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Mechanical amplification in the inner ear
The ear is not a passive receiver of sound ; an active process instead endows our hearing with its remarkable features. The active process amplifies sounds by more than a hundredfold, greatly enhancing our sensitivity to weak stimuli. The ear’s intrinsic amplifier additionally tunes our responsiveness to specific frequencies of stimulation, thus facilitating the recognition of sound sources and the discrimination of speech. The active process next allows us to analyze acoustic signals over a millionfold range of magnitudes, compressing responses so that we can appreciate both soloist and orchestra. Most remarkably, the ear’s intrinsic hearing aid—like an electronic device—can become unstable, leading to the spontaneous emission of sounds from the ear.
In addition to responding to mechanical stimuli, the hair cell’s transduction organelle, the hair bundle, accounts for the active process in non-mammalian tetrapods. Owing to the effects of ion-channel gating, this structure exhibits negative stiffness, a mechanical instability that poises it to amplify inputs. Active hair-bundle motility results from the interaction of negative stiffness with the myosin‑1c motors that underlie the bundle’s adaptation to protracted stimuli. In the active process of mammals, the mechanical activity of the piezoelectrical protein prestin extends the range of responsiveness to 100 kHz.
Sound evokes a traveling wave that propagates along the basilar membrane of the cochlea, growing progressively as successive hair cells add mechanical energy to the oscillation. Focal inactivation of the active process by a light-activated blocker of electromotility reveals the magnitude and spatial extent of this effect. Theoretical and experimental analyses of active hair-bundle motility indicate that its operation near a Hopf bifurcation explains many of the characteristics of hearing. In particular, the dependence of response amplitude on stimulus force follows the expected power law with an exponent of one-third. Operation near a Hopf bifurcation also produces distortion products with the characteristics observed for human hearing. Finally, a critical oscillator can become unstable, providing a natural explanation for spontaneous otoacoustic emissions.