Anatomy & Basic Physiology of Vestibular System
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Balance of our body during static or dynamic positions is maintained by: 1. Vestibular apparatus (inner ear) [Fastest system]. 2. Vision (Eye) 3. Somatosensory/ proprioceptive system These inputs then integrated at the level of - Brain stem - Cerebellum - With influence from cerebral cortex (frontal, parietal & occipital lobes) Results in various motor & perceptual outputs.
Vestibular function contributes to perception of: -
Self-motion Head position in relation to motion to maintain clear vision Stabilizing body position to avoid falling Spatial orientation relative to gravity (ranging from simple tasks like reading or walking to more complex ones like driving or sports). To correct for posture at the limits of stability, & Adaptation to new balance tasks & Compensation to restore learned tasks after vestibular diseases. ** These tasks are achieved by sensory integration of 3 balance sensory modalities (V..V..S..) at level of vestibular nuclei.
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As cochlea transduces sound into electrical activity, Vestibular system transduces mechanical energy into electrical activity
Types of movement: 1. Translational (Linear) 2. Rotational 1. Translational motion: in these planes (linear acceleration and static displacement of the head) is the primary concern of the otolith organs. 2. Rotational motion: The three degrees of rotational freedom refer to a body's rotation relative to the x, y, and z axes: -
X axis: front-backward → rotation around it called (( Roll )).
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Y axis: Horizontal axis (rt-lt) → rotation around it called (( Pitch )).
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Z axis: vertical axis (up down) → rotation around it called (( Yaw )).
SCCs are primarily responsible for sensing rotational velocity around these three axes.
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Motion sensing is a "mission critical" task -- for example, vestibular system is needed to walk reasonably safely in the dark.
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The vestibular system incorporates considerable redundancy.
Vestibular system is formed of bony & membranous labyrinth with 5 sensory end organs: 1. 3 crista ampularis in 3 SCC → for angular velocity. 2. 2 otolith organs: Utricle & Saccule → for linear acceleration **The information these organs deliver is proprioceptive in character, dealing with events within the body itself (stimuli that are produced and perceived within an organism), rather than exteroceptive, dealing with events outside the body, as in the case of the responses of the cochlea to sound.
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Gross & Microscopic Anatomy of Vestibular System TEMPORAL BONE: -
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The temporal bone contributes to the base and lateral wall of the skull, forms part of the middle and posterior fossae. It is divided into four parts: the squamous, tympanic, petrous, and mastoid areas. The ear is divided into three anatomic parts: the external, middle, and inner ear. Except for the auricle and soft tissue portion of the external auditory canal, the ear is enclosed within the temporal bone of the skull. The petrous portion, or pyramid, contains the sense organs of the inner ear.
INNER EAR (LABYRINTH) Vestibular Relations: - The vestibular apparatus is enclosed within a bony labyrinth, in the petrous portion of the temporal bone. - The five vestibular end organs, along with the cochlea, are contained within an endolymphfilled membranous labyrinth, the endolymphatic space, which is contained in the perilymphfilled bony labyrinth, the perilymphatic space. - The vestibule is situated between: a) the internal auditory meatus anteromedially and b) the middle ear cavity laterally c) The entrance to the mastoid antrum, the aditus ad antrum, is just lateral to the horizontal semicircular canal. d) The cochlea sits anterior to the vestibule and is connected to the vestibule by the narrow ductus reuniens. e) Posterior and lateral to the vestibule are the mastoid air cells. f) Directly medial is the posterior cranial fossa, into which the endolymphatic duct and sac extend beneath the dura.
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The bony labyrinth: consists of an anterior cochlear part and a
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posterior vestibular part. The vestibule is a central chamber (about 4
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mm in diameter) marked by the recesses of the utriculus and sacculus (i.e., the macules). The superior and posterolateral walls
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contain openings for 3 semicircular canals, and anteriorly the vestibule is continuous with the scala vestibule of cochlea. Medial to the bony labyrinth is the internal
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auditory canal. Halfway between the canal and the sigmoid sinus, the slit-like aperture of the vestibular aqueduct contains the endolymphatic sac, a structure important in the exchange of endolymph. The second opening is that of the cochlear aqueduct , at the same level as the auditory canal but on the inferior side of the pyramid. The labyrinthine opening of this channel is located in the scala tympani, providing a connection between the subarachnoid and the perilymphatic spaces. Infection or blood in the cerebrospinal fluid (CSF) can make its way into the inner ear through this channel. A space containing perilymphatic fluid, a supportive network of connective tissue, and blood
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vessels lies between the periostium of the bony labyrinth and the membranous labyrinth; the spaces within the membranous labyrinth contain endolymphatic fluid.
