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The hair-cell bundle as a sensory antenna and mechanical amplifier for hearing
The technical specifications of the human ear are remarkable. We can detect sounds that
span six orders of magnitudes of sound-pressure levels, with exquisite sensitivity and sharp
frequency selectivity to weak sound stimuli. These characteristics emerge from nonlinear
amplification of mechanical vibrations by sensory hair cells of the inner ear. The hair cell’s
mechano-receptive antenna, the hair bundle, can function as a force generator and even oscillate
spontaneously. We have performed micromechanical experiments on single hair bundles
from the bullfrog’s sacculus. They revealed that an oscillatory instability, called a Hopf
bifurcation in the theory of dynamical systems, can arise from the interplay between negative
hair-bundle stiffness, the activity of adaptation motors and Ca2+ feedback on the force
that the motors can produce. As any dynamical system that operates near a Hopf bifurcation,
an oscillatory hair bundle functions as a frequency-selective, nonlinear amplifier that
qualitatively recapitulates the key features of the auditory amplifier. Thermal fluctuations,
however, jostle the response of a single hair bundle to weak stimuli and seriously limit amplification.
Because in vivo most hair bundles are mechanically coupled by overlying gelatinous
structures, we have recently assessed the effects of mechanical coupling on the hair-bundle
amplifier. We found that elastic coupling of a hair bundle to neighbors effectively reduces
noise and enhances amplification to a level that can accord with in vivo measurements. Auditory
detection by “critical” oscillators, be they implemented solely by active hair-bundle
motility or by a more complex process, thus provides a useful framework to apprehend the
dazzling performances of mammalian hearing.
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