Dr. Daibhid O Maoileidigh
Rockefeller University

The Babel Fish: Active Mechanics in the Ear

The auditory system's exquisite sensitivity, sharp frequency tuning, broad dynamical range, and tendency to emit sounds spontaneously are thought to result from an active mechanical process within the ear. All vertebrate ears contain sensory hair cells on whose apices reside active force generators known as hair bundles. In mammals, the hair cell bodies also transduce electrical stimuli into mechanical forces by functioning as piezoelectric actuators. I will present a physical description of the mammalian inner ear that, by amplifying the power of an external input, acts as a sensitive, sharply tuned, and nonlinear signal detector. The system's state diagram resembles a fish with a loop of Hopf bifurcations and exhibits generic properties which accord with experimental observations when it operates near one of these critical points. In addition, I will show that most observations of hair bundle dynamics can be understood in terms of a simplified model with a more general fishlike state diagram. This fish diagram predicts that the bundle can operate as a low-pass filter, quiescent resonator, active oscillator or bistable switch depending on how it is mechanically loaded. I will describe how mechanical loading is used to adjust the signal detection properties of the hair bundle in different organs and organisms so it can serve diverse roles in various sensory systems including those associated with hearing, balance, and fluid flow detection.