Labyrinthine Fluids: -
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The endolymph-filled membranous labyrinth is completely immersed in perilymphatic fluid and suspended by fine trabecular filaments within a cartilaginous or osseous labyrinth. Compartmentalization of the fluids plays two roles. 1. minimizes sensitivity of the inner ear to atmospheric pressure modulations. 2. provides for the electrochemical gradients between the endolymph, perilymph, and the intracellular compartments necessary for hair cell transduction and neural transmission. The perilymph is rich in Na+ ions and deficient in K+ ions and has a chemical composition very similar to cerebrospinal fluid. The endolymph has a K+ concentration higher than that found in any other fluid in the body. There is high protein content in the endolymphatic sac, compared with that in the rest of the endolymphatic space, is consistent with the sac’s role in the resorption of endolymph. The difference in protein concentration between perilymph and CSF argues against a free communication between the compartments of these two fluids and in favor of an active process of perilymph production.
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The electrolyte composition of the endolymph is critical for normal functioning of the sensory organs bathed in fluid. Rupture of the membranous labyrinth in experimental animals causes destruction of the sensory and neural structures at the site of the endolymph–perilymph fistula.
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Tight junctions at the apical face of hair cells and adjoining supporting cells form a barrier that separates the labyrinthine fluids such that the basolateral surfaces of hair cells are bathed in perilymph whereas the apical faces are bathed in endolymph.
Fluid Dynamics -
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Perilymph is thought to be a filtrate of CSF and from blood vessels in the ear. Filtration from blood vessels within the perilymph space, since blocking the cochlear aqueduct does not appear to affect inner ear morphology or function. As noted previously, the CSF communicates directly with the perilymphatic space through the cochlear aqueduct, a narrow channel 3 to 4 mm long with its inner ear opening at the base of the scala tympani. In most instances, this channel is filled with a loose net of fibrous tissue continuous with the arachnoid. The size of the bony canal varies from individual to individual. The main sites to produce endolymph are the marginal cells of the stria vascularis of the cochlea and the dark cells of the vestibular labyrinth. Endolymph production is tightly coupled to K + secretion Na-K-Cl cotransporter expressed in the basolateral membrane of marginal and dark cells pumps K + into these cells to high levels. Potassium channels at the apical surface of the marginal and dark cells allow K + accumulating in the cells to flow back into the endolymph, thus maintaining the high K + concentration and the generator potential mutations in the genes that code for the NaK-Cl cotransporter protein or the apical K + channel proteins lead to a failure to produce endolymph and deafness and imbalance. Cellular water channels, aquaporins, are essential for the fluid regulation of several organs (e.g., kidney, lung, and brain), and aquaporins are widely expressed in the inner ear but their role in labyrinthine fluid dynamics is yet to be defined. Three theories have been proposed regarding the regulation of endolymph volume: a) The longitudinal, or Guild, theory assumes that endolymph is produced in the cochlea and vestibular labyrinth and flows toward the endolymphatic sac, where it is resorbed. b) The radial theory assumes a local transverse and active diffusion process between endolymph and perilymph. c) The dynamic theory, a combination of both. The pressure of the inner ear fluids has been shown by direct measurements to be different from the atmospheric pressure of the middle ear. The perilymph and endolymph are both at an equal positive pressure of approximately 7 to 10 cm of H 2 O.
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When the pressure in the intracranial cavity or the labyrinth increases to above normal, the pressure will tend to equilibrate between the two compartments The round window elasticity provides a measure of protection for pressure regulation in the inner ear. Destruction of the epithelium lining the endolymphatic sac or occlusion of the duct results in an increase of endolymphatic volume in experimental animals. Leading to an expansion of cochlear and saccular membranes, which may completely fill the perilymphatic space.
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Blood Supply: -
Labyrinthine artery (or IAA), the sole blood supply to the inner ear, either branches off from the AICA (most common) or from the basilar artery directly. anterior inferior cerebellar artery the internal auditory artery. Within the internal auditory canal, the internal auditory artery irrigates the ganglion cells, nerves, dura, and arachnoid membranes and divides into two main branches: a) The common cochlear artery further branches into the - main cochlear artery supplies the apical ¾ of the cochlea - vestibulocochlear artery, the posterior vestibular artery to the inferior part of the saccule and the ampulla of the posterior SCC.
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cochlear ramus the cochlear ramus irrigates the basal one fourth of cochlea. b) The anterior vestibular artery supplies the utricle, superior part of the saccule, and ampullae of the anterior and horizontal SCC. -
Venous drainage *Internal Auditory vein, *veins of cochlear aqueduct & vestibular aqueduct into inferior petrosal & sigmoid sinus.
