The Vestibular System

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The Vestibular System

The vestibular apparatus is the sensory organ for detecting sensations of equilibrium located in the petrous portion of the temporal bone. It is composed mainly of three semicircular canals; and two large chambers, the utricle and saccule each one of them is composed of bony labyrinth. Within this bony labyrinth is membranous labyrinth.

The utricle and saccule

Located on the inside surface of each utricle and saccule, is a small sensory area called a macula. The macula of the utricle lies mainly in the horizontal plane (on the floor of the utricle) and plays an important role in determining orientation of the head when the head is upright. Conversely, the macula of the saccule is located mainly in a vertical plane (in the wall of the saccule) and signals head orientation when the person is lying down

Each macula is covered by a gelatinous layer in which calcium carbonate crystals called statoconia (ear dust , otoconia) are embedded. Also in the macula are thousands of hair cells,The bases and sides of the hair cells synapse with sensory endings of the vestibular nerve.

Each hair cell has small cilia called stereocilia, plus one large cilium, the kinocilium .The weight of the statoconia bends the cilia in the direction of gravitational pull, bending of stereocilia in the direction of the kinocilium, This opens several hundred channels in the cell membrane which conduct large numbers of positive ions. Therefore, causing depolarization. Conversely,bending of stereocilia in the opposite direction (away from kinocilium) ; this closes the ion channels thus causing hyperpolarization.

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