cross to the opposite side to form the lateral corticospinal tract – Diagram (a). 3. The remaining fibres (about 10%) do not cross at this point; these uncrossed fibres constitute the anterior (ventral) corticospinal tract – Diagram (b). The fibres in this tract descend through the spinal cord and, ultimately, do decussate at the spinal cord segment where they are to exit. (Refer to the diagram again.)

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Brainstem

Brainstem

Spinal cord

Spinal cord

Spinal cord

Spinal cord

4. The tracts leave the spinal cord at the appropriate segment depending on which muscles are to innervated. Thus for the arm and finger muscles, the nerve fibres will exit from segments higher in the spinal cord than those for the leg and toe muscles. Important note: Because two diagrams (a and b) are shown, this might suggest that two bundles of fibres leave M1. This is done for the sake of clarity. There is actually just one bundle leaving M1. But this divides into two in the medulla as stated in points 2 and 3 above. So after this, there are two tracts. You will have to imagine a single diagram with parts (a) and (b) superimposed with 90% of the original bundle of nerve fibres in the lateral corticospinal tract and 10% in the anterior corticospinal tract. The tracts, as mentioned, control voluntary movements of the trunk but different parts of limbs. The lateral corticospinal tract is responsible for the distal parts of the limbs. (Distal means 'farther away from some point', the point here being the spinal cord.) Thus they serve the hands and fingers (and perhaps lower arms) and the feet and toes (and perhaps lower legs). The anterior corticospinal tract moves mainly the proximal segments of the arms and legs. Proximal means 'closer to some point', the point here again referring to the spinal cord. Examples are the upper

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arm, thigh and the back muscles that move the vertebral column. Both corticospinal tracts also plays a role in the voluntary control of axial muscles. Axial means axis, which here refers to the spinal cord, and so includes trunk muscles, such as the shoulder muscles. (All the muscles of the face, neck and head are also axial muscles, but these are controlled by the corticobulbar tract – see later when discussing sensations and movements involving the face.) Damage to the corticospinal tracts If the spinal cord is severed at the lower back, most people lose the use of their legs. The only neural pathway between the brain and the legs has been cut, so such people can neither move their legs nor feel them. Other people are even more unfortunate, losing the use of their arms as well as their legs when their spinal cords are severed at neck level. Reaching for a cup: When someone grasps an object such as a cup, the motion can actually be broken down into two separate actions: reaching the arm out toward the object, and closing the hand around it. A lesion in the lateral corticospinal tract will interfere mostly with closing the hand around the object, but will have little effect on the reaching motion, which is controlled more by the anterior corticospinal tract.

2. Corticopontocerebellar tract [Corticopontocerebellar = cortico- (cerebral cortex) + -ponto- (pons) + -cerebellar (cerebellum)] This tract descends from the cerebral cortex to the cerebellum and not to the spinal cord. So, unlike the corticospinal tracts, it does not control the muscles directly. This pathway allows the cerebral cortex to maintain close communication with the cerebellum. The tract starts again mainly in the primary motor and primary sensory areas of the cerebral cortex (that is M1 and S1). The fibres descend to the pons in the brainstem. Fibres from the pons then project to the cerebellum. Note: It is interesting to note that 90% of all nerve fibres leaving the cerebral cortex join this pathway, whereas only 10% join the corticospinal pathways, suggesting that the cerebellum plays a very important role in motor control. As discussed above, the motor cortex (M1) sends commands for a voluntary movement down the corticospinal tracts. But it also provides the cerebellum with a 'copy' of this information concerning the planning or programming signals of voluntary movements that are transmitted to the muscles. The cerebellum receives feedback from these muscles from proprioceptive receptors in the muscles and acts as a 'comparator' to check if all the movements are going according to plan. If not, the cerebellum sends messages back to M1 along the cerebellothalamic tract (#8 of the ascending tracts above) so that corrective action can be taken. The corrective commands then pass down the corticospinal tracts again.

Tracts working together Several tracts work together to execute accurate voluntary movements. Take the example of what happens when you move a leg. Refer to the steps below and to the diagrams. (I have drawn the diagrams in two ways but they show the same steps. Try to match them.)

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Thalamus

Cerebral cortex (M1) 1 & 5 Lateral

Thalamus

corticospinal tract

2

Corticopontocerebellar tract

4 Cerebellothalamic tract

Brainstem

Cerebellum

Spinal cord

1 & 5 Lateral corticospinal tract

2 Corticopontocerebellar tract

4 Cerebellothalamic tract

3 Spinocerebellar tracts

Pons

Step 1. Initial commands for movements pass from M1 down the corticospinal tracts to the muscles.

3 Spinocerebellar tracts

Step 2. A copy of this information is sent via the

(proprioceptive feedback)

corticopontocerebellar tract to the cerebellum. Step 3. Proprioceptive feedback from the moving muscles (from both muscles and Golgi organs) passes up the two spinocerebellar tracts to the cerebellum. (Look back at ascending tracts 5 and 6).

Step 4. Corrective feedback, if needed, is passed from the cerebellum back to M1 up the cerebellothalamic tract. Step 5. Any necessary corrective commands are sent down the spinocerebellar tracts to the muscles to correct the voluntary movement. Note: Descending tracts are always two-neuron tracts whereas ascending tracts are always three-neuron tracts. This cycle is continually repeating itself to ensure that our muscles don't do anything stupid when they move and we are blissfully unaware of all this. Identify these in the diagrams.

3. Rubrospinal tract The rubrospinal tract is smaller and less important than the corticospinal tract for motor control of voluntary movements. Although it is a major pathway in many animals, it is relatively minor in humans. The tract begins in a part called the red nucleus (rubro- = red) in the midbrain of the brainstem. The red nucleus receives input from the same area of the cerebral cortex as the corticospinal tracts, so it is not perhaps surprising that their actions are similar. This input passes along a separate tract called the corticorubral tract (cortico- = cortex [M1] + -rubral = rubro/red) – see diagram. – 56 –

The nerve fibres in the rubrospinal tract decussate immediately in the midbrain (see diagram) and travel in the company of the lateral corticospinal tracts. The tract influences the muscles of the neck, upper limbs and lower limbs. However, the tract can compensate to some extent for damage to the corticospinal tract, but will take several weeks or longer to adapt to this role. The one ability it is unable to compensate for is the ability to use the fingers individually. Evidently individual finger movements are the sole province of the corticospinal system.

Notes: 1. Some sources suggest that the rubrospinal tract terminates in the cervical cord, in which case it would control just the upper limbs, such as the upper arms, but not lower limb control. 2. The corticorubral tract from M1 to the red nucleus is not part of the rubrospinal tract; it just connects with it. 3. In the above diagram, if the corticorubral and rubrospinal tracts are combined, the tract does look similar to the corticospinal tracts, though the point of decussation differs. Look back at the diagrams for the corticospinal tracts and see if they look similar. As well as receiving connections from the motor cortex (M1), the rubrospinal tract also receives connections from the cerebellum (again, see diagram). The information received is possible learned motor commands (stored in the cerebellum) that are passed on to muscles as feedback when movements are not going according to plan (refer again to Level I article and discussion above for comments about this feedback).