Cochlear function is affected within 15–30 sec but can recover even after 5–10 min of complete blood flow obstruction. If the dysfunction is of a longer duration, the damage is irreversible and associated with pathological inner ear changes, including sensorineural degeneration and even new bone formation destroying the inner ear spaces. Shorter intervals of ischemia produce mixed functional and morphological changes. Interfering with endolymph circulation (experimental hydrops), and thus increasing inner ear pressure, can impair labyrinthine blood flow.
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CELLULAR MORPHOLOGY OF VESTIBULAR SENSORY EPITHELIUM: -
The sensory epithelium is composed of several different elements: hair cells, supporting cells, afferent nerve fibers and their synaptic terminals, and efferent nerve fibers and their synaptic boutons. - Two basic cell types are present within the sensory epithelium: supporting cells and hair cells. a) Supporting cells: o extend from the basement membrane to the apical surface, and o their nuclei are usually found just above the basement membrane and below the hair cells. o In sections taken tangential to the apical surface, several supporting cells can be seen to form a ring around an individual hair cell. o Contain well-developed Golgi complexes, many mitochondria, and occasional lipid droplets. o Top surface is covered with microvilli. o The upper part contains large numbers of round or ovoid granules, and these secretory granules are responsible for the formation of the cupula and otolithic membrane. o Supporting cells can differentiate into new hair cells following destruction of the sensory epithelium. This was initially seen in the cochlea of chicken after acoustic trauma and then in the cochlea and vestibular labyrinth of mammals after drug ototoxic exposure b) Hair cells: o contain a bundle of stereocilia attached to their apical surface and grouped in a staircase arrangement. o These stereocilia are densely packed with longitudinally oriented actin filaments that extend into the hair cell and are anchored in a thickened region near the apical surface; this is termed the cuticular plate, a dense filamentous meshwork of randomly oriented actin filaments that fills up the area just under the apical surface of the cell except for the region of the kinocilium. A basal body and many large vesicles are usually in the region of the kinocilium. o Hair cells are surrounded by supporting cells, as mentioned earlier, and form tight junctions and desmosomes with the supporting cells; these separate the endolymphatic space, in which endolymph bathes the stereocilia above the cells, from the perilymphatic space below the apical surface. Another general feature that applies to hair cells is that they are presynaptic to the afferent nerve fibers that they contact. Hair cells make synaptic contacts by means of synaptic specializations termed synaptic ribbons or bars, electron-dense structures with synaptic vesicles clustered around them.
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The vestibular hair cells are of two types: **Type I cells: -
have a rounded body “flask-shaped cells” enclosed by a nerve calyx or chalice → Phasic receptors (fire when move). When a calyx surrounds a single type I hair cell, is termed a simple calyx ending. Measuring 10 μm in humans. Efferent nerves synapse on the outside surface of the calices Sometimes, one calyx surrounds two to four type I hair cells; this is termed a complex calyx ending. Complex calyx endings are much more common in the central zone (striola) than in the periphery (extrastriola). Histologically, the existence of complex endings is a criterion that can be used to define the central zone.
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**Type II cells: -
have a cylindrical body with nerve endings at the base → Tonic receptors (fire all the time).
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contacted at their basal surfaces by numerous afferent and efferent synaptic boutons. Afferent boutons contain mitochondria and few vesicles and receive synaptic contacts from the hair cell; they transmit the impulse centrally to the vestibular nuclei and are postsynaptic to the hair cell. Efferent boutons contain many vesicles and smaller mitochondria and vesicles than those found in afferent boutons. They form synapses with hair cells and afferents and transmit impulses from the efferent group of neurons located in the brainstem. Type II hair cells in the periphery (extrastriola) make synaptic contact with many afferent boutons, each of which receives one synapse from the adjacent hair cell. Type II hair cells in the central zone (striola) make synaptic contact with relatively few afferent boutons but make multiple contacts with each of these. In addition, type II hair cells in the central zone make synaptic contacts with the outer surface of the calyx endings that surround type I hair cells, a type of synapse rarely found in the peripheral zone.
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They form a mosaic on the surface of the maculae, with the type I cells dominating in a curvilinear area (the striola) near the centre of the macula and the cylindrical cells around the periphery. The significance of these patterns is poorly understood, but they may increase sensitivity to slight tiltings of the head.
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TIP-LINKS: -
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The basic elements of a tip link include an ion channel, an interconnecting tether, and a motor protein. The motor protein moves along actin filaments and is critical for adaptation to prolonged stimuli. Tether that couples the channel to intracellular or extracellular structures so that stretch on the tether opens the channel. The tethers are attached to stiff elements within the cell such as microtubules or actin filaments or outside the cell to protruding parts of the same or other cells or the extracellular matrix. Mutations in the rare tip link motor protein, myosin VIIa, cause vestibular and auditory loss in humans.
