Hair bundles teaming up to tune the mammalian cochlea

Abstract

The mammalian inner ear separates complex sounds into individual frequency and amplitude components with remarkable precision. The active process is a term coined to encompass the mechanisms involved in establishing both frequency selectivity and sensitivity of the living cochlea (1). The underlying molecular mechanisms that establish the active process are under debate and incorporate two distinct schools of thought. The first and more widely accepted is based on outer hair cell (OHC) somatic motility. The second theory hinges on active hair bundle motility. In this issue of PNAS, Dierkes et al. (2) create a biophysical model of outer hair cells where hair bundles are tethered to each other by the tectorial membrane, the overlying membrane in which OHC hair bundles are embedded. This association enhances both the frequency selectivity and the gain provided by the hair bundle active processes. This model is significant because it demonstrates a realistic mechanism by which hair bundle force generation can be used in tandem to make a significant contribution to the active process and because it provides a basis in which the disparate mechanisms of the active process might be unified. That is, although the model does not explicitly include outer hair cell motility or the resonant properties of the tectorial membrane, considering these additional variables in the framework presented may provide a means by which both measured frequency and sensitivity can be reproduced theoretically.