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Flexors and extensors: Many muscles in the body work together but perform opposite jobs (such muscles are termed antagonists). Flexor and extensor muscles are one example of such opposites. A flexor muscle bends a joint, whereas an extensor muscle extends (i.e. straightens) a joint. Look at the picture on the right. The biceps muscle is a flexor as it bends the arm, whereas the triceps is an extensor as it straightens the arm. So, when a biceps muscle contracts and causes the arm to bend, the opposing muscle – the triceps – must be inhibited to prevent it from working against the biceps muscle that is working. Activation of the rubrospinal tract causes excitation of flexor muscles and inhibition of extensor muscles. Another important pair of antagonistic muscles are ones in the leg: the hamstring muscle at the back of the upper leg (flexor for bending the lower leg), and the quadriceps at the front of the upper leg (extensor for straightening/extending the lower leg). Tracts and evolution Over the course of primate evolution, the role of the rubrospinal tract has diminished. With the evolution of the cerebral cortex, the corticospinal tracts developed and assumed more and more of the responsibility for motor control. That is why the rubrospinal tract does not begin in the cerebral cortex – simpler animals did not have one (or had only a very simple one).

4. Reticulospinal tracts [These are tracts for which I have had a lot of difficulty sourcing comprehensible material that is useful to me. So I will just make some brief comments and leave it at that, especially as their function overlaps with those of the vestibular tracts that are discussed next.] There are two reticulospinal tracts, both originating in the brainstem, one in the pons area and the other in the medulla. As with the rubrospinal tract, the two reticulospinal tracts first receive nerve fibres from the pre-motor cortex (M1); this initial tract, logically enough, is called the corticoreticular tract. It terminates in a loosely defined area of the brainstem called the reticular formation to form the two reticulospinal tracts. The tract that begins in the pons is the medial reticulospinal tract (MRST). It does not decussate and descends down one side to the entire extent of the spinal cord. The other tract begins in the medulla and is

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called the lateral reticulospinal tract (LRST). It descends on both sides (i.e. bilaterally) again to all levels of the spinal cord. I have not included diagrams for these two pathways but they are similar to that for the rubrospinal tract except that one originates in the pons and the other in the medulla, whereas the rubrospinal tract begins in the midbrain. I will leave the reader to construct the two pathways and compare them with diagrams on the Internet. Two functions 1. Both tracts seem to help in the initiation of voluntary movements, thus assisting the corticospinal tracts. In fact, it seems that, like the rubrospinal tract, they can be an alternative to the corticospinal tract (though I don't know much about this). 2. They are involved in helping to maintain posture (upright position) and balance during movements, for example, when extending a limb towards an object or when holding a heavy object. They activate the so-called anti-gravity muscles to prevent us from losing balance or falling over. In this, they are similar to the vestibular tracts, which are discussed next, and for which a lot of comprehensible material is available.

5. Vestibular tracts The vestibular tracts are important. Their primary role is to maintain balance and an upright posture of the body and head. So before we discuss the tracts, some background on posture and balance. Some more terminology: Vestibular: Is derived from the word 'vestibule' meaning an enclosed space, such as a lobby in a building. Here it refers to an enclosed space (called the vestibule cavity ) in the inner ear. Excitatory and inhibitory nerve signals: Excitatory signals muscles to contract/pull/work. Inhibitory signals cause muscles to relax, that is, to stop working. Note that a muscle cannot receive both excitatory and inhibitory signals at the same time as it would not know whether to work or to stop working! Posture and balance Posture is the position the body has while a person is standing, walking, sitting or lying down. Balance means the body having a position so that it does not fall. Posture and balance are not the same but are related. Compare the two photographs. The posture of the lady on the left is not upright, so is regarded as poor, but she still maintains her balance. The vestibulospinal tracts are involved in restoring balance and posture. For example, if while walking, we trip and lose balance, these tracts help to restore our balance. Also, for the lady in the photographs, these tracts are involved to get her to stand up straight and look ahead. Note: Posture is also directly related to gravity and our bodies have 'anti gravity' muscles to control posture. More of this later. – 59 –

Organs of balance and posture For the control of body balance and posture: 1. Proprioception in joints and muscles, especially those of the feet, trunk, neck. (Proprioception was introduced in the Level I article. You might like to refer back to this.) 2. Vestibular apparatus (or vestibular organs) located in the inner ears. These are the most important parts for determining head position and straight line movement of the body. 3. Eyes: The eyes control posture in subtle ways, which we will not be discussing. Dysfunction in any one of these systems can result in loss of balance. 1. Proprioception for balance and posture Proprioception is the body's knowledge of the position of its head, trunk and limbs. For balance and posture, proprioception involves sense receptors in the muscles and joints of the neck, torso/trunk, legs and feet. Proprioceptive information is obtained when we do things such as turn our heads, bend over, run, and even walk on different surfaces. The proprioceptive information travels via ascending pathways in the spinal cord (discussed earlier) to the cerebellum (which is unconscious) and some to the cerebral cortex (which allows us to be conscious/aware of the position of our body parts.) 2. Vestibular apparatus In the front of the inner ear are the cochlea, involved in hearing; in the rear are semicircular canals and otolith organs, which affect balance and posture (see diagram right; ignore the other terms unless you are interested in looking them up). The semicircular canals have sensory receptors that detect rotation of the head. Otolith organs (oto- = ear, lithos = stone) detect horizontal movement (in a straight line), vertical movement (e.g. when in a lift), and head position (e.g. upright, tilted as in photograph above). From these organs, messages of head position and straight-line body movementpass to the midbrain. In less than a second, the brain sends these messages to the muscles needed to maintain balance and posture. These are reflex actions and are extremely fast so we are (usually) unaware of them. Otolith organs and chalk! Otoliths are small crystals of calcium carbonate, the same stuff that chalk is made of! It is the presence of these crystals that allows the otolith organs to sense gravity and linear (straight line) movement. The photograph here is a coloured scanning electron micrograph of crystals of calcium carbonate found in these inner-ear organs.

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The vestibular pathways ?

There are four vestibular pathways: 1. Vestibulo-ocular reflex (VOR) 2. Vestibulospinal tracts: (a) Lateral vestibulospinal tract

4. Vestibulothalamic tract

(b) Medial vestibulospinal tract

?

cerebellum

3. Vestibulo-cerebellar connections 4. Vestibulothalamic tract

1. VOR

Sensory inputs from the inner ear

vestibular nerve

pass (via the vestibular nerve) to the brainstem (to an area at the border of

3. Vestibulo-cerebellar connections

INPUT

the pons and medulla called the

2 (b) Medial vestibular tract

vestibular nuclei) There is also input to the brainstem from the cerebellum. The output – remember – controls posture and balance and passes down four tracts called vestibular pathways (or tracts). Refer to the diagram on the right for

2 (a) Lateral vestibular tract

the input and output pathways. The four pathways are discussed below (with the numbers corresponding to those of the diagram). All these tracts except number 4) involve automatic reflex actions that

are not under conscious control. Number 4 involves conscious control, that is, awareness of what happens. 1. Vestibulo-ocular reflex (VOR) The main function of the VOR is to produce eye movements that counter head movements, thus allowing the gaze to remain fixed on a particular point. Take the example in the diagram (with the numbers as in the diagram on the next page): 1. The head turns to the left. 2. The semicircular canals – one set on left side of the head, the other on the right – detect opposite rotations. 3. Signals from the left and right semicircular canals pass to the vestibular nuclei in the brainstem. The vestibular nuclei then send signals to the eyes which are either excitatory (+) or inhibitory (-). 4. Input from the left ear results in excitatory signals to the muscles of both eyes that turn them to the right. Input from the right ear results in inhibitory signals being sent to the muscles of both eyes that turn them to left.

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5. Thus, only the eye muscles that move the eyes to the right are activated. 6. The eyes turn to the right to compensate for the head turning to the left so that the eyes are still fixed on whatever they were viewing.