RIBBON SYNAPSE -
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The basal portion of the hair cells makes contact with afferent nerve terminals by way of ribbon synapses. These structures that are efficient in converting Ca 2+ influx into neurotransmitter release are only seen in the inner ear and eye. There are approximately 10 to 20 synapses per hair cell, and each synapse contains 100 to 200 synaptic vesicles. Of these about 15 vesicles are docked beneath the ribbon ready to release their contents into the extracellular space. Within each receptor central hair cells of both types have more synaptic ribbons than do peripheral hair cells. About 13 % of the transduction channels are open at rest, resulting in a resting current flow through the hair cells and a resting firing rate of the afferent nerves.
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Relationship between the Direction of Force and Hair Cell Activation -
The adequate stimulus for hair cell activation is a force acting parallel to the top of the cell, resulting in bending of the hairs (a shearing force). A force applied perpendicular to the cell surface (a compressional force) is ineffective in stimulating the hair cell. Deflection of the hairs toward the kinocilium decreases the resting membrane potential of the sensory cells (depolarization). Bending in the opposite direction produces the reverse effect (hyperpolarization). The effect is minimal when hair deflection is perpendicular to the axis of maximal excitation.
Mechanism of Hair Cell Activation: -
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The top surface of hair cells in mammals faces the endolymph, a fluid rich in K + (like the intracellular space), while the basolateral membrane is surrounded by perilymph rich in Na+ (like the extracellular space). In the vestibular labyrinth, the positive potential is slightly more smaller in relation to intracellular flhuid ( + 5 to 10 mV). There is a nonlinear behavior of the hair cell transduction mechanism, it is not surprising that the modulation of the spontaneous neuronal firing rate is likewise nonlinear. Responses to excitatory stimuli are more than those to inhibitory stimuli. This asymmetry in response is of great physiological and clinical significance, as will be shown later.
Signal Processing at the Hair Cell/Afferent Nerve Junction: -
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Signal processing must be interposed between the hair cell and the afferent nerve to account for the wide range of afferent nerve responses. This signal processing can be traced to at least four different processes: (1) neurotransmitters released by the hair cells, (2) neurotransmitters released by efferent terminals, (3) adaptation at the ribbon synapse, and (4) a diversity of receptors and ion channels in the afferent terminals. Glutamate is the main neurotransmitter at the hair cell–afferent nerve junction, but other transmitters, including gamma-aminobuteric acid (GABA), are also released. Acetylcholine (Ach), released by the efferent system, modulates afferent nerve firing through both presynaptic and postsynaptic mechanisms. The number of vesicles (both total and docked) at the ribbon synapse can be up and down regulated based on a number of factors, including synaptic activity. The afferent nerve terminals express AMPA, NMDA, and GABA type B receptors along with a wide range of ion channels, all of which can be up and down regulated.
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THE INNER EAR VESTIBULAR RECEPTORS: -
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The vestibular system monitors the forces associated with angular and linear accelerations of the head by means of five organs located within the labyrinthine cavities of the temporal bones on each side of the skull. The saccular and utricular macules sense linear acceleration, and the cristae of the three semicircular canals sense angular velocity of the head.
Semi Circular Canals 1. Function: -
The SCCs provide sensory input about head velocity, which enables the VOR to generate an eye movement that matches the velocity of the head movement enabling clear vision. Neural firing in the vestibular nerve is proportional to head velocity over the range of frequencies in which the head commonly moves (0.5–7 Hz). In engineering terms, the canals are “rate sensors.” & report their magnitudes & orientations to brain.
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A second dynamic characteristic their response to prolonged rotation at constant velocity. Instead of producing a signal proportional to velocity, as a perfect rate sensor should, the canals respond reasonably well only in the first second or so, because output decays exponentially with a time constant of about 7 seconds. This behavior is due to a spring like action of the cupula that tends to restore it to its resting position.
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Each of the canals acts as an integrating accelerometer; the necessary stimulus for the canal is an angular acceleration, but the information encoded by the firing of the afferent nerve fiber is more closely related to angular velocity.
2. Posterior, superior & lateral SCCs are orthogonal to each other, each canal is maximally sensitive to rotations that lie in the plane of that canal. 3. ** Lateral canal inclined to 30 degrees. This canal is accordingly sensitive to rotations about a vertical axis, for example to twisting the neck. ** Superior/posterior canals 45° off of sagittal plane. The plane in which each anterior vertical SCCs lies is slanted about 45° with respect to the coronal plane, so that the lateral extreme of each canal lies rostrally to the medial edge. The planes of the two posterior vertical canals are canted approximately 45° in the opposite direction.
