The Cochlear Amplifier is a Surface Acoustic Wave Resonator

Abstract

A companion paper (Bell, 2001) formulated a model of the cochlea as a surface acoustic wave (SAW) resonator. This supporting paper seeks to give a working account of the sensing elements in the ear and how they operate together to create a SAW resonator. A key feature of any such device is that the interdigital transducers alternate in polarity, an arrangement ideal for launching and detecting surface waves. Translated to the ear, the three rows of outer hair cells (OHCs) are conjectured to be the interdigital transducers. In the simplest (degenerate) SAW resonator, only a single set of three electrodes is required to create resonance between the fingers, a situation presumed to apply in the cochlea, where OHC2 is assumed to respond in antiphase to OHC1 and 3. The antiphasic response is not to displacement, but to intracochlear fluid pressure. An examination of the literature interprets OHCs as responding directly to pressure via their cell bodies, and two populations, with opposite response polarities, are observed. Whether an OHC behaves in one way or the other depends on its membrane potential and turgor pressure, so it is conjectured that OHC1/3 operate at a membrane potential of about –70 mV, whereas in OHC2 it is about –50 mV. At low sound pressure levels, two mechanisms for creating an electrical response in OHCs are identified: one involves the piezoelectric response of the OHC wall to pressure, the other a transient sodium current which acts as a biological ‘transistor’ to amplify the transducer voltage.