Note: Imagine the result if all the signals were excitatory (the eyes would try to turn left and right at the same time) or all inhibitory (no movement of the eyes at all). Question How does the brain decide which signals are excitatory and which are initiatory? That is way beyond the scope of this article. 2. Vestibulospinal tracts These are the main descending tracts from the brainstem that pass down the spinal cord. There are two such tracts: (a) the lateral vestibulospinal tract, and (b) the medial vestibulospinal tract. Their main function is to produce changes in posture in response to environmental (or self-induced) disturbances of body position and stability to ensure a rapid compensatory response to any postural instability. This is done by controlling the head, neck, limb and trunk muscles so that they work together to maintain an upright and balanced posture, to stand erect and look straight ahead, and to – 62 –

walk upright. Again they are reflex actions so happen automatically without our conscious awareness. (a) Lateral vestibulospinal tract. This tract descends the whole length of the spinal cord. This pathway controls muscles of the trunk and legs that help us walk upright, and to compensate for tilts and movements of the body; this is especially important while the body is moving. [Memory tip: L for Lateral tract and Legs.] Input to this tract comes first to the vestibular nuclei from two sources: (i) the otolith organs in the inner ears and (ii) the cerebellum. The otolith organs detect straight line movements of muscles such as when the body tilts forward or backward. or when we walk. The cerebellum receives proprioceptive information about movements from muscles and joints in the trunk and legs and compares this with its stored knowledge of whether corrective changes need to be made; if so, it then sends this to the vestibular nuclei. Then, the vestibular nuclei send signals down the lateral vestibular tract to the leg and trunk muscles to make any adjustments to correct posture and balance. This is done by contracting extensor/ straightening muscles and relaxing flexor muscles. These two actions straighten the body to re-establish balance and an upright posture. For example, while walking, suppose the body trips and so leans forward. To pull the body back to an upright position, the soelus muscle (one of the calf muscles in the rear of the lower leg) contracts/pulls the leg upright. The muscle at the front the the lower leg (the tibialis anterior) , which normally does the opposite and pulls the leg down, relaxes thus allowing only the soelus to operate. (Don't worry about the names of the muscles for now; just try to understand how they work.) Note: I have read that it is the inner ear that provides excitatory inputs, that is, signals that cause muscles to activate/contract, while it is the cerebellum that provides inhibitory inputs. So the excitatory signals must activate the soelus muscle and the inhibitory signals the tibialis anterior muscles, though any more than that I do not understand. We will return to these muscles later in the article when we discuss 'anti-gravity' muscles and posture. (b) Medial vestibulospinal tract. This tract is shorter and descends only to the cervical level of the spinal cord (C6) (see diagram again at beginning of 'vestibular pathways'). It controls neck and shoulders muscles which affect head movements, to ensure that the head remains stable while walking and does not wobble about. For example, during a downward pitch of the body (e.g., tripping), the neck muscles reflexively pull the head up with this tract (as well as the calf muscles pulling the body upright at the same time as described above for the lateral tract). Input is again from (i) the inner ear – but this time the from the semicircular canals (which detect head rotation) – and (ii) the cerebellum allowing it to influence the position of the head. (Again because the

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cerebellum stores learned knowledge of head position, if there are postural errors, it will provide corrective signals to adjust the head position.) The action is very similar to that described above for the lateral vestibular tract. That is: Information from semicircular canals & cerebellum → vestibular nuclei in the brainstem → medial vestibulospinal tract → muscles in neck → head upright Note: When there are no posture or balance problems with the body, these two vestibulospinal pathway are normally strongly suppressed even though they still receive input from the inner ear and cerebellum. If they were not suppressed, they could interfere with normal muscle movements. 3. Vestibulo-cerebellar connections [vestibulo- = vestibular nuclei (in brainstem); -cerebellar = cerebellum. So it refers to nerve fibres from the brainstem to the cerebellum.] Our discussion of these connections summarises much of what has already been said about the cerebellum. As mentioned earlier, sensory information received from the inner ear passes from the vestibular nuclei in the midbrain to the cerebellum. Corrective messages from the cerebellum return to the vestibular nuclei in the midbrain from where is passes down the other tracts to make postural adjustments. In this way, the cerebellum regulates the activity of neurons in many parts of the body that relate to position, posture and balance including the following three: Eyes: The cerebellum exerts control over the position of the eyes. Changes in head position result in activation of the cerebellum, which, in turn, transmits signals back to the vestibular nuclei in the midbrain that can modify the position eyes via the VOR. Head: Information in the midbrain received from the cerebellum passes down the medial vestibulospinal tract, which in the pathway that influences the position of the head. Trunk and legs: Likewise, information from the cerebellum passes down the lateral vestibulospinal tract, which is the pathway then sends signals to control limb and trunk muscles resulting in the straightening of the truck and legs to give an erect posture. 4. Vestibulothalamic tract [vestibulo- = vestibular nuclei; -thalamic = thalamus. This path is from the vestibular nuclei in the midbrain to the thalamus and from there to the cerebral cortex.] As mentioned, all the vestibular tracts except this one (number 4) involve automatic reflex actions that are not under conscious control. Number 4 involves conscious control, for example, when we consciously stand/sit up straight. The cerebral cortex is for conscious awareness of postural movements and changes whereas the cerebellum is for the automatic correction of movements for posture and balance. The vestibulothalamic tract is the main pathway from the vestibular nuclei in the midbrain to the cerebral cortex, via the thalamus. There is debate over exactly where in the cerebral cortex it goes to:

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some people put it close to the auditory (hearing) area in the temporal lobe, others put it in S1 – hence the '?' drawn on the cerebral cortex in the diagram at the beginning of this section. This pathway is not well understood and seems to be very complicated.

Posture and anti-gravity muscles (an introduction) Posture means the position of the body while standing, walking, sitting or lying down (at least for humans, but not other animals). A good posture allows the body to maintain an upright alignment and usually results in the least amount of energy being expended. The picture on the right shows the ideal upright alignment. Bad habits and posture: Behaviour, such as as sitting in front of a computer with a certain posture for hours on end (as I am doing right now!), or tilting the head forwards while looking at a mobile phone (and which I also do to look at the computer monitor!), results in the body getting used to being in those positions. They become the new normal 'posture'. Gravity and its effect on the body In addition to bad habits, gravity also affects posture. Gravity is the force pulls things downwards which Isaac Newton – supposedly – discovered when he observe an apple fall to the ground). That includes the human body. Gravity drags parts of us to the ground. Most of the time, this results in a saggy stomach or butt. As we age, heads fall forward, butts disappear, stomachs protrude and breasts sag. Gravity also tends to pull the body forward over the feet resulting in a stooped posture. As we age, we also become shorter. Gravity compresses the discs between the vertebrae in the spine. After the age of 20, we lose an average of about 1.5 cm in height every 20 years. Over a lifetime, a person can permanently lose up to 5 cm in height! Gravity also causes the internal organs begin to fall from their rightful place in the body. Organ function then becomes less efficient. Anti-gravity muscles Anti-gravity muscles are the primary muscles used to hold the body upright and resist the downward pull of gravity. They are also called postural muscles. They are mainly the muscles of the neck, shoulders, back, and legs. The diagram (right) shows the major antigravity muscles and their positions. All these muscles work to support an upright posture. (You can ignore the other muscles.)

