Damping Properties of the Hair Bundle

TL;DR

This study models the mechanical damping in hair bundles, revealing how their structure reduces energy loss and enhances sensitivity in hearing through specific cellular interactions and fluid dynamics.

Contribution

The paper introduces a detailed finite-element model of hair bundles, showing how their morphology and internal forces optimize damping and sensitivity in auditory transduction.

Findings

Grouping stereocilia reduces total drag

Tip links induce dissipative relative motions at low frequencies

Horizontal top connectors restrain relative movements, aiding low-frequency response

Abstract

The viscous liquid surrounding a hair bundle dissipates energy and dampens oscillations, which poses a fundamental physical challenge to the high sensitivity and sharp frequency selectivity of hearing. To identify the mechanical forces at play, we constructed a detailed finite-element model of the hair bundle. Based on data from the hair bundle of the bullfrog's sacculus, this model treats the interaction of stereocilia both with the surrounding liquid and with the liquid in the narrow gaps between the individual stereocilia. The investigation revealed that grouping stereocilia in a bundle dramatically reduces the total drag. During hair-bundle deflections, the tip links potentially induce drag by causing small but very dissipative relative motions between stereocilia; this effect is offset by the horizontal top connectors that restrain such relative movements at low frequencies. For higher frequencies the coupling liquid is sufficient to assure that the hair bundle moves as a unit with a low total drag. This work reveals the mechanical characteristics originating from hair-bundle morphology and shows quantitatively how a hair bundle is adapted for sensitive mechanotransduction.