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4. In contrast, linear accelerations of the head produce equal forces on the two sides of the cupula, so the hair bundles are not displaced. 5. Each canal forms two-thirds of a circle, with a diameter of 6.5 mm and a cross-sectional diameter 0f 0.4 mm. The receptor organ of SCCs is (the crista ampullaris(. 6. At the anterior opening of the horizontal and anterior canal and the inferior opening of the posterior canal, each tube enlarges to form the ampulla that open in vestibule. 7. It rests on the bone of the canal and consists of sensory epithelium lying on a mound of connective tissue, where blood vessels and nerve fibers reach the sensory receptor area. In the human vestibular organ, there are approximately 23,000 hair cells (type I and type II) in the three cristae and about 52,000 in the two macules. The number of neurons innervating the three cristae is approximately 5700 and the two macules, approximately 8600, for a total of approximately 14,300 nerve fibers.
END ORGAN ANATOMY:
Ampulla, contains the neuroepithelium (crista ampullaris), cupula, supporting cells, connective tissue, blood vessels, and nerve fibers.
The crista is a saddle-shaped, raised section of the wall that extends across the floor of the ampulla at right angles to its long axis. The crista has been found to be divided into two zones, central (near the apex) and peripheral (on the slope), based on the morphology and physiology of vestibular afferents that supply the different regions.
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The shape of the crista facilitates maximal packing of the specialized mechanoreceptor hair cells.
The hair cells and the supporting cells are modified columnar epithelial cells that have microvilli on their apical surfaces.
In the hair cells, many of these microvilli are elongated to form stereocilia, which are grouped in an organ pipe–like arrangement.
In addition, each hair cell has a single long kinocilium, * a true cilium * shows the 9-plus-2 arrangement of microtubules. * longer than the stereocilia * is eccentrically located, which imparts a certain polarization to the hair cell that has important functional implications.
In the cristae, the kinocilium on each hair cell is located on one end of the cell.
In the horizontal cristae, the kinocilia are located on the side of the hair cell that is closest to the utricle. In the vertical cristae, the kinocilia are located on the side of the hair cell furthest from the utricle, the canalicular side.
Hair cells have about 50-100 stereocilia and a single kinocilium, originate from the apical surface of the sensory hair cells and extend up to 80 µm into the cupula. The cupula spans the cross section of the ampulla and conforms approximately to the receptor region of the crista. Cupular displacement distribution that resembled a diaphragm attached to the ampullary wall along its entire perimeter, with a maximum displacement near the center of the ampulla that decreased monotonically to zero around the periphery.
Stereocilia are not true cilia, they are graded in height with tallest nearest the kinocilium. Kinocilium is located on one end of cell giving each cell a polarity. Lacks inner dynein arms, and central portion of microtubules not present near ends → may mean they are immobile or weakly mobile. The cupula is a gelatinous mass that consists of mucopolysaccharides within a keratin meshwork, which extends from the surface of the cristae to the roof and lateral walls of the membranous labyrinth to form a fluid-tight partition, dividing the interior of the ampulla into two approximately equal parts.
The subcupular filaments are connected to the cupular filaments on one side and to the sensory epithelium on the other side.
A distinct subcupular space in the region of the cupula overlies the apex of the center of the crista. This subcupular space is believed to provide space for freedom of movement
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and more sensitive responses to endolymph flow for the stereocilia on the hair cells in the central zone.
The specific gravity of the cupula is approximately 1, which is about the same as that of the endolymph. This matching of the specific gravity of the cupula and the endolymph is necessary to prevent the cupula from floating upward in certain head positions and causing an enduring nystagmus. Disruption of this match in specific gravity is likely the cause of alcohol nystagmus.
Thus, the cupula is attached at its base to the crista but is free to incline toward or away from the utricle in response to the slightest flow of endolymph or a change in pressure.
The tips of the stereocilia are tightly connected to the subcupular meshwork and sometimes the longest stereocilia or the kinocilia protrudes into the cupula and is connected to the cupular filaments.
The subcupular meshwork may help transmit the shearing force of the cupula to the ciliary bundle and dampen unwanted vibrations.
1. The ampullae of the horizontal and superior canals lie close together, just above the oval window, but the ampulla of the posterior canal opens on the opposite side of the vestibule. 2. The membranous labyrinth enclosing the endolymph varies between 15 and 50 µm in thickness. It is comprised of fibrous ectodermal epithelium that consists of three layers: an outer layer of fibrous tissue containing blood vessels, a thick middle layer containing many papilliform projections, and an inner layer comprised of epithelial cells. The stiffness of the membranous tissue is orders of magnitude greater than that of the cupula. Based on this, one would expect pressure gradients within the ampulla to have a much larger impact on cupular deformations than on membranous duct deformations. 3. The perilymphatic space is enclosed by a nearly continuous osseous canal that is lined with periosteum. Fine connective tissue filaments, referred to as trabeculae, span the perilymphatic space and suspend the membranous endolymphatic duct from the periosteum within the osseous canal The filaments serve to anchor the membranous labyrinth to the temporal bone such that the gravitoinertial acceleration experienced by the sensory organs could be expected to be nearly identical to that experienced by the temporal bone. 4. For the canals, the lower corner frequency is determined by the slow (long) time constant, and the upper corner frequency is determined by the fast (short) time constant.
