Otoliths are hard stones of calcium carbonate that sit on the otolith( otoconial) membrane above the hair cells of the maculae of the utricle and saccule. They weigh down the otolith membrane and keep it in contact with the sensitive cilia of the macular hair cells. Otoliths thus function to help the membrane bend the cilia as the pull of gravity changes with changes of the position of the head in space. These changes in straight line speed or acceleration of the head are reported to the brain by the vestibular nerve ; in response appropriate motor adjustments are made by higher neural centers Otoliths, therefore , have an important function in the maintenance of posture or position in space.
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Otoliths are found covering the gel-like otolith membrane that covers the hair cells of maculae in utricle and saccule in the inner ear.. The maculae are small ares of tissue that have supporting cells , several stereocilia, and one kinocilium each. There is a basement membrane beneath the macula parenchyma and supporting cells. Above the macula is a membranous covering of the consistency of a gel--the otolith membrane. The cilia of the macula are engaged/embedded in this gel-like membrane. The macula of the utricle is horizontal, and the hair cells and cilia are vertical. Therefore utricular cilia respond to movements in the horizontal plane-- like tilting the head.. On the other hand, the saccular macula is vertical, its hair cells and cilia project horizontally into the otolith membrane. Consequently, the hair cells of the saccular macula are sensitive to up and down movements like jumping up and down or elevator rides. The hair cells of the maculae are innervated by neurons from the vestibular ganglion. The sensory stimuli are carried to the brain by the vestibular nerve ( vestibulocochlear, cranial nerve VIII).