Abstract
Human hearing is enhanced by an active process with four cardinal characteristics. The active process amplifies the ear’s mechanical inputs, thus increasing sensitivity by more than a hundredfold. The second feature is enhanced frequency selectivity, which permits us to distinguish tones that differ in frequency by only 0.2%. Next, compressive nonlinearity telescopes a millionfold range of sound amplitudes into only a hundredfold range of responses. Finally, spontaneous otoacoustic emissions emerge from ears in a very quiet environment, an indication that the active process can be so exuberant as to become unstable.
Mechanoelectrical-transduction channels underlie the active process. Because the stereocilia of a hair bundle move as a unit, the gating of a given channel influences the probability that any other channel opens or closes. This mechanical cooperativity confers negative stiffness on the hair bundle, causing dynamical instability. Adaptation motors consisting of myosin‑Ic molecules can drive a bundle into the domain of instability, thus powering both spontaneous oscillations and active mechanical amplification. Experiments on hair bundles from the bullfrog’s sacculus have shown that this mechanism accounts for the four characteristics of the active process.
We have developed a novel feedback system, the mechanical-load clamp, with which to impose on a hair bundle a specified load stiffness and constant force. In this system a master computer controls the experiment while a slave computer rapidly calculates the appropriate feedback signals delivered to the bundle by a piezoelectrical stimulator. As the imposed load varies, the bundle progresses from passivity to spontaneous oscillation and exhibits a broad range of sensitivity to mechanical stimulation. We can then construct phase diagrams that characterize the bundle’s responsiveness as a function of the two control parameters, stiffness and force. The results accord with the predictions of a nonlinear dynamical model of hair-bundle activity; in particular, we find that a hair bundle can experience an instability termed the Hopf bifurcation. The bundle’s operation near this critical point yields optimal sensitivity and phase locking to external stimuli. These experimental results buttress the argument that the ear benefits from operation in the vicinity of a Hopf bifurcation and that active hair-bundle motility underlies this phenomenon.
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