E5 Transduction: Inner Hair Cells and Receptor Potential

Topic

When the basilar membrane vibrates at the position of maximum amplitude corresponding to a specific frequency, that movement displaces the fluid surrounding the stereocilia of the inner hair cells resting at that exact location on the membrane. Stereocilia are thin, rigid filaments protruding from the top of each inner hair cell in bundled groups; the fluid displacement caused by the basilar membrane's vibration makes the stereocilia in each bundle bend together toward the same side—a movement known as stereociliary deflection.

Deflecting the stereocilia to one side stretches tiny filaments connecting the tip of each stereocilium to its neighbor, thereby opening mechanically gated ion channels located at the tips; conversely, when the stereocilia bend toward the opposite side, these same filaments slacken, and the mechanically gated ion channels close. The stereocilia of each inner hair cell are bathed in endolymph—the same fluid that fills the semicircular canals in the context of vestibular dysfunction. The endolymph surrounding the cochlear stereocilia has a very high ion concentration and a positive electrical charge, driving these ions into the inner hair cell the moment the mechanically gated ion channels open.

Each time the mechanically gated ion channels open, ions flow from the endolymph into the inner hair cell, generating an ionic current that alters the voltage within the cell. This voltage change within the inner hair cell is called the receptor potential; the greater the deflection of the stereocilia toward the side that opens the mechanically gated ion channels, the more channels open simultaneously, the greater the inward ionic current, and the larger the receptor potential generated within the inner hair cell.

The receptor potential generated within the inner hair cell represents the first signal in this sequence that is neither a mechanical movement nor a pressure difference, but rather an electrical change; thus, the receptor potential acts as the interface between the mechanics described by earlier models—such as the basilar membrane's traveling wave and stereocilia deflection—and the auditory neurophysiology described by subsequent models. When the receptor potential within the inner hair cell reaches a certain magnitude, it triggers the release of neurotransmitter at the cell's base onto the auditory nerve fibers with which it synapses, thereby initiating the auditory nerve's electrical activity—a process further addressed by models of evoked potentials and sensorineural dysfunction.

Through this chain of events—extending from the basilar membrane's vibration at its point of maximum amplitude, through stereocilia deflection and the opening of mechanically gated ion channels, to the generation of the receptor potential within the inner hair cell—each inner hair cell converts the specific mechanical movement associated with its location on the basilar membrane into an electrical signal that the auditory nerve can transmit. Without this mechanotransduction step, all the precision achieved by the preceding models in delivering each frequency to its specific location on the basilar membrane would be lost at the interface between ear mechanics and the electrical activity of the nervous system, because no nerve can fire directly in response to mechanical movement without a preceding receptor potential to translate it into ionic current.

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