The otolithic membrane is a gelatinous membrane located in the vestibular apparatus of the inner ear, and plays an essential role in the brain's interpretation of equilibrium. The brain receives neural impulses from the saccule and utricle, which are 2 by 3 mm patches of hair cells in the brain. The macula sacculi lie nearly vertical on the wall of the saccule and the macula utriculi lie nearly horizontal on the floor of the utricle. Each of these hair fibers have 40 to 70 stereocilia and a true cilium called a kinocilium.
The tips of the stereocilia and kinocilium are embedded in the gelatinous otolithic membrane, thus providing the common name of "the otoliths" to the utricle and saccule.
The membrane is weighted with crystalline protein-calcium carbonate granules called otoliths. The otoliths greatly add to the inertia of the membrane. When the head is erect, the otolithic membrane bears down directly on the hair cells, and stimulation of the cells remains in their resting state. When the head is tilted, however, the heavy otolithic membrane sags and bends the stereocilia, stimulating the hair cells. At any given head orientation, the brain is receiving constant neural signals from the hair cells which the brain integrates with information from the visual field and stretch receptors of the neck to determine if only the head of if the entire body is tilted.
The otolithic membrane also plays a critical role in the body's ability to sense linear acceleration. As the entire body accelerates, such as in an aircraft on take off, or de-accelerates, such as a car coming to a stop sign, the entire otolithic membrane in the macula utriculi is shifting against the force, bending the stereocilia backwards or forwards. The brain also uses the otolithic membrane of the macula sacculi to detect vertical linear acceleration, such as the sensation felt when an elevator comes to a quick stop at a floor.