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5. Between the two corner frequencies, the model predicts that the viscosity of the endolymph flowing within the slender duct mechanically integrates the angular acceleration of the head to produce a cupular volume displacement proportional to angular velocity of the head. 6. Below the lower corner frequency, the response is predicted to be attenuated by the stiffness of the cupula. 7. Above the upper corner frequency, the response is predicted to be attenuated by the inertia of the endolymph within the slender portion of the duct, where the kinetic energy is highest. The upper corner frequency, however, is likely to be in error due to the fact that frequency dependence of the velocity profile and membranous duct deformability—both of which increase with increasing frequency.
SCC & Detection of angular acceleration: dynamic equilibrium: -
Each canal is composed of 2 chambers: * bony one contain Perilymph (↑Na & ↓ K) & * membranous labyrinth contain endolymph (↑K & ↓Na).
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SCC system acts as a precise angular (rotational) speedometer. its neural output is directly proportional to the angular (rotational) velocity of head movements, by combining the input from each of the three canals, the brain can create a representation of the vector which describes the instantaneous speed of head rotation relative to 3D space: a 3D speedometer! due to the very small size of the canal (diameter of about 0.3 mm), which results in a large increase in the viscous properties of the fluid. Ampulla of the SCCs → Dilated end of canal → Contains sensory neuroepithelium, cupula, supporting cells
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Each afferent neuron has a baseline firing rate “Resting potential”. -
Hair cells exhibit a constant "resting discharge activity" even in the absence of a stimulus, about 60 to 90 spikes per second Thus, stimulation is sensed by the CNS as a change in this resting, "spontaneous" discharge rate this explains why it is possible to function reasonably well after the loss of one labyrinth.
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Deflection of stereocilia toward kinocilium results in an ↑ increase in the firing rate of the afferent neuron.
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Deflection away causes a ↓ decrease in the firing rate
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Neurons in the vestibular nuclei typically receive bilateral inputs from the coplanar SCC (in the same plane) (direct projections from afferent nerve fibers on the ipsilateral side and commissural projections from central vestibular neurons from the contralateral side) with excitation of peripheral afferents that cause increased firing rates of the neurons in the ipsilateral vestibular nucleus. Morphological polarization: is the orientation of kinocilium: - kinocilia are located closest to utricle in lateral canals and are on canalicular side in other canals - Utriculopetal/or ampullopetal flow (toward the utricle) excitatory in lateral canals, inhibitory in superior/posterior canals - Utriculofugal/or ampullofugal flow (away from the utricle) has opposite effect.
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- “Push-Pull organization”: SCCs are paired: – Horizontal canals – Right superior/left posterior – Left superior/right posterior – Allow redundant reception of movement – Explains compensation after unilateral vestibular loss -
Advantages to the push-pull arrangement of coplanar pairing: a. Pairing provides sensory redundancy. If disease or surgical intervention affects the SCC input from one member of a pair (e.g., as in vestibular neuritis, or canal plugging for benign paroxysmal positional vertigo), the central nervous system will still receive vestibular information about head velocity within that plane from the contralateral member of the coplanar pair). b. Such a pairing allows the brain to ignore changes in neural firing that occur on both sides simultaneously, such as might occur due to changes in body temperature or chemistry changes are not related to head motion and are “common-mode noise.” The engineering term for this desirable characteristic is “common mode rejection.” c. Push-pull configuration assists in compensation for sensory overload.
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Angular Velocity: 1. within the first few seconds of rotation, Because of its inertia, the endolymph tends to lag behind and therefore rotates within SCC in a direction opposite that of the head. -
A cup of coffee can demonstrate the motion of endolymph in a semicircular canal. While gently twisting the cup about its vertical axis, one can observe a particular bubble near the fluid's outer boundary. As the cup begins to turn, the coffee tends to maintain its original orientation in space and thus counterrotates in the vessel. At the conclusion of the turning motion, when the cup decelerates, the coffee moves in the opposite direction.
2. another 5 to 20 seconds, the cupula slowly returns to its resting position in the middle of the ampulla because of its own elastic recoil. 3. When the rotation suddenly stops - The endolymph continues to rotate while the semicircular duct stops - the cupula bends in the opposite direction, causing the hair cell to stop discharging entirely.
