Enhancement of sensitivity gain and frequency tuning by coupling of active hair bundles.

The vertebrate inner ear possesses an active process that provides nonlinear amplification of mechanical stimuli. A candidate for this process is active hair bundle mechanics observed, for instance, for hair cells of the bullfrog's sacculus. Hair bundles in various inner ear organs are coupled by overlying membranes. Using a stochastic description of active hair bundle dynamics, we study the consequences of an elastic coupling on the properties of amplification. We report that collective effects in arrays of hair bundles can enhance the amplification gain and the sharpness of frequency tuning as compared with the performance of an isolated hair bundle. We also discuss the transient response elicited by the sudden onset of a periodic stimulus and its relation to temporal integration curves. Simulations of systems with a gradient of intrinsic frequencies show an enhanced amplification gain while preserving a frequency gradient, provided the coupling strength is similar to the hair bundle stiffness. We relate our findings to the situation in the bullfrog's sacculus and the mammalian cochlea.