The Long Outer-Hair-Cell RC Time Constant: A Feature, Not a Bug, of the Mammalian Cochlea

Abstract

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 RC (resistance-capacitance) time constant of the hair-cell membrane appears inconveniently long, producing 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 roughly two orders of magnitude over the frequency range of mammalian hearing, casting doubt on the hypothesized role of cycle-by-cycle OHC-based amplification in mammalian hearing. Here, we review published data and basic physics to show that the “RC problem” has been magnified by viewing it through the wrong lens. Our analysis finds no appreciable mismatch between the expected magnitude of high-frequency electromotility and the sound-evoked displacements of the organ of Corti. Rather than precluding significant OHC-based boosts to auditory sensitivity, the long RC time constant appears beneficial for hearing, reducing the effects of internal noise and distortion while increasing the fidelity of cochlear amplification.

Rate Dependent Cochlear Outer Hair Cell Force Generation: Models and Parameter Estimation

“…Second, if basolateral membrane capacitance is too large, then the MET current-induced transmembrane voltage may be too small to generate sufficient somatic force to drive the active feedback process [12]. This concern arises because the OHC operates at frequencies where capacitance, not resistance, dominates the basolateral membrane impedance [13][14][15], even under the estimates of conductance and capacitance most favorable to high-frequency amplification [2]. Due to these over 30-year-old challenges, other theories of cochlear enhancement continue to be actively pursued [16][17][18], and it has been speculated the somatic-based CA may work cooperatively with other modes of enhancement, or operate in different regions along the cochlear spiral (e.g., 4).…”

Fluctuating Flexoelectric Membranes in Asymmetric Viscoelastic Media: Power Spectrum through Mechanical Network and Transfer Function Models

“…In addition, the frequency voltage and tension dependent lipid mobility in the OHC plasma membrane has been investigated using an electro-mechanical model [14][15][16][17]. Electro-motility in the cochlea system and the stereociliary columns in the mammalian cochlear hair have been studied at length using complex mathematical models [15][16][17][18][19][20]. From a mechanic-electrodynamic point of view, the mechanism effects of the electric and magnetic fields on cells have been analyzed [21].…”