The otolith organs, the utricle and saccule, each have a patch of specialized epithelium called a macula that consists of hair cells and supporting cells. The hair cells project into the endolymph-filled membranous labyrinth, with their hairs embedded in a gelatinous otolithic membrane (fig. 10.14). The otolithic membrane contains microscopic crystals of calcium carbonate (otoliths) from which it derives its name (oto = ear; lith = stone). These stones increase the mass of the membrane, which results in a higher inertia (resistance to change in movement).
Because of the orientation of their hair cell processes into the otolithic membrane, the utricle is more sensitive to horizontal acceleration and the saccule is more sensitive to vertical acceleration. During forward acceleration, the otolithic membrane lags behind the hair cells, so the hairs of the utricle are pushed backward. This is similar to the backward thrust of the body when a car quickly accelerates forward. The inertia of the otolithic membrane similarly causes the hairs of the saccule to be pushed upward when a person descends rapidly in an elevator. These effects, and the opposite ones that occur when a person accelerates backward or upward, produce a changed pattern of action potentials in sensory nerve fibers that allows us to maintain our equilibrium with respect to gravity during linear acceleration.
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