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Below are brief comments on a few of these muscles. I won't guarantee that everything written here is correct as it turns out that muscles and their action is a very complex subject. Neck muscles Many muscles – their action is very complicated! When these muscles contract, they pull the head to an erect position and hold it there preventing it from rolling forwards (such as when looking at a mobile phone!). Quadriceps These muscles are located in the front of the thigh. They consist of four muscles (quad- = four). The upper ends of the muscles are attached to the top of the thigh bone (femur) and the lower ends are attached to the patella, or kneecap (red line in the diagram). When the quadriceps contract (and so become smaller), they pull and extend/straighten the knee. Note: If you lift your leg in front of your body, the quadriceps pull you leg forwards (but this only happens when the foot is not on the ground.) Try it! Gluteus maximus / Glutes (butt muscles) This is a very big muscle. It is connected to the tailbone and the top of the thigh bones (femur). The muscle is responsible for straightening/extension of the hip and thigh bones so that the body becomes upright (as shown in the diagram). Standing up from a sitting position, climbing stairs, and staying in an erect position are all aided by the gluteus maximus. Hamstrings [Seems to be very complicated! The 'experts', both in texts and videos, explain things in a way that I do not fully understand.] The hamstrings are a large set of three muscles (or four if you count the two divisions of one muscle) at the back of the thighs. The tops of the muscles are attached to the bottom of the pelvis/hip and the lower ends of the muscles are attached to the top of the tibia (the thicker of the two lower leg bones). Their anti-gravity effect on posture seems to be in straightening the trunk after it is bent forwards and while the feet are on the ground (refer to diagram). The hamstrings act as anti-gravity muscles by pulling on the hip, thus extending the trunk into an upright position. Note: The hamstrings also bend the knee, but I think this only happens when the foot is lifted above the ground, as when

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walking and especially when running. This is easy to understand. But I do not see what this has to do with posture and anti-gravity muscles. Perhaps nothing! Exercise Stand upright with both feet on the floor. 1. Lift one leg slightly so that your foot is just off the floor but you are still upright. Now bend your knee backwards. Explain how the hamstring muscles bend the knee. 2. Now lower your knee. What muscle does this? Calf muscles The calves consist of two muscles – the gastrocnemius (the upper part of the calf) and the soleus (the lower part of the calf). Both are located in the rear of the lower legs. But their connections and functions differ.

The gastrocnemius is joined to the thigh bone (femur) and to the Achilles tendon which is attached to the ankle. This muscle is especially active in fast movements such as running and jumping. Its primary action is plantar flexion of the foot (see note below and Diagram A). The soleus runs between the top of a lower leg bone (the fibula – the thinner of the two lower leg bones) and the ankle bones again via the Achilles tendon. In diagram B, when the soleus muscle contracts, it shortens and tries to pull the foot down (as in Diagram A), but because the foot is already on the ground and cannot bend downwards, the legs straighten instead. But the result is the same because whether the foot is on the ground or above the ground (as when walking or running) the angle between the legs and the front of the foot increases (or between the legs and the back of the foot decreases), as shown in the diagram on the left. This seems to be the same as the action of gastrocnemius, but occurs during slow movements as soleus is the more active of the two muscles when standing still or walking slowly. It is also used for plantar flexion. The soleus muscle also plays an important role in maintaining a vertical standing posture. It does this by preventing the body from falling forward; if not for its constant pull, the body would fall forward. (See Diagram B again.)

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Another example for gastrocnemius: Pulling an object to move it (as in the picture). The gastrocnemius lowers the feet, but in this case cannot do so as they are already on the ground, so the body bends backwards to maintain a suitable posture for pulling the object. But the angle between the feet and legs is still increased as in the previous example. (Note: This example seems to contradict what I wrote above that gastrocnemius is used for fast actions!) Plantar flexion: (planta- = sole (of foot), flex- = to bend) Point your toes, or when standing, lift your heels off the floor. These movements decrease the angle between the sole of the foot and the back of the leg, causing the foot to bend/point down. For example, the movement when depressing a car pedal or standing on the tiptoes is plantar flexion. The photograph shows a ballerina demonstrating plantar flexion of the foot when standing on her toes. leg

angle decreases

Note: Flexion always decreases the angle when a joint bends. So, during plantar flexion when the foot points downwards,

foot

it is the angle between the foot and the back of the leg that decreases. Logical! (See diagram at left for the angle during plantar flexion.)

How do anti-gravity muscles work? [This is essentially a repetition of what has been said earlier.] Proprioception and the balance organs in the inner ear play important roles in maintaining a good posture. 1. Proprioceptors detect the tension in the anti-gravity muscles and send this information up the spinocerebellar tracts to the cerebellum. 2. The vestibular apparatus in the inner ear, as we have seen, responds to changes in the body's position in space, such as head tilting, trunk stooping as well as head rotation. This information is sent from the inner ear to the vestibular nuclei in the midbrain and to the cerebellum. The cerebellum is the part of the brain that stores learned knowledge about automatic (non-voluntary) movements of muscles. (This was discussed in the Level I article. For example, by repeatedly practising serving a ball, a tennis player learns to optimise balance control during that movement; this knowledge becomes automatic and is stored in the cerebellum.] But here we are discussing posture and balance and not 'normal' movements. Still, the same happens for the things we learn on how to maintain a good posture. The cerebellum receives information about posture and balance from the inner ear and proprioceptive information (from the muscles and joints involved in bad posture or loss of balance) and compares this with its stored knowledge of posture and balance and sends any necessary corrective instructions back to the vestibular nuclei in the midbrain. These corrective signals, now in the midbrain, are then sent along the relevant vestibular tracts/pathways (i.e. numbers 1, 2a or 2b) to the parts of the body where action is needed to restore balance and posture. The medial vestibulospinal tract (2b) transmits signals that activate neck muscles to lift the head to

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correct a person's posture and balance. Signals are also sent to shoulder muscles to pull them back from a bent position. The lateral vestibulospinal tract (2a) transmit signals to activate anti-gravity muscles mainly in the legs, such as the gluteus, hamstrings and calves. These cause the body to straighten so that a person can walk upright. Revision Remember that for normal movements – not related to posture and balance, such as the tennis example above – the cerebellum sends any necessary corrective information up the cerebellothalamic tract to the to the M1 which then sends updated signals down the corticospinal tracts to correct the voluntary movement. Look back at Ascending tract #8 for the cerebellothalamic tract, and Descending tracts #1(a) and #1(b) for the corticospinal tracts. Question The cerebellum seems to store knowledge about the muscles for good posture. Does tit also store knowledge about poor posture? Perhaps. If so, how does it sort out a possible conflict between its knowledge of good and bad posture. I don't know. Maybe the cerebral cortex has to send a command for a person to sit/stand upright. Given such a command, the cerebellum presumably would use its stored knowledge of good posture to correct any deviation from this command. Summary for descending tracts to the body Tract

Origin

End

Primary function

1. (a) Lateral corticospinal Voluntary movement of trunk + distal tract Cerebral cortex (M1) Whole spinal limbs, e.g. feet, hands (b) Anterior corticospinal cord Voluntary movement of trunk + tract proximal limbs, e.g. arms, legs 2. Corticopontocerebellar tract 3. Rubrospinal tract* 4. (a) Medial reticulospinal tract* (b) Lateral reticulospinal tract* 5. Vestibular tracts (1) VOR (2) (a) Lateral vestibulospinal tract (b) Medial vestibulospinal

Cerebral cortex (M1, S1) Red nucleus (in midbrain) Pons Midbrain

Semicircular canals

Cerebellum

Compare planned vs actual movements of muscles

Whole spinal Additional movement of neck, upper cord and lower limbs Whole spinal cord

Eyes

Otolith organs + Whole spinal cerebellum cord Semicircular canals Upper spinal + cerebellum cord – 69 –

Initial of voluntary movements + helps in balance and posture – (as for MRST) –

Eye movement to fix gaze Posture of trunk, legs Posture of upper body

tract (3) Vestibulo-cerebellar connections (4) Vestibulothalamic tracts

Brainstem (vestibular nuclei) Midbrain (vestibular nuclei)

Cerebellum Cerebral cortex

Position of eyes, head, trunk and legs posture^ Conscious posture and balance

* Connected by tracts from the cerebral cortex, so initial input from cerebral cortex. ^ Correct position of eyes via VOR, of head via medial vestibulospinal tract, position of trunk and legs via lateral vestibulospinal tract.

