<P> Nerve fiber innervation is much denser for inner hair cells than for outer hair cells . A single inner hair cell is innervated by numerous nerve fibers, whereas a single nerve fiber innervates many outer hair cells . Inner hair cell nerve fibers are also very heavily myelinated, which is in contrast to the unmyelinated outer hair cell nerve fibers . The region of the basilar membrane supplying the inputs to a particular afferent nerve fibre can be considered to be its receptive field . </P> <P> Efferent projections from the brain to the cochlea also play a role in the perception of sound . Efferent synapses occur on outer hair cells and on afferent axons under inner hair cells . The presynaptic terminal bouton is filled with vesicles containing acetylcholine and a neuropeptide called calcitonin gene - related peptide . The effects of these compounds vary, in some hair cells the acetylcholine hyperpolarized the cell, which reduces the sensitivity of the cochlea locally . </P> <P> Research on the regrowth of cochlear cells may lead to medical treatments that restore hearing . Unlike birds and fish, humans and other mammals are generally incapable of regrowing the cells of the inner ear that convert sound into neural signals when those cells are damaged by age or disease . Researchers are making progress in gene therapy and stem - cell therapy that may allow the damaged cells to be regenerated . Because hair cells of auditory and vestibular systems in birds and fish have been found to regenerate, their ability has been studied at length . In addition, lateral line hair cells, which have a mechanotransduction function, have been shown to regrow in organisms, such as the zebrafish . </P> <P> Researchers have identified a mammalian gene that normally acts as a molecular switch to block the regrowth of cochlear hair cells in adults . The Rb1 gene encodes the retinoblastoma protein, which is a tumor suppressor . Rb stops cells from dividing by encouraging their exit from the cell cycle . Not only do hair cells in a culture dish regenerate when the Rb1 gene is deleted, but mice bred to be missing the gene grow more hair cells than control mice that have the gene . Additionally, the sonic hedgehog protein has been shown to block activity of the retinoblastoma protein, thereby inducing cell cycle re-entry and the regrowth of new cells . </P>

Hair cells in the inner ear are activated