Energy Output from a Single Outer Hair Cell

Abstract

Electromotility of outer hair cells (OHCs) has been extensively studied with in vitro experiments because of its physiological significance to the cochlear amplifier, which provides the exquisite sensitivity and frequency selectivity of the mammalian ear. However, these studies have been performed largely under load-free conditions or with static load, while these cells function in vivo in a dynamic environment, receiving electrical energy to enhance mechanical oscillation in the inner ear. This gap leaves uncertainties in addressing a key issue, how much mechanical energy an OHC provides. This study is an attempt of bridging the gap by introducing a simple one-dimensional model for electromotility of OHC in a dynamic environment. This model incorporates a feedback loop involving the receptor potential and the mechanical load on OHC, and leads to an analytical expression for the membrane capacitance, which explicitly describes the dependence on the elastic load, viscous drag, and the mass. The derived equation of motion was examined in a mass-less model system with realistic parameter values for OHC. It was found that viscous drag is more effective than elastic load in enhancing the receptor potential that drives the cell. For this reason, it is expected that OHCs are more effective in counteracting viscous drag than providing elastic energy to the system.

Outer hair cell electromotility is low-pass filtered relative to the molecular conformational changes that produce nonlinear capacitance

“…similar to the previous treatment for the special case without inertial loading (Iwasa, 2016). Here, the viscoelastic relaxation frequency is defined by ωη=k/η. It is essentially an equation for viscoelastic relaxation, adding a low pass filter to the motile mechanism.…”

“…Actually, the magnitude of the “gating compliance” is quantitatively consistent with a report that voltage dependence of axial OHC stiffness is absent (Hallworth, 2007). Indeed, a previous treatment of a viscoelastic process involving the OHC does not lead to voltage dependence of the viscoelastic frequency for a voltage driven stimulus, while it does for a force stimulus (Iwasa, 2016).…”

Kinetic Membrane Model of Outer Hair Cells

“…The present paper assumes that the intrinsic rate is faster than the auditory range, to evaluate the maximal power production, consistent with the previous 1D model [12,13]. Then, a kinetic equation is derived by extending the membrane model [28], all parameters of which have been determined by in vitro experiments, unlike a shell model with more parameters [29].…”

“…While the movement of the motile molecule's charge increases the membrane capacitance of the cell (nonlinear capacitance) under load-free condition, it is sensitive to mechanical load. Indeed, elastic load can reduce the membrane capacitance [12]. Moreover, nonlinear capacitance can turn negative and eliminate the membrane capacitance altogether near resonance frequency in the presence of inertial load [13].…”

“…Previous theoretical efforts have addressed the issue of high frequency response [14,15] and the effect of elastic load [15]. However, these efforts did not consider the effect of load on the membrane capacitance, nor inertial load with an exception of some cochlear models [16,17] until the preceding reports [12,13], which evaluated the effectiveness of OHCs by comparing the power output of OHC with energy dissipation in the subtectorial gap, essential for mechanoreception of the ear, in view of the importance of energy balance involving OHCs in vivo [18,19]. Power output of OHCs was obtained by constructing the equation of motion of OHC with mechanical load.…”