4. After another few seconds, the endolymph stops moving and the cupula gradually returns to its resting position, thus allowing hair cell discharge to return to its normal tonic level.
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- “Hydrostatic concept”: -
in 1870, the German physiologist Friedrich Goltz postulated that SCCs are stimulated by the weight of the fluid they contain, the pressure it exerts varying with the head position.
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In 1873 the Austrian scientists Ernst Mach and Josef Breuer and the Scottish chemist Crum Brown, proposed the “hydrodynamic concept,” which held that head movements cause a flow of endolymph in the canals and that the canals are then stimulated by the fluid movements or pressure changes.
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The hydrodynamic concept was proved correct by later investigators who followed the path of a droplet of oil that was injected into SCC of a live fish. At the start of rotation in the plane of the canal the cupula was deflected in the direction opposite to that of the movement and then returned slowly to its resting position. At the end of rotation, it was deflected again, this time in the same direction as the rotation, and then returned once more to its upright stationary position. These deflections resulted from the inertia of the endolymph, which lags behind at the start of rotation and continues its motion after the head has ceased to rotate. The slow return is a function of the elasticity of the cupula itself.
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In view of the complexity of the sensory stimuli associated with seemingly simple everyday acts, one may better appreciate why infants need many months of training to support bipedal locomotion. Even as adults we must work diligently to incorporate into reflexes the new patterns of vestibular stimulation associated with new experiences, for example, piloting an airplane. It also seems likely that the need for continual practice by athletes results from constant fine-tuning of vestibular pathways and the associated motor outflows.
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Ewald’s three laws: is the compensatory reflex eye movement in the plane of canals being stimulated & is opposite to direction of acceleration “VOR”.
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Adaptation:
Macula Little adaptation
Crista Ampullaris Rapidly adapting
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Ewald’s laws, were ( 1 ) the eye and head movements always occurred in the plane of the canal being stimulated and in the direction of endolymph flow, ( 2 ) ampullopetal endolymph flow in the horizontal canal caused a greater response (i.e., induced movements) than did ampullofugal endolymph flow, and ( 3 ) ampullofugal endolymph flow in the vertical canals caused a greater response than did ampullopetal endolymph flow.
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Otolith Organs The vestibule is oval in shape, connecting to the membranous semicircular canals via five openings. The saccule lies on the medial wall of the vestibule in a spherical recess inferior to the utricle with which it is in contact but without direct connection. It communicates with the endolymphatic duct by the utricular & saccular duct and with the cochlea by the ductus reuniens.
1. Utricle is in horizontal plane (+30°) 2. Saccule is in vertical plane. -
Because they respond to gravitational forces, they are also called gravity receptors. So, they are sensitive to *the direction and magnitude of gravity With no head movement: Hair cells only respond to the pull of gravity static response code * Linear acceleration or deceleration of the head also displaces otolithic membrane
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each of which consists of an ovoidal sac of membranous labyrinth about 3 mm in the longest dimension.
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Macula: - Definition: is a roughly thickened ectodermal elliptical patch, elongated and resembles the letter “J”, in which the hair cells of each organ is localized, is about 2 millimetres (0.08 inch) in diameter → monitors the position of the head. -
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The human utricle contains about 30,000 hair cells, while the saccule contains some 16,000. In the utricle the macula projects from the anterior wall on the floor of that tubular sac and lies primarily in the horizontal plane. In the saccule the macula is in the vertical plane and directly overlies the bone of the inner wall of the vestibule. Each macula consists of neuroepithelium → a layer that is made up of supporting cells and sensory cells, as well as a basement membrane, nerve fibres and nerve endings, and underlying connective tissue……………… The sensory cells are called hair cells because of the hair like cilia stiff, non-motile stereocilia and flexible, motile kinocilia—that project from their apical ends. The hair bundle at the apex of each hair cell extends into the endolymphatic space of the utricle or saccule, where the bundle's top is attached to a gelatinous sheet, the otolithic membrane, that covers the entire sensory macula.
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Embedded within and lying on the otolithic membrane are fine, dense particles, blanket of rhombohedral crystals, the otoconia (“ear dust”) or “statoconia”; consisting of calcium carbonate in the form of the mineral calcite. Otoconia are typically 0.5-20 μm long and millions of these particles fill the endolymphatic cavities of the utricle and the saccule. Because of the prominence of otoconia, the utricle and saccule are named the otolithic organs, their specific gravity is almost 3 times that of the membrane and the endolymph—and thus add considerable mass to it. Otoconia are bound to the underlying amorphous gelatinous layer by fibrous proteins. The rapid deposition of calcium carbonate on the protein matrix requires adequate Ca2 + and CO3– ions. The plasma membrane calcium ATPase 2 (PMCA2) is the primary calcium pump for maintaining endolymph Ca 2 + levels and carbonic anhydrase generates CO3– .
