Transduction Process in Cochlear Hair Cells

Stereocilia of IHCs are in functional contact with an overlying auxiliary structure called the tectorial membrane. The base of the hair cell is in synaptic contact with the distal ends of auditory nerve axons. Sound waves that reach the inner ear set the basilar membrane, and hence the organ of Corti, into motion. This causes a shearing motion between the tectorial membrane and the tops of the hair cells that, in turn, displaces the stereocilia and triggers the flow of transducer currents. These changes in the receptor potential are mediated by the opening and closing of mechanically gated ion channels (transduction channels) at the tips of the stereocilia. This action leads to opening and closing of voltage-gated ion channels distributed over the basolateral surface of the cell body and then to the release of neurotransmitter at the afferent synapses at the base of the hair cell. The hair bundle is "polarized," which means that displacement of the stereocilia in the direction of the kinocilium (or basal body) results in hair cell depolarization and an increase in firing of auditory nerve fibers, whereas displacement in the opposite direction leads to hyper-polarization and a decrease in firing (Fig. 3). Thus, the modulation of neurotransmitter release, and, as a consequence the pattern of action potentials in the auditory nerve, is linked tightly to the intensity, frequency, and temporal structure of sound waves entering the ear.

Inner ear structures are easily damaged by intense sound, drugs, viruses, and bacteria, and there are genetic causes of inner ear malformation. The resulting hearing loss is called sensorineural, and in such cases no treatment has been found to fully restore normal inner ear function. Some functional hearing may be restored, however, by electrically stimulating surviving spiral ganglion neurons through a cochlear prosthesis.

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