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Sensations and Movements involving the Face: Brainstem Tracts We have discussed ascending and descending tracts for the body (i.e. without head and neck). But what about the facial area? It too sends sensory information and receives motor signals to move the facial muscles. But the pathways to and from the face do not involve the spinal cord. Instead, they involve the brainstem.

Sensory pathways The facial area also has sense receptors which detect touch, heat, pain, etc. For the face/facial area, this sensory information passes to the brainstem. The main (only?) pathway used to carry this information is called the trigeminal system. [Note: a system = parts that work together.] The main part of this system is the trigeminal nerve. This nerve actually consists of three parts (as shown in the diagram): 1. The ophthalmic nerve (V1), which serves the nose area to the top of the head (shown in blue), 2. The maxillary nerve (V2) for the cheek and upper jaw lip and teeth (shown in green), and 3. The mandibular nerve (V3) for the chin and lower jaw lip and teeth (shown in orange). Through each are conveyed sensations of touch, pain, temperature and proprioception. Remember that a 'nerve' is actually a bundle of fibres and not merely a single nerve. So in each of V1, V2 and V3, there will be a mixture of fibres – some of the fibres carry touch signals, some pain, temperature and some proprioception. (V1, V2 and V3 refer to the three parts of cranial nerve V – see below.) Cranial nerves In the section on the spinal cord, it was pointed out that there are 31 pairs of spinal nerves that enter/leave the spinal segments and carry signals to/from the body. The brainstem also has a number of nerves that enter/leave the brainstem and carry signals t/from the facial area. These are called cranial nerves. Some of the cranial nerves bring information from the sense organs in the face area to the brain; others send messages to control the muscles of the face. There are 12 pairs of cranial nerves (numbered using Roman numbers, I – XII).

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The trigeminal nerve is cranial nerve V (CN V), located in the pons area (see diagram above). Information both enters and leaves here. Sensory information (as mentioned above) for touch, pain, temperature and proprioception enters the brainstem. Motor nerves, which control the muscles for chewing leave here. (CN V also happens to be the largest of the cranial nerves.) The diagram shows two other cranial nerves, both located between the pons and the medulla: The facial nerve CN VII: Information both enters and exits. Sensory information for taste from the tongue (just the front 2/3 of the tongue) enters the brainstem here. Motor nerves, which control/move the muscles used in facial expression leave at CN VII (refer to the discussion on the corticobulbar tract later). The vestibular nerve CN VIII: The vestibular nerve, discussed above, is part of CN VIII. So sensory information for posture and balance from the inner ear enters the brainstem at this point. (Go back to the earlier diagram showing the vestibular nerve.) The arrow showing where it enters the brainstem is pointing to CN VIII. Note: The diagram above shows the location of six cranial nerves and not all 12, and the numbers and names for only three of them, namely, CN V, CN VII and CN VIII. The others do not concern us. The part labelled CN V is actually made up of three parts, one for each of V1, V2 and V3, which is shown more clearly in the diagrams in the discussion below. Why is it CN V called the trigeminal nerve? The word 'trigeminal' comes from tri-, meaning three and -geminus, meaning twin; three, because the nerve has three parts (V1, V2 and V5), and twin, because nerves enter the brainstem on two sides (just as they do in the spinal cord) .

The trigeminal nerve, CN V The three parts of the trigeminal nerve (V1, V2 and V3) enter the brainstem in the mid-pons. The diagram (right) shows more clearly than the one above the three parts of CN V. (Compare this diagram with that above to see that they are almost the same, though the one here is a lot simpler.) From the mid-pons, the input fibres fan out to their different targets, depending on which sense is involved. Ultimately though, they all go to the thalamus and then the cerebral cortex, again in a chain of three neurons. Unlike the spinal tracts, none go to the cerebellum. In classical anatomy most sensory information from the face is carried by the fifth cranial nerve (trigeminal nerve CN V), but sensation from parts of the tongue, mouth (and some other parts of the face area) is also carried by other cranial nerves such as CN VII, as mentioned above for the tongue. We will now look at the different kinds of sensory information carried by the three parts of the trigeminal nerve CN V. As with the spinal cord tracts, each pathway have three neurons – the first from the sensory

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area in the face to the brainstem, the second-order neuron from the brainstem to the thalamus and the third-order neuron from the thalamus to the cerebral cortex. Trigeminal nerve: Discriminative touch [Discriminative touch is the very sensitive sense of touch.] The three-neuron pathway is shown in the diagram (right). The example is for V2, the area that includes the cheek. Touch your right cheek. Here is what happens: A touch receptor in your right cheek is stimulated and the signal is conducted along the first-order neuron (shown in red) into the mid-pons part of the brainstem (which is where CN V enters the brainstem. Like many of the neurons in the spinal cord tracts, the second-order neuron (shown in black) decussates (crosses) to the opposite side. The neuron then joins the trigeminothalamic tract (also called the trigeminal lemniscus tract) and continues to the thalamus. From the thalamus, the third-order neuron (shown in light blue) terminates in the cerebral cortex (S1). [Remember from the discussion in the Level I article, the information in S1 is incorporated in our mental model of the environment – in this case touching our cheek – and is passed to the frontal lobe where we become conscious of the feeling of touch.] The diagram above labels the first-order neuron as Aβ (Abeta) neuron, which the type of neuron used for touch. See notes below for more on this. Exercise An alternative diagram – perhaps a little more realistic – for the discriminative touch pathway is shown on the right. Compare the pathway here with that in the diagram above to see that they are identical. Some notes 1. Some sources say that pressure and vibration are also carried along V2; other say they aren't! Who is

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correct? I don't know. This is part of the frustration in reading multiple sources! 2. There are actually two trigeminothalamic tracts called the anterior/ventral trigeminothalamic tracts or anterior/ventral trigeminal lemniscus tracts. (We will not discuss these.) 2. One source says that some of the second-order neurons do not decussate/cross to the opposite side. Instead they ascend in a parallel path to the thalamus and cerebral cortex but on the same side (not shown in the diagram). Therefore, because both sides of the cerebral cortex receive signals, it is very unusual for a person to lose all sensation on one side of the face from damage to the brain. So damage to the left side of the brain will leaves the right side functioning. 3. For the technically minded, the neurons in this pathway go to a different part of the thalamus (called the ventral posteromedial nucleus or VPM) than neurons in spinal cord tracts which go to the ventral posteromedial nucleus (VPM). If not technically minded, ignore it! Revision: Location of the cell bodies of first-order neurons Recall that all neurons have a cell body (soma). The diagram shows the symbol for a sensory neuron with the black dot representing the cell body: cell body first-order

neuron: axon

As discussed under 'Terminology' at the beginning of this section on Sensations and Movements involving the Body and Face, for almost all sensory neurons, the rule is that the cell body is outside the CNS, that is, outside the spinal cord or brainstem. Look back at the diagrams for the ascending spinal tracts on the previous few page – notice that the sensory cell body is in the dorsal ganglion, just before it enters a spinal segment, and so is located outside the spinal cord. The same is true in most of the diagrams involving the face. Also look at first-order neurons shown in the two diagrams above (one in red, the other in green). The cell body of the first-order neuron is located just outside the pons in the trigeminal ganglion and so is outside the CNS. (If it were inside the pons, it would be inside the brainstem, which is part of the CNS.) However, there always seems to be an exception to any rule! And the exception here is for proprioception nerves (discussed below); their cell bodies are inside the brainstem/CNS (in the midbrain area). They are the only exception to the rule. Trigeminal nerve: Pain and temperature The example here also involves V2, the part of the face that includes the cheek and lower nose. Imagine you stick a pin into the skin of your right cheek or touch it with something hot. Here is what happens: Pain or hot/cold receptors in the right cheek are stimulated and their signals conducted along the firstorder neurons (one of which is shown in red in the diagram on the next page) into the mid-pons part of the brainstem. At this point the neurons do something unusual – they turn and travel down the pons to the base of the medulla where they decussate. The second-order neurons join the trigeminothalamic tract (the same

