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Vestibular System- Mechanism and Functions

It was Mach in 1875 that identified the role of the vestibular apparatus in the perception of motion. This consists of functional subdivisions –

Semicircular canals-Sense of head rotation.
Otolith organs-Stimulated by gravity and linear acceleration of the head.

Physiologically, the vestibular labyrinth transduces mechanical energy (linear and angular) into electrical activity (nerve action potential) which is interpreted by the brain to allow conscious awareness of the position and movement. Transduction and coding-common to both maculae and cristae.

Mechanical event is bending of hair cells.
When the macula surface is tilted, otoliths slide down carrying gelatinous membrane and attached cilia. On head rotation, the endolymph pushes the cupula which carries cilia with it.

Bioelectric Events

There is a constant resting or tonic discharge in many of the afferent fibers from the receptors. Bending the cilia modulates the discharge. On bending of the stereocilia towards the kinocilium depolarization with increase in impulse frequency while bending away from the kinocilium causes hyper polarization with decrease in impulse frequency. Thus, membrane deformation of hair shearing surface alters its electrical conductance.

Reflex Responses Distributes as Follows:

1. Superior vestibular nucleus: Receives impulses from cristae of semicircular canal and cerebellum.
2. Lateral vestibular nucleus: Receives impulses from macula of utricle and cerebellum.
3. Medial vestibular nucleus: Input from cristae and cerebellum.
4. Descending vestibular nucleus receives the impulses from macula of utricle and saccule.

Efferent Activity

Vestibular nuclei connect with five main systems.
1. Oculomotor nuclei by way of MLF and multisynaptic connections in reticular formation.
2. Motor part of spinal cord by reticulospinal,vestibulospinal and inferior part of MLF.
3. Cerebellum.
4. Autonomic nervous system.
5. Cerebral cortex by multisynaptic pathways.

Functions of Vestibular Nuclei

1. Superior nucleus: Control of semicircular canal-ocular reflex.
2. Lateral nucleus: Vestibulospinal activity.
3. Medial nucleus: Co-ordinating eye, head and neck movements through medial longitudinal bundle.
4. Descending nucleus: Mainly to cerebellum and recticular formations.
5. Subjective awareness is by vestibulocortical projections.

Functions of Utricle and Saccule

Utricle and saccule respond to the slightest tilt and to fine acceleration of the head. Such a movement results in compensatory ocular reflexes where by the visual axis is fixed when the head is deviated slightly.
Macula of the saccule is at right angles to the macula of the utricle and may serve linear acceleration.

Functions of Semicircular Ducts

They respond to angular acceleration. The horizontal pair are for rotation about a vertical axis. Posterior and superior canals respond to tipping displacement of the head about a horizontal axis.
Movements of endolymph within the ducts stimulate cristae causing reflex nystagmus. The slow component is vestibular and the fast component cerebral. In clinical practice, nystagmus is named in the direction of the fast component.

Postural Reflexes

Labyrinthine reflexes maintain the posture.
1. Static reflexes: It is the postural reaction at rest. Together with reflexes from muscles, joints and others. They include the following labyrinthine (utricular) reflexes:
• Tonic labyrinthine reflexes-with effect on the limbs, neck, trunk and eyes.
• Labyrinthine righting reflexes

Restore the body to normal position when it is brought to rest in abnormal position.
2. Kinetic reflexes: Postural reactions of the body during movement.
• Angular as in movement of rotation in any plane.
• Progressive as in movement in a straight line.
In general, kinetic reflexes bring the body into normal stance while it is maintained by activation of various static reflexes.

Vestibular Physiology

Saccule and utricle are known as static labyrinth whereas semicircular canals are known as dynamic labyrinth because it is associated with kinetic balance.
Saccular macula responds to the tilting movement of the head, i.e. if the head is tilted to the right side, the right saccular macula will get stimulated whereas the left saccular macula will remain static. The utricular macula responds to forward and backward movement of the head. The cristae of the semicircular canals respond to turning of the head, i.e. angular acceleration.

The bending of hair cells towards the kinocilium causes depolarization with increase in impulse frequency, whereas bending towards the steriola will produce less stimulation due to hyper polarization. The hair cells of the utricle and saccule are divided by an arbitrary line called striola. The bending of utricular hair cells away from striola causes hyper stimulation due to depolarization whereas the bending of saccular hair cells towards the striola causes decrease stimulation.

Bending of hair cells of the cristae towards utricle (ampullofugal) movement in the lateral semicircular canal causes hypo stimulation. However, some authors believe that the ampullofugal movement in the lateral semicircular canal causes hyper stimulation whereas ampullopetal movement in the vertical semicircular canal causes less stimulation.
Fibers, both afferent and efferent, form a plexus of unmyelinated fibers from which myelinated fibers arise and go to the bipolar cells in Scarpa’s ganglion of the internal auditory canal.

