Dual contribution to amplification in the mammalian inner ear

TL;DR

This paper explains how the mammalian inner ear achieves high mechanical gain through a dual amplification mechanism involving hair bundles and pressure waves, supported by analytical and numerical analysis.

Contribution

It introduces a model showing how hair bundles and pressure waves interact to produce large amplification in the cochlea, clarifying the underlying mechanism.

Findings

Large gain (>4000) explained by combined effects of hair bundles and pressure waves.

Ratchet mechanism separates hair-bundle forces from pressure wave feedback.

Analytical WKB approximation aligns with numerical solutions.

Abstract

The inner ear achieves a wide dynamic range of responsiveness by mechanically amplifying weak sounds. The enormous mechanical gain reported for the mammalian cochlea, which exceeds a factor of 4,000, poses a challenge for theory. Here we show how such a large gain can result from an interaction between amplification by low-gain hair bundles and a pressure wave: hair bundles can amplify both their displacement per locally applied pressure and the pressure wave itself. A recently proposed ratchet mechanism, in which hair-bundle forces do not feed back on the pressure wave, delineates the two effects. Our analytical calculations with a WKB approximation agree with numerical solutions.