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one as for touch above), thence on to the thalamus. (See note below for Adelta (Aδ) and C fibres.) Again, the third-order neurons proceed to the cerebral cortex. So the trigeminothalamic tract carries a mixture of sensory nerve fibres, some for touch, some for pain and some for temperature. trigeminothalamic

Exercise The diagram (above left) is an alternative diagram for the transmission of pain when a pin pricks the right cheek. Compare the pathway here with that in the diagram above to see that they are identical. Trigeminal nerve: Proprioception and the jaw reflex As a reminder, proprioception means 'awareness of oneself' and keeps us aware of the location and movements of body parts. Proprioception comes from the stretch in muscles and the tendons that join them to bones. Proprioception also occurs with receptors in the muscles of mastication (i.e. chewing). This causes a jaw reflex to occur, that is, an automatic reaction that causes the muscles of the lower jaw to close so that we can chew food without having to think about it. (This reflex is the same as described in Level I but involves the midbrain and not the spinal cord.) Here is what (I think!) happens: (Refer to the diagram on the right) The first-order neurons from the jaw area in V3 (which is for the chin and lower jaw including the mouth) (shown in orange) enter the brainstem (but see note below),

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turn and head up to the midbrain. The neurons then immediately turn and head down towards the pons area again! Most of these neurons then synapse (connect) with second-order motor neurons. These return to the jaw area where their signals cause the muscles of mastication to contract and the jaw to move up and chew the food. As this happens automatically without us having to think about it, it is a reflex action. Of course, we can consciously control our chewing, which means that a second-order sensory neuron, not shown in the diagram, links with the first-order neuron (in orange) and ascends via the thalamus, from where a thirdorder neuron terminates in the cerebral cortex. Could you draw this pathway? Notes on the diagram 1. Unusually, the proprioceptive neurons do not actually go through the trigeminal openings of V3 as the neurons for touch (V1) and pain and temperature (V2) do. Instead, they enter the brainstem via a small branch (shown in orange) that bypasses the V3 and then turns up towards the midbrain. 2. The proprioceptive axons from the face have a strange characteristic unique among somatosensory neurons: their cell bodies are inside the CNS. They are the only exception to the rule. In the diagram, the cell bodies ( orange blob) are in the midbrain (which is part of the CNS). Experiment: Jaw reflex The jaw reflex in the face behaves exactly like reflexes in the body, such as tapping below the knee causes the lower leg to automatically extend. The same thing happens with the jaw reflex. Place your thumb (or a finger) on the chin of a volunteer. Tap your thumb (finger) lightly with a hammer (a small one!). In a normal person, this may produce a twitch (though with some people nothing happens). An exaggerated jaw jerk, however, implies brain disease somewhere above the level of the pons. A, C, β, δ, γ nerve fibres In the diagrams above, you will notes these symbols Aα, Aβ, Aδ and C. As they are included in these diagrams, I will make a few comments on them instead of deleting them. These symbols show one way to classify neurons. A-alpha (Aα) fibres carry proprioceptive information. A-beta (Aβ) fibres carry touch information. A-delta (Aδ) fibres carry 'fast pain' information, that is, the sharp, acute, specific pain we first feel very quickly. 'C' fibres carry 'slow pain', that is, the more diffuse, dull pain we feel a few seconds after the sudden pain (carried by A-delta fibres). C fibres are estimated to account for about 70% of all nociceptive (i.e. pain) fibres. The speed at which the impulses travel also differ; myelinated fibres transmit the impulse faster than unmyelinated fibres. Of the above four, only the C fibres are unmyelinated.

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Relative speeds: A-alpha (Aα): 80-120 m/s, about that of a jetliner. A-beta (Aβ): 35-90 m/s, about that of a racing car. A-delta (Aδ): 5-40 m/s, about that of a racing cyclist. C: 0.5-2 m/s, about walking speed. So, because the Aβ neurons for touch are heavily myelinated, they transmit signals very quickly. Also, because the 'slow pain' C fibres are unmyelinated, they send impulses slower than the (thinly) myelinated 'fast pain' A-delta (Aδ) fibres. There are also A-gamma (Aγ) nerve fibres but these are motor fibres and not sensory fibres.

Motor pathway: the corticobulbar tract [cortico- = cerebral cortex; -bulbar = bulb. 'Bulbar' refers to the brainstem. The ancient anatomists thought that the medulla in the brainstem looked like a plant bulb.] The corticobulbar tract passes from the cerebral cortex (M1) to the medulla in the brainstem. The nerve fibres exit the medulla at CN VII and go to the facial muscles. (The diagram for this pathway is shown on the next page.) The corticobulbar tract along with the corticospinal tract and the most important motor tracts. Both are descending tracts that control voluntary movements. The corticospinal tract controls voluntary movements of muscles of the body – that is, all parts of the body below the neck. The corticobulbar tract does the same thing but for the muscles of the facial area – that is, the face, head and neck. Both tracts, like all motor tracts, are two neuron descending pathways. (Unlike ascending pathways, the thalamus is not involved.) In the corticospinal tract, the first neuron carries motor signals from the cerebral cortex (M1) down to the spinal cord. There, in one of the spinal segments it synapses with a second nerve (a spinal nerve) that exits the segment and passes to the muscles. (Refer to the diagrams of this tract earlier in the article.) In the corticobulbar tract, the first neuron carries motor signals from M1 to the brainstem (and not the spinal cord). There it synapses with the facial nerve which is the seventh cranial nerve (CN VII). It emerges from the brainstem between the pons and the medulla (see earlier diagram again) and passes to the face area where it controls the muscles of facial expression. The diagram on the right shows the facial nerve (in yellow) as it divides and spreads to different parts of the face after leaving the brainstem at CN VII. The picture also shows the trigeminal nerve (CN V), which as we saw above, brings sensory information from the sense organs into the brainstem. The diagram below shows a simplified diagram of the path of the corticobulbar tract involving the facial nerve (CN VII).

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The pathway is: 1. The first-order neuron (shown in black) descends from the right side of the pre-motor cortex (M1) to the pons of the brainstem (i.e. where CN VII is located). It decussates and terminates on the left side of CN VII. (Of course, there is also another neuron from the left side of the brain, which, for clarity, is not shown.) 2. At CN VII, the first-order neuron synapses with the facial nerve (the second-order neuron, shown in red) and passes to the left side of the brain. There it goes to whichever part of the face is activated in M1, for example, for a movement of the left cheek. Note that the thalamus (shown in dark red) is not involved in this pathway.)

