JACOPIN / SCIENCE PHOTO LIBRARY JACOPIN / SCIENCE PHOTO LIBRARY
Mechanism of the macula. The macula is the organ of static balance (position of the head) and participate in the dynamic balance (recognition of the linear accelerations). Located at the level of the inner ear, the macula is a thickening of the inner walls of the saccule and the utricle (see images 0228706 and 0228806). It is composed of hair cells (in orange), constitute the sensory receptors, and supporting cells (in pink). Each hair cell is possessed of 40 to 70 stereocilia (microvillosities) and a unique kinocila, cilium much longer. The supporting cells secrete a gelatinous substance forming the otolithic membrane, in which dig into the stereocilia and kinocilia. This membrane is recovered by a layer of aragonite cristals (calcium carbonate), the otoliths (in grey). Each hair cell forms a synapse with a sensitive neuron (in yellow) and a motor neuron (in green) of the vestibular nerve, branch of the auditory nerve. When a sudden acceleration lead the head forward, the inertia causes a sliding backwards ( in the inverse sense of the movement) of the otolithic membrane and the otoliths, that move the stereocilia and kinocilia with them. This generate a stimulation of the vestibular nerve, enabling the recognition of the movement.
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