Otolith Physiology

Otoliths are stretch receptors. When a tangential force, regardless of its origin is applied over the macula, the otoconia slide over the surface of the sensory epithelium stimulating the hair cells. The position of otoliths changes whenever the head position is changed or during head displacement with a component of linear acceleration.
The Dynamics of the otoliths is comparable to those of a low pass filter i.e., displacement due to linear acceleration is greater for lower frequencies, including stimulation with constant acceleration, than for higher frequencies.

The nerve fibers innervating the macular organs are sensitive to changes in position of the head. Each fiber has a preferred direction of tilt to which it responds maximally. Thus the movements of head are appreciated along a vector in a three dimensional space. The mechanical deformation of the hair bearing surface alters its electrical conductance. The endolymph has a +40mV potential compared to the -5Om V in the substance of sensory epithelium. Electrical modulation of the resting discharge arises when electrical current leaks from the top of the cell membrane.

Saccules are affected by lateral tilt of the head. When head is tilted to right, macula of right saccule hangs downwards – pulls on its macula and maximal stimulation is produced. Whereas the macula of the left saccule points upwards, rests on its macula producing minimal stimulation.
Ventral/dorsal deflection of the head affects macula of utricle when the head is straight, macula of the utricle points upwards producing minimal stimulation. Forward or backward bending makes the macula pendant, pulling on the macula producing maximal stimulation.

A tilt of as little as 2.50 stimulates the appropriate macula. The frequency of the impulse generated by the hair cells is directly related to the strength of the stimulation. They show little adaptation.

Mode of Action of Semicircular Canals

Effective stimulus to each semicircular canal is rotation of head in the plane of its canal.
When the head starts to rotate, e.g.: to the right, the endolymph in the semicircular canals, which lie at right angles to the axis of rotation tends to lag behind the movement and it moves to the left initially.
With regard to the flow of the endolymph, the left ampulla is Leading and the right one Trailing.

Rotation to right-endolymph moves to left. After the initial inertia is overcome, endolymph no longer lags behind the movement of head. It flows in the same direction.
Cupula swings back because of its elasticity, to its normal position, as long as the movement exists in the same velocity and direction and resting discharge is resumed. When the movement ceases, the endolymph tends to continue to rotate and cupula is displaced in the opposite direction and hair cells are bent in that direction. Swinging in a particular direction in any canal increases the stimulus while in the opposite direction decreases the stimuli. Thus in any movement there is an increase in frequency of impulse in one ampula and decrease in its opposite side ampulla.

This combination of increased and decreased stimuli forms the basis of interpretation of the direction of the movement. Accordingly in the above example-when the movement ceases, since the chain of events occur in a way opposite to that at the start, a subjective impression of motion in the reversed direction occurs. Finally the cupula regains the resting potential and sense of movement ceases.The various impulses produced by the hair cells are transmitted to the vestibular nucleus via vestibular fibers. The vestibular system has diffuse connections with central nervous system like with ocular nuclei complex, anterior horn cells of spinal cord, cerebellum, and higher centers in the brain.

Ewalds Law
The vestibular physiology was well clarified by Ewald’s experiments. He came out with three major observations.
1. Head and eye movement always occur in the plane of the canal being stimulated and in the direction of the endolymph flow.
2. In the horizontal canal, ampullopetal endolymph flow causes a greater response than ampullofugal flow.
3. In the vertical canals ampullofugal flow causes greater response than ampullopetal flow.

Vestibular Tract

Some salient features and points to ponder.
1. Vestibular nuclear complex occupies the dorsolateral region of the rostral medulla and caudal pons.
2. Vestibular nuclear complex-Afferent­ efferent connections.
3. Reflex responses:

Stimulation of the vestibular sensory epithelium causes Two reflex responses.
1. Vestibulo-ocular reflexes a means by which humans stabilize gaze so that image can be fixed on the fovea of the retina during head movement.
2. Vestibulospinal reflex to maintain the normal posture of the head, trunk and limbs.

Vestibulo-ocular Reflex

Utriculopetal stimulus in one labyrinth is matched by an equal but opposite, utriculofugal displacement in the functionally paired canal of the other ear. Accordingly the lateral canals form one pair while the posterior canal pair with the opposite side superior canal.
Taking as an example – utriculopetal deviation of the cupula of the right horizontal canal, occurs with an utriculofugal movement of the cupula of the left horizontal canal.

This results in an increase in the firing rate of the right afferent nerve and an equal but opposite response in the left afferent nerve. The right afferent information exerts an excitatory influence on the agonist muscles and an inhibitory influence on the antagonist muscles. The left afferent information reduces the excitatory influence on the antagonist muscles and dis-inhibits the agonist muscles. This results in contraction of the left lateral rectus and the right medial rectus muscles producing deviation of the eyes to the left.

If the stimulus is large, such that the compensatory movement cannot be obtained within the confines of the orbit, a fast movement in the opposite direction occurs. This fast movement is central, initiated and mediated by the reticular formation, cutting off the incoming flow from the vestibular nucleus and reticular activating neurons directing the ocular muscle nuclei to return the eyeballs to the point of gaze at which the slow component began.

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