THE NERVOUS SYSTEM IN THE HORSE

Equine Ed: Anatomy & Physiology Manual, written by VJ Hawkins

THE NERVOUS SYSTEM IN THE HORSE The nervous system is the system, which correlates the adjustments and reactions of an organism to internal and external conditions. It can be compared to our telephone system, whereby a complex network of peripheral nerve fibres (similar to telephone wires) connects all the parts of the body with a Central Nervous System (CNS) (which serves as the switchboard). The impulses or messages, which travel over these live wires at a tremendous rate of speed, enable us to make rapid adjustments to a variety of conditions. In contrast to the nervous system, the ENDOCRINE SYSTEM resembles the slower moving postal service but also plays an important part in general communication. The nervous system consists of two main divisions - central and peripheral (named due to their location)

THE CENTRAL NERVOUS SYSTEM (CNS) - consists of the brain and the spinal cord.

THE PERIPHERAL NERVOUS SYSTEM (PNS) - Consists of the nerves, which connect the CNS to the rest of the body and are further divided into two sections depending on their destination. i) The Somatic system consists of nerves, which connect the CNS to structures of the body wall (soma) eg. Skeletal muscles and the skin, often referred to as the Voluntary Nervous System (VNS) as it is under voluntary control. It includes both motor and sensory nerves. ii) The Visceral system consists of nerves, which connect the CNS to smooth muscle, cardiac muscle and glands. These structures are not under our conscious control so are usually referred to as the Autonomic Nervous System (ANS). iii) The ANS is further specialised into two subsidiary groups known as sympathetic and parasympathetic., which are controlled by the hypothalamus in the brain and are in constant operation to adjust the body processes. iv) Sympathetic nerves function to excite the body in response to danger or other challenges. v) Where as Parasympathetic nerves keep things under control and acts as a dampener to calm things. All in all the nervous system contributes to the maintenance of HOMEOSTASIS.

Equine Ed: Anatomy & Physiology Manual, written by VJ Hawkins

STRUCTURE The structural features of the Nervous System include Nerve cells - Neurons and supporting cluster cells - Neuroglia which is very delicate connective tissue eg brain and spinal cord. Neurons are microscopic units designed for the transmission of impulses and vary considerably in length. Like other cells they consist of a cell body which contains the basic cell structure such as a nucleus, Mitochondria, Golgi apparatus and protoplasm ( neuroplasm). They also contain a network of fine fibrils known as neurofibrils. The unique structure of the neuron is in its “tree - like” appearance, with branches of the cell body called DENDRITES, and a trunk known as an AXON. Sensory neurons differ in shape to motor neurons. The neuron may develop a sheath or membrane covering known as neurolemma, or remain uncovered Peripheral nerves such as the sciatica are composed of millions of nerve fibres, arranged in bundles. Similar to muscle fibres they are bound to one another by delicate connective tissue called endoneurium and then bundled together and further surrounded by a stronger sheath called perineurium. When several nerve fibres are grouped together they are bound by epineurium and called a nerve. If neurons die they can never be replaced, therefore the vital portion of each neuron is located for protection in the grey matter of the brain and spinal cord. Neurons are classified into three groups - afferent, efferent and central. a) Afferent neurons transmit impulses away from the periphery to the CNS.  Somamatic afferent neurones (sensory neurons)  Visceral afferent neurones b) Efferent neurons transmit impulses away from the CNS to the periphery.  Somatic efferent neurons (motor neurons)  Visceral efferent neurons - secretary (stimulate glands) - excitary smooth muscle - inhibitory and cardiac. c) Central internuncial interneuron or association neurons are those which transmit impulses within the CNS

Equine Ed: Anatomy & Physiology Manual, written by VJ Hawkins

FUNCTION The two outstanding characteristics of nerve tissue (similar to muscle tissue) are excitability and conductivity. Both are inseparably related to depolarisation (interior pos, exterior neg) and repolarisation (interior neg -ve, exterior pos +ve) which provide for stimulation and the conduction of impulses. The entire nervous system is composed of chains of neurons but there are no actual physical links between them. There are however, functional links known as synapses. The secretion of excitatory or inhibitory substances at these junctions provide a passage for impulses When an impulse reaches the neuromuscular junction (at the muscle) a specific chemical mediator called acetylcholine is released which initiates the impulse (depolarisation) in the muscle cells. In order for the synapse to return to its resting state - ready for another stimulus, the acetylcholine must be de-activated. This is performed by secretion of the enzyme Cholioesterase, which splits the acetylcholine into acetic acid and choline. Various drugs can be used to alter the transmission of nerve impulses and are used in anaesthetics. NB: Neuritis - is inflammation or degenerative lesions of a nerve, which may result in pain, hypersensitivity, loss of sensation, muscular paralysis or atrophy. Neuralgia is a condition in which there is severe paenoxysmal (recurring suddenly) pain along the course of the nerve.

Equine Ed: Anatomy & Physiology Manual, written by VJ Hawkins

THE CENTRAL NERVOUS SYSTEM THE BRAIN The brain is made up of three main parts - the brainstem - the cerebellum - the cerebum The brainstem controls involuntary actions such as the heart beat and breathing. The cerebellum co-ordinates movements of the muscles so that actions are smooth and balanced. The cerebum is the largest part of the brain and is divided into two halves or hemispheres the left and right. The outer layer has special areas to receive messages regarding sight, touch, hearing and taste while other areas control movement, intelligence and personality. Twelve pairs of nerves leave the brain and travel to various parts of the body. They are referred to by Roman numerals from I - XII. (Refer to Plate 78 in text)

THE SPINAL CORD The Spinal Cord is that part of the CNS which occupies the canal running the entire length of the vertebral column and ending at the caudal half of the second sacral vertebra From this extends the cauda equine (horses tail) which innervates the tail and hindquarters. Forty-two pairs of spinal nerves emerge from the cord at intervals corresponding to the vertebrae. Thus there are eight cervical, eighteen thoracic, six lumbar, five sacral and five coccygeal nerves.

THE SENSORY MECHANISM The Sensory Mechanism is a group of structures, which detect change in the environment and give rise to a sensation. Sense organs (receptors) are the peripheral endings of dendrites of afferent neurons. The pathway of these neurons to the brain are programmed to specialised areas of the brain known as sensory areas in the cerebral cortex. Senses can be grouped into: - Cutaneous senses ; touch, pressure, heat, cold and pain. - Muscle sense (proprioceptive or kinesthetic ) : body awareness - - Visceral sense : hunger, thirst, nausea distended bladder and bowel. - - Olfactory (smell) and gustatory (taste) - – Vision - – Hearing Without sense organs animals would be completely helpless and unable to survive.

Equine Ed: Anatomy & Physiology Manual, written by VJ Hawkins