The sensory role of hair cells is accomplished by activating the auditory or vestibular nerve fibers that carry information into the brain. Neurotransmitter release at an afferent synapse is regulated by changes in the membrane potential of the hair cell in response to bending its stereocilia bundle. The bending of the bundle is a mechanical event which is why hair cells are mechanoreceptors (photoreceptors and chemoreceptors change their membrane potential in response to light and chemicals respectively, the change in the membrane potential of a sensory receptor cell in response to an appropriate stimulus is called a receptor potential). The stereocilia of each hair cell are arranged in a precise geometry. This arrangement is asymmetrical and polarized because the stereocilia are arranged in rows of short, intermediate and tall stereocilia. There is a stepwise progression from the shortest row to the tallest row that gives the bundle the appearance of a pipe organ. The bundle’s organization from short to tall rows is related to the functional consequences of bending the bundle on the cell's membrane potential. Bending the bundle in the direction of the tallest row leads to an increase in neural activity to the brain. Bending the bundle in the opposite direction leads to a decrease in activity. Each hair cell therefore codes the direction and degree of stereocilia bundle bending by either increasing or decreasing the firing rate of the postsynaptic afferent fiber in proportion to the magnitude of the bend.
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