How hearing happens

Tuesday, 26. Februay, 8:30 h, H 0105

Our hearing is remarkable for its physical capabilities. We can discern sounds at frequencies up to 20 kHz, a bandwidth one-thousandfold that of our vision. At the auditory threshold, our ears detect vibrations of only 0.3 nm, an atomic dimension. Finally, our hearing responds over twelve order of magnitude in intensity, a range unmatched by other sensory systems or indeed by manmade detectors. Uniquely among vertebrate sensory receptors, the ear’s mechanoreceptive hair cells amplify their inputs by means of an active process that increases responsiveness to sound, sharpens frequency selectivity, and compresses the dynamic range of hearing. An overly exuberant active process can even cause the spontaneous emission of sound from an ear! Each hair cell uses its hair bundle, an elaborate array of tiny biological strain gauges, both to transduce mechanical stimuli and to implement the active process. Application of the fluctuation-dissipation theorem confirms that a hair bundle can contribute energy to its inputs. Mechanical amplification is accomplished through the interaction of negative hair-bundle stiffness with the activity of the motor protein myosin-1c. The operation of the active process near a Hopf bifurcation explains many of the characteristics of our hearing.

James Hudspeth
Rockefeller University, New York, USA