Notes on the diagram: 1. For the sake of clarity, the diagram is very, very simplified (see below). 2. Because the first-order neuron decussates (in the pons), M1 on the right side of the brain sends its signals to the left side of the face. 3. The corticobulbar tract shown in the above diagram sends neurons to one side of CN VII in the brainstem. But there is a second tract which sends neurons to the other side of the brainstem, which is why CN VII is labelled twice. However, again for clarity, this second pathway is not drawn – but is shown in the diagram below under 'Extension'. This second pathway serves the right side of the face. Extension: More on the corticobulbar tract From the above discussion, it appears that these pathways control the whole face. But this is not correct. There are actually pathways from M1 to the upper half of the face (UF, above the level of the eyes) and another to the lower half of the face (LF). So there are in total four corticobulbar tracts.

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This happens because the pons area for cranial nerve VII is actually divided into two halves – an upper half and a lower half. So, the neurons from one corticobulbar pathway go to the upper half of CN VII while those from the other pathway go to the lower half of CN VII. And because there are two sides to the brain, there are another two pathways to give a total of four areas of CN VII served (i.e. upper. lower, left and right). These four pathways are shown in the diagram on the right. (Ignore the thick black and red bars for the moment.) Consider the neurons that serve the upper face (UF): Neurons for the upper face muscles (UF) leave M1 on both sides of the brain and descend to the brainstem where they enter both the left and right CN VII. They then synapse with motor neurons (red lines) that serve the muscles of the upper face. Now consider the neurons that serve the lower face (LF): Neurons again leave M1 on both sides of the brain and descend to the brainstem. But here they differ. They do not enter both sides of CN VII. The LF neuron from the left side of the brain descends and enters CN VII on the opposite/right side while the neuron from the right side of the brain enter the opposite/left side of CN VII. They then synapse with the motor neurons (red lines) the serve the muscles of the lower face. Lesions to the corticobulbar tract [lesion = damage to part of the body through injury or disease.] Upper neuron lesion: Suppose the brain has a lesion represented by the black bar in the above diagram. This lesion cuts off the UF motor signals from the right side of the brain to the brainstem. Is this serious? Not too much as the signals from M1 on the other side of the brain still get through and so can control the muscles of the upper face. The muscles may be a little weaker but they are not paralysed. But now consider what happens to the lower face. No motor signal gets through to the brainstem from the right side of the brain. So, no signal gets to the lower left side of the face. Thus these muscles are either completely paralysed (cannot move at all) or are very weak. The shaded part in the diagram on the right indicates this facial weakness. The other side of the lower face however is unaffected as the signal from M1 on the left side of the brain still gets through.

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Bell's palsy In 2009, I suffered from an affliction called Bell's palsy. This is a condition where the facial nerve that moves the muscles of on one side (usually) of the face becomes inflamed and swollen. The swelling causes pressure on the nerve and results in the total loss of control of half the face. The cause of Bell's palsy is not known. It is named after Scottish anatomist and Edinburgh graduate Charles Bell (1774–1842), who first described it. The red bar represents a lesion to the lower motor neurons (facial nerves). It affects the muscles on the same side as the injury as shown by the shaded part in the diagram on the right. Symptom of Bell's palsy include: A sudden weakness or paralysis that causes the side of the face, and especially the corner of the mouth, to droop, that is, to bend or hang downwards limply (this is the main symptom). This makes smiling difficult. There may be a drooping eyelid and difficulty in closing the eye on the side of the face affected. There may also be a smoothing of the forehead and the inability to wrinkle the brow. Can you identify these symptoms in the face of the lady shown here? And which side of her face is affected? For most people, Bell's palsy is not permanent. Mine lasted a month or two (I can not now remember exactly how long). Some people are born with it; an example is the actor Sylvester Stallone. Can you see evidence of Bell's palsy in his picture (right)? George Clooney also suffered from Bell's palsy as a child. Another is the former Canadian Prime Minister Jean Chretien, who had Bell's palsy as a child. Once, during a leadership campaign, he referred to his affliction, saying that he was "One politician who didn't talk out of both sides of his mouth."

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References The following is a list of websites that I referred to, for text, diagrams or both, in the preparation of these two articles. Some of the websites are very good, others marginally so. Websites that turned out to be of little or no use are not included here. Some of the websites are not too difficult to understand; others are very difficult. There are millions of other websites available on the Internet., but while some of these may be very good and have been missed, it is just too time consuming to have to search through so many. In the list below, I begin with sites that include many large block of links. Then I list individual sites. As well as providing the Internet links, I give the occasional comment on some of them. McGill University: Body movement and the brain A very good source of information. Contains sites at different levels of difficulty from beginner to intermediate to advanced. There are also what are called 'Level of Organisation' which go from Neurological to Cellular to Molecular levels. My focus was at the neurological and cellular levels. Here is the site to begin with: http://thebrain.mcgill.ca/flash/d/d_06/d_06_cr/d_06_cr_mou/d_06_cr_mou.html University of Texas: Neuroanatomy websites Again, a large number of websites available on many topics. Sometimes a little difficult to understand, but still very useful. Many of the sites have useful animations. Here is the link for the index: http://neuroscience.uth.tmc.edu/toc.htm The following are chapters relevant to the brain and movement: Section 2: Sensory Systems Chapter 1: Overview of the Nervous System Chapter 2: Somatosensory Systems Chapter 3: Anatomy of the Spinal Cord Chapter 4: Somatosensory Pathways Chapter 5: Somatosensory Processes Chapter 10: Vestibular System: Structure and Function Chapter 11: Vestibular System: Pathways and Reflexes (deals with ascending and descending tracts) Section 3: Motor Systems Chapter 1: Motor Units and Muscle Receptors Chapter 2: Spinal Reflexes and Descending Motor Pathways Chapter 3: Motor Cortex Chapter 4: Basal Ganglia Chapter 5: Cerebellum Chapter 6: Disorders of the Motor System Chico State University http://www.csuchico.edu/~pmccaffrey/syllabi/CMSD%20320/620index.html Chapters 1, 2, 4, 6, 7, 8, 9, 10

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Tutis (University of Western Ontario) http://www.tutis.ca/NeuroMD/index.htm Session 4: Cerebellum & Basal Ganglia Chapter 5: Muscle Receptors, Spinal Reflexes and Muscles Session 6: Motor Cortex Chapter 7: Touch Chapter 8: Vestibular System and Eye Movements What-When-How This contains a large number of relevant websites which contain useful material and diagrams. However, the site is very hard to navigate and the home page is not helpful. For this reason, I am providing the individual links. There will probably be overlap in these links. Neuroscience (provides a list of topics): http://what-when-how.com/category/neuroscience/ Overview of the Central Nervous System (Gross Anatomy of the Brain) Parts 1 – 3 Begin with the following link to an overview in Part 1: http://what-when-how.com/neuroscience/overview-of-the-central-nervous-system-gross-anatomy-of-thebrain-part-1/ Cerebral cortex lobes: http://what-when-how.com/neuroscience/overview-of-the-central-nervous-system-gross-anatomy-of-thebrain-part-2/ Basal ganglia, cerebellum, diencephalon: http://what-when-how.com/neuroscience/overview-of-the-central-nervous-system-gross-anatomy-of-thebrain-part-3/ The neuron – types of neurons: http://what-when-how.com/neuroscience/histology-of-the-nervous-system-the-neuron-part-2/ Synaptic transmission: http://what-when-how.com/neuroscience/synaptic-transmission-the-neuron-part-1/ Neurotransmitters: http://what-when-how.com/neuroscience/neurotransmitters-the-neuron-part-1/ Spinal cord: http://what-when-how.com/neuroscience/the-spinal-cord-organization-of-the-central-nervous-systempart-1/ Spinal segments, dermatomes: http://what-when-how.com/neuroscience/the-spinal-cord-organization-of-the-central-nervous-systempart-2/ Ascending tracts (this link and the next): http://what-when-how.com/neuroscience/the-spinal-cord-organization-of-the-central-nervous-systempart-2/ Descending tracts (this link and next): http://what-when-how.com/neuroscience/the-spinal-cord-organization-of-the-central-nervous-system– 82 –

