Electro-mechanical waves in isolated outer hair cell

Recent in vitro and in vivo data have drawn attention to the presence of high-frequency electro-mechanical resonances in the electrically evoked response of the cochlear partition and analogous resonances in isolated cochlear outer hair cells (OHCs). Resonances in isolated OHCs are similar to those present in damped piezoelectric structures and therefore it has been suggested that the behavior may result from the interplay of electro-mechanical potential energy and mechanical kinetic energy. In OHCs, the total potential energy includes both mechanical and electrical terms associated with the lateral wall while the kinetic energy accounts for the inertia of the moving fluids and tissues entrained by the moving plasma membrane. We applied first principles of physics to derive a model of OHC electromechanics consisting of an electrical cable equation directly coupled to a mechanical wave equation. The model accounts for the voltage-dependent capacitance observed in OHCs by means of a nonlinear piezoelectric coefficient. The model predicts the presence of electromechanical traveling waves that transmit power along the axis of the cell and underlie high-frequency resonance. Findings suggest that the subsurface cisterna (SCC) directs current from the transduction channels to the lateral wall and slows the phase velocity of the traveling wave. Results argue against the common assumption of space-clamp in OHCs under physiological or patch clamp conditions. We supplemented the traveling wave model with an empirical description of transduction current adaptation. Results indicate that the so-called RC paradox isn't paradoxical at all; rather the capacitance of the OHC may work in concert with transduction current adaptation and electro-mechanical wave propagation to achieve a relatively flat frequency response.