Sound-induced motion of outer hair cells: new observations and implications for the mechanisms of dynamic range compression

In the cochlea, mechano-electrical transduction by inner hair cells is preceded by a stage of dynamic range compression. A ~90-dB range of sound intensities is compressed to ~30 dB of intracochlear vibration magnitude to fit the limited dynamic range of transduction by inner hair cells. Outer hair cells (OHCs) and their voltage-dependent length changes, known as electromotility, play a central role in this compression process, but the exact mechanisms are poorly understood. Here we review old and new experimental findings, and show that (1) just audible high-frequency tones evoke an ~1-μV AC receptor in basal OHCs; (2) any mechanical amplification of soft high-frequency tones by OHC motility would have an adverse effect on their audibility; (3) the corner frequency of OHC motility is several octaves below the frequencies it is supposed to amplify (4) OHC receptor currents display a substantial rectified (DC) component; (5) mechanical DC responses (baseline shifts) to acoustic stimuli, while insignificant on the basilar membrane, are comparable in magnitude to AC responses when recorded in the organ of Corti, both in the apex and the base. In the basal turn, the DC component may even exceed the AC component, indicating that OHCs act as envelope detectors. We interpret these findings in a framework in which local dissipation is regulated by OHC length changes