1:30-3:00 How hearing happens: Mechanical amplification by ion channels and myosin molecules in the inner ear
A. James Hudspeth, Rockefeller University
Human hearing is enhanced by an active process that amplifies the ear's mechanical inputs several hundredfold, sharpens frequency tuning to allow the discrimination of tones differing in frequency by less than 0.2%, and compresses six orders of magnitude in the amplitude of sounds into only two orders of magnitude in neural output. In addition, 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. Cooperativity between mechanoelectrical-transduction channels confers negative stiffness on the hair bundle, which together with myosin-based adaptation motors elicits a dynamical instability that underlies the active process. Experiments on individual hair bundles indicate that the bundle's operation near this instability, a Hopf bifurcation, accounts for the four characteristics of the active process.