The outer-hair-cell RC time constant: A feature, not a bug, of the mammalian cochlea

The cochlea of the mammalian inner ear includes an active, hydromechanical amplifier thought to arise via the piezoelectric action of the outer hair cells (OHCs). A classic problem of cochlear biophysics is that the long resistance-capacitance ( RC ) time constant of the hair-cell membrane produces an effective cut-off frequency much lower than that of most audible sounds. The long RC time constant implies that the OHC receptor potential—and hence its electromotile response—decreases by several orders of magnitude over the frequency range of hearing. This “ RC problem” is often invoked to question the role of cycle-by-cycle OHC-based amplification in mammalian hearing. Here, we use published data and simple physical reasoning to show that the RC problem is, in practice, a relatively minor physical issue whose importance has been unduly magnified by viewing it through the wrong lens. Indeed, our analysis indicates that the long RC time constant is actually beneficial for hearing, reducing noise and distortion while increasing the fidelity of cochlear amplification.