ACETYLCHOLINE, OUTER HAIR CELL ELECTROMOTILITY, AND THE COCHLEAR AMPLIFIER

The dominant efferent innervation of the cochlea terminates on outer h air cells (OHCs), with acetylcholine (ACh) being its principal neurotr ansmitter. OHCs respond with a somatic shape change to alterations in their membrane potential, and this electromotile response is believed to provide mechanical feedback to the basilar membrane. We examine the effects of ACh on electromotile responses in isolated OHCs and attemp t to deduce the mechanism of ACh action. Axial electromotile amplitude and cell compliance increase in the presence of the ligand. This resp onse occurs with a significantly greater latency than membrane current and potential changes attributable to ACh and is contemporaneous with Ca2+ release from intracellular stores. It is likely that increased a xial compliance largely accounts for the increase in motility. The mec hanical responses are probably related to a recently demonstrated slow efferent effect. The implications of the present findings related to commonly assumed efferent behavior in vivo are considered.