part-3/ Reflex actions (this link and next): http://what-when-how.com/neuroscience/the-spinal-cord-organization-of-the-central-nervous-systempart-5/ Upper motor neurons: http://what-when-how.com/neuroscience/the-upper-motor-neurons-motor-systems-part-1/ Basal ganglia and pathways: http://what-when-how.com/neuroscience/the-upper-motor-neurons-motor-systems-part-1/ http://what-when-how.com/neuroscience/the-basal-ganglia-motor-systems-part-2/ Basal ganglia and Parkinson's disease: http://what-when-how.com/neuroscience/overview-of-the-central-nervous-system-gross-anatomy-of-thebrain-part-3/ Cerebellum (Parts 1 – 4) Part 1: http://what-when-how.com/neuroscience/the-cerebellum-motor-systems-part-1/ Glossary (Neuroscience) A – Z: In 13 parts: http://what-when-how.com/category/neuroscience/page/11/ First page of glossary: http://what-when-how.com/neuroscience/glossary-neuroscience-part-1/ You will have to flick through the pages of the glossary to get to the term you want. However, more simply, go to the link for Page 1 then change the number at the end of the link displayed in the search box. Studying the brain Boundless site: https://www.boundless.com/psychology/textbooks/boundless-psychology-textbook/biologicalfoundations-of-psychology-3/psychology-and-the-brain-406/studying-the-brain-149-12684/ The Nervous System Overview (News Medical site). A good overview: http://www.news-medical.net/health/What-is-the-Nervous-System.aspx Introduction (Boundless site): https://www.boundless.com/psychology/textbooks/boundless-psychology-textbook/biologicalfoundations-of-psychology-3/the-nervous-system-34/introduction-to-the-nervous-system-146-12681/ Functions of the nervous system (News Medical site): http://www.news-medical.net/health/Function-of-the-Nervous-System.aspx Structure and function of the brain (Boundless site): https://www.boundless.com/psychology/textbooks/boundless-psychology-textbook/biologicalfoundations-of-psychology-3/structure-and-function-of-the-brain-35/ CNS and PNS McGraw Hill Education: highered.mheducation.com/sites/dl/free/0070960526/323541/mhriib_ch11.pdf

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School of Nursing site. Very good diagrams: http://www.austincc.edu/apreview/PhysText/CNS.html#cerebrum Boundless site: https://www.boundless.com/psychology/textbooks/boundless-psychology-textbook/biologicalfoundations-of-psychology-3/the-nervous-system-34/the-central-nervous-system-cns-147-12682/ Parts of the brain Lobes of the brain (Boundless site): https://www.boundless.com/psychology/textbooks/boundless-psychology-textbook/biologicalfoundations-of-psychology-3/structure-and-function-of-the-brain-35/cerebral-hemispheres-and-lobes-ofthe-brain-153-12688/ Midbrain, Cerebellum (Boundless site): https://www.boundless.com/psychology/textbooks/boundless-psychology-textbook/biologicalfoundations-of-psychology-3/structure-and-function-of-the-brain-35/lower-level-structures-151-12686/ Spinal cord Diagrams of the spinal cord and parts http://www.anatomy-diagram.info/cross-section-of-spinal-cord-labeled/ Vertebrae of the Spine Cedars-Sinai site. Good diagrams: http://www.cedars-sinai.edu/Patients/Programs-and-Services/Spine-Center/The-Patient-Guide/Anatomyof-the-Spine/Vertebrae-of-the-Spine.aspx Neurons McGraw Hill Education site: highered.mheducation.com/sites/dl/free/0070960526/323541/mhriib_ch11.pdf Boundless site: https://www.boundless.com/psychology/textbooks/boundless-psychology-textbook/biologicalfoundations-of-psychology-3/neurons-33/introducing-the-neuron-141-12676/ Neurotransmitters Excitatory and inhibitory neurotransmitters (McGill University site): http://thebrain.mcgill.ca/flash/d/d_01/d_01_m/d_01_m_ana/d_01_m_ana.html Neurotransmitters (McGill beginner site; you can then proceed to higher levels): http://thebrain.mcgill.ca/flash/d/d_01/d_01_m/d_01_m_ana/d_01_m_ana.html Four steps in neurotransmitter transmission (McGill site. Simple diagrams): http://thebrain.mcgill.ca/flash/d/d_01/d_01_m/d_01_m_fon/d_01_m_fon_2a.swf Structure and function of the brain Columbia University site: http://www.columbia.edu/cu/psychology/courses/1010/mangels/neuro/anatomy/structure.html Neuroscience for Kids A lot of good and digestible content. Go to 'explore' (click top lhs) for different topics: – 84 –

http://faculty.washington.edu/chudler/neurok.html Experiments on touch: http://faculty.washington.edu/chudler/chtouch.html Brain facts One page general introduction on more complex movements http://www.brainfacts.org/sensing-thinking-behaving/movement/articles/2012/more-complex-movements/ Parkinson's disease http://www.brainfacts.org/diseases-disorders/degenerative-disorders/articles/2012/parkinsons-disease/ The anatomy of movement Brain connection site. http://brainconnection.brainhq.com/2013/03/05/the-anatomy-of-movement/ University of Minnesota – motor control system http://www.d.umn.edu/~jfitzake/Lectures/DMED/MotorControl/Organization/SystemDesign.html Slide Player Slides 4 – 23 cover the nervous system, neurons and types of neurons, CNS and PNS: http://slideplayer.com/slide/6095767/ Sensory and motor tracts – ascending and descending: http://slideplayer.com/slide/4875971/ Biology A-Level notes Nice simple overview of nervous system, neurons and reflex arc: http://biology4alevel.blogspot.hk/2015/09/115-control-and-co-ordination-in.html Basal ganglia pathways (one page) NeuroLove site. http://neurolove.tumblr.com/post/2745681497/lets-breakdown-the-basal-ganglia-pathways-there Substantia niagra (NeuroLove site): http://neurolove.tumblr.com/search/Substantia+Nigra+ Cerebellum and feedforward control Frontiers in Human Neuroscience site. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4069484/ Sensory and Motor tracts (i.e. Ascending and descending) tracts of the spinal cord Southeast Community College site. Quite a good site. http://www.napavalley.edu/people/briddell/documents/bio%20218/15_lecture_presentation.pdf Sensory pathways (Ascending) CNS Clinic site http://www.humanneurophysiology.com/sensorypathways.htm Neuro quizlets Many useful notes plus explanatory diagrams on all topics covered in the two articles. Spend some time – 85 –

looking at them. Example: Cerebellar motor circuits (scroll down to the relevant quizlets) https://quizlet.com/11011908/neuro-exam-3-chp-8-flash-cards/ Videos (some are complicated) CNS and PNS https://www.youtube.com/watch?v=lh4pdaWYu7A Basal Ganglia https://www.youtube.com/watch?v=69YkcDWbxiU Basal Ganglia: Direct pathway https://www.youtube.com/watch?v=noBTt1tsAMs Basal Ganglia: Indirect pathway https://www.youtube.com/watch?v=VZer0w7foLg Basal Ganglia: Both direct and indirect pathways https://www.youtube.com/watch?v=82oIHBGDoiI https://www.youtube.com/watch?v=h6La63khW2I https://www.youtube.com/watch?v=NcIWYCkKwVA

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