JACOPIN / SCIENCE PHOTO LIBRARY JACOPIN / SCIENCE PHOTO LIBRARY
Functionning of the macula. The macula is the organ of the static equilibrium (position of the head) and participates to the dynamic equilibrium (recognition of the linear accelerations). Located at the level of the inner ear, the macula is a thickening of the saccule and utricle inner walls (see. It is composed of hair cells (in orange), constituting the sensorial receptors, and of supporting cells (in pink). Each hair cell is possessed of 40 to 70 stereocilia (microvillosities) and of a single kinocilium, longer true cilium. The supporting cells secrete a gelatinous substance forming the otolithic membrane, in which embed the stereocilia and kinocilia. This membrane is covered with 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. During a sharp acceleration leading the head forward (during the ascension in the roller coaster, for example), the inertia causes a sliding movement backwards (in the reverse direction of the movement) of the otolithic membrane and the otoliths, that move the stereocilia and kinocilia with them. This leads to a stimulation of the vestibular nerve, enabling the recognition of the movement. See.
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