Sensors, motors, and tuning in the cochlea: interacting cells could form a surface acoustic wave resonator

The outer hair cells of the cochlea occur in three distinct and geometrically precise rows and, unusually, display both sensing and motor properties. As well as sensing sound, outer hair cells (OHCs) undergo cycle-by-cycle length changes in response to stimulation. OHCs are central to the way in which the cochlea processes and amplifies sounds, but how they do so is presently unknown. In explanation, this paper proposes that outer hair cells act like a single-port surface acoustic wave (SAW) resonator in which the interdigital electrodes-the three distinctive rows-exhibit the required electromechanical and mechanoelectrical properties. Thus, frequency analysis in the cochlea might occur through sympathetic resonance of a bank of interacting cells whose microscopic separation largely determines the resonance frequency. In this way, the cochlea could be tuned from 20 Hz at the apex, where the spacing is largest, to 20 kHz at the base, where it is smallest. A suitable candidate for a wave that could mediate such a short-wavelength interaction-a 'squirting wave' known in ultrasonics-has recently been described. Such a SAW resonator could thereby underlie the 'cochlear amplifier'-the device whose action is evident to auditory science but whose identity has not yet been established.