Active processes and sensing in the cochlea

One key question in the biophysics of the mammalian cochlea is determining the relative contribution to cochlear amplification by the two active processes present in the outer hair cell, namely prestin-based somatic motility and hair bundle (HB) motility. In the biological cochlea, these two effects are intimately coupled as HB force generation is linked to calcium-dependent adaptation of the transduction current and somatic force generation is driven by the depolarization caused by the same transduction current. To separate these effects, we construct a global mechanical-electrical-acoustical mathematical model of the cochlea. The global cochlear model is coupled to linearizations of nonlinear somatic motility and HB motility. We find that the active HB force alone is not sufficient to power high frequency cochlear amplification while somatic motility can perform this task. We discuss the limitations to this mathematical approach along with existing seminal experiments and proposed experiments (both in the cochlea and in the auditory nerve) to map future directions for uncovering the micromechanical contributions to the system level response of the cochlea. Describing the relation between the microfluidic flow stimulating the inner hair cell HB and the active processes in the cochlea and is an important future research direction. [Support: NIH-NIDCD.]