Tonotopy of the mammalian cochlea is associated with stiffness and tension gradients of the hair cell’s tip-link complex
Frequency analysis of sound by the cochlea relies on sharp frequency tuning of mechanosensory hair cells along a tonotopic axis. To clarify the underlying biophysical mechanism, we have investigated the micromechanical properties of the hair cells mechanoreceptive hair bundle in the rat cochlea. We studied both inner and outer hair cells, which send nervous signals to the brain and amplify cochlear vibrations, respectively. We find that tonotopy is associated with gradients of stiffness and resting mechanical tension, with steeper gradients for outer hair cells, emphasizing the division of labor between the two hair-cell types. We demonstrate that tension in the tip links that convey force to the mechano-electrical transduction channels increases at reduced Ca2+. Finally, we reveal tonotopic gradients in stiffness and tension at the level of a single tip link. We conclude that intrinsic mechanical gradients of the tip-link complex help specify the characteristic frequency of the hair cell.