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Inhibitors of calcification in other areas prevent more generalized calcification. Once formed there is normally a low rate of turnover of otoconial calcium in adult mice. Patients with osteoporosis have abnormal otoconia and may have an increased risk of developing benign positional vertigo. Abnormal or reduced otoconia have been produced in humans by: aminoglycosides, phenytoin, carbonic anhydrase inhibitors, prostaglandins, and ethacrynic acid. **It appears that maintenance of normal otoconia requires the maintenance of normal hair cells. Aging also lead to loss of otoconia. Alteration in endolymphatic ion concentration may be the cause of both hair cell and otoconial degeneration. Utricular and saccular macules are responsive to static tilt and dynamic linear acceleration. However, the sacculus can be stimulated by loud sounds, which is the basis for the vestibular evoked myogenic potentials (VEMP) test. In the sacculus and utricle, the striola divides the hair cells into populations with opposing hair bundle polarities.
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When the head undergoes linear acceleration, the membranous labyrinth moves along as well because it is fixed to the skull. The otoconial mass, however, is free to shift within the receptor organ. Because of its inertia this mass lags behind movement of the head. The motion of the otoconia is communicated to the gelatinous otolithic membrane → This motion in turn deflects the hair bundles that link the otolithic membrane to the macula, thus exciting an electrical response in the hair cells.
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During head displacement, the calcified otolithic membrane is affected by the combined forces of applied linear acceleration and gravity and tends to move over the macule, which is mounted in the wall of the membranous labyrinth. The otolithic membrane is restrained
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in its motion by elastic, viscous, and inertial forces analogous to the forces associated with cupular movement. Displacements due to sinusoidal linear acceleration would be greatest at low frequencies, including static head tilts. At greater frequencies, the otolithic membrane displacement would decrease by one-half each time the frequency was doubled.
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- Detection of linear acceleration: static equilibrium -
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The left and right utricular maculae are in the same, approximately horizontal plane and because of this position are more useful in providing information about the position of the head and its side-to-side tilts when a person is in an upright position. The saccular maculae are in parallel vertical planes and probably respond more to forward and backward tilts of the head. Both pairs of maculae are stimulated by shearing forces between the otolithic membrane and the cilia of the hair cells beneath it. The otolithic membrane is covered with a mass of minute crystals of calcite (otoconia), which add to the membrane’s weight and increase the shearing forces set up in response to a slight displacement when the head is tilted. The hair bundles of the macular hair cells are arranged in a particular pattern with Kinocilia facing * toward (in the utricle) steriola or * away from (in the saccule) a curving midline “Steriola” that allows detection of all possible head positions. These sensory organs, particularly the utricle, have an important role in the righting reflexes and in reflex control of the muscles of the legs, trunk, and neck that keep the body in an upright position. Some investigators have suggested that saccule is responsive to vibration as well as to linear acceleration of the head in the sagittal plane. Of the two receptors, the utricle appears to be the dominant partner. There is evidence that the mammalian saccule may even retain traces of its sensitivity to sound inherited from the fishes, in which it is the organ of hearing.
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- In the utricular macule, tilting of the head ipsilaterally results in an increase in firing of units on the medial side of the striola and a decrease in firing of units on the lateral side of the striola. With the subject in the upright position, most of the units are at baseline because the vector orientation of the utricular macule is orthogonal to the gravitational vector. Because of the curvature of the macule, some afferent units are sensitive to forward and backward tilting of the head.
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- Since the saccular macule is oriented in the sagittal plane, the vectors of most of the functional units are parallel to gravity when the head is in the upright position. Because of the push–pull relationship of hair cells on either side of the striola, some functional units are excited, whereas others are inhibited. The striola of the saccular macule has less curvature than that of the utricle and therefore most of the units have an orientation in the rostrocaudal direction. Baseline firing of neurons innervating the saccule and macule are about the same.
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AUTONOMIC SUPPLY OF THE PERIPHERAL VESTIBULAR APPARATUS: -
Superior cervical ganglion Postganglionic sympathetic fibers also innervate the vestibular end organs, and they are of two types, nonvascular and perivascular. The nonvascular sympathetic fibers run among the myelinated afferent fibers. The terminals of these fibers are found as free endings near the cells of the Scarpa ganglion, distal to the ganglion and beneath the sensory epithelium. They do not seem to penetrate the basement membrane into the sensory epithelium and do not seem to have a direct effect on the hair cells or afferent fibers. Their functional role remains unexplored.
Best Regards )(و ق ل رب ارحمهما كما ربيانى صغيرا
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