vestibular system

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An organ in the membranous labyrinth of the inner ear mediating sensations of balance, orientation, and movement, containing the otolith (or statolith) organs that are responsive to linear acceleration (including gravity) and the semicircular canals that are responsive to angular acceleration. When the head is accelerated rapidly in a straight line, a gelatinous membrane covered with heavy otoconia crystals of calcium carbonate tends to remain stationary through inertia, bending or displacing hair cells that project into the fluid at different angles from the maculae of the utricle and saccule, causing the hair cells to trigger nerve impulses that are sent to the brainstem and cerebellum, leading to reflex movements of muscles of the eyes, neck, limbs, and trunk, and exciting neurons in the central nervous system that mediate the perception of motion and orientation in space. The otolith organs also respond to gravity, which according to the general theory of relativity is equivalent to accelerating an object upwards at 9.8 metres per second squared. The three semicircular canals detect angular acceleration by means of hair cells that project into the gelatinous membrane of the cupula where a fluid (endolymph) acts as an inertial mass resisting angular acceleration, displacing the hair cells in the direction opposite to the direction of movement and causing adjoining neurons to send nerve impulses to the brainstem and cerebellum. Also called the vestibular apparatus. See also Aubert effect, kinaesthesis, mechanoreceptor, proprioception, vestibular nerve, vestibular nystagmus, vestibular sense, vestibulo-ocular reflex. [From Latin vestibulum a forecourt or entrance]

Subjects: Psychology.

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