VPB 121: Veterinary Physiology - II (2+1)

VESTIBULAR APPARATUS

• The vestibular apparatus located in internal ear that detects sensation related to equillibrium (linear accelaration of head).
• Popularly known as OTOLITHIC organ.
• It consist of bony labyrinth (semicircular canal with utricule and saccule) and membranous labryinth. This is the functional part of the vestibular apparatus and is composed of cochlear duct, three semicircular canals, the utricle and saccule.
• Membranous labryinth with an enlargement at one end called as is filled with endolymph and surrounded by perilymph.
• The ampulla is the enlargement of the semicircular canals at it base and is filled with endolymph.
• The bag-like structures (vestibular sacs) the utricle and saccule have sensory area known as maculae. This detects the orientation of the head according to the direction of gravitational pull.

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• Utricule and saccule are located within the petrous portion of the temporal bone.
• The maculae is covered by a gelatinous layered and has otoconia or statoconia which are small crystals of calcium carbonate.
• The gelatinous layer has numerous hair cells projecting with their cilia with synapse with the sensory axons of vestibular nerve.
• Change in the orientation of head alter the bending of the celia due to the weight of otoconia accordingly signals are transmitted to cerebellum.
• Maculare detect linear acceleration of the head and act as the receptor for righting reflex.

• The ampullae of the semicircular canals do not have otoliths, but has the cristae which contains hair cells inside the cup shaped gelatinous mass called as cupula. This houses a sensory organ called crista ampullaris that will be excited by the flow of endolymph.
• Hair cells do project their cilia into cupula and the base of these hair cells are connected to axons of vestibular nerve.
• Bending of cupula with the flow of endolymph alters the position of the head.

• The cristae detect angular/rotary acceleration of the head, while the ampullae of the vestibular sacs detect linear acceleration. Most of the vestibular nerve fibres from the macula and semicircular canals end in vestibular nuclei present at the junction of medulla and pons; some fibres pass directly to cerebellum.
• The fibres that end in the vestibular nuclei send second order neurons to the cerebellum, cerebellar cortex, vestibulospinal tract and other areas of the BSRF.

Process of equilibrium maintenance

• When the head is tiled to one side, the otoliths and gelatinous mass of the ampullae of the vestibular sacs induce a bend in the hair cells. This excites or stimulates the hair cells.
• Change in head movements causes movement of otoliths and inertia cost subsequently induces generation of action potential and through vestibular nerve cerebellum receives the information relating the position of head in space.
• The macula of the utricle is placed horizontally in the ear and therefore can detect movement in the horizontal plane.
• The macula of the saccule is placed in vertical position and therefore detects vertical movement of the head.
• But within these maculae, the hair cells are oriented in opposite direction and thereby detect movements of forward, backward, and side to side directions.

• Since the hair cells placed in one plane are stimulated to tilt of the head in one particular directfion, the brain can perceive the direction of the movement, which sends motor signals to maintain the body in posture to maintain the position of the body.
• The semicircular canals are located at right angles to each other and therefore the ampulla in these canals detects angular movement of the head. Each semicircular canal detects the acceleration in one single plane pertaining to it.
• If the head is turned in one particular plane of one particular canal, then there is fluid motion inside the canals causing push on the ampulla and stimulating the hair cells.
• Since each canal is placed in one plane, the degree of movement is sensed by the stimulation of the hair cells in the canals.
• The whole body position is maintained by inputs from visual, auditory and proprioceptive systems.

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