Compressive nonlinearity in the hair bundles active response to mechanical stimulation P. Martin* and A. J. Hudspeth* Proc Natl Acad Sci USA 2001 Dec 4;98(25):14386-91 *Howard Hughes Medical Institute and Laboratory of Sensory Neuroscience,
The Rockefeller University, 1230 York Avenue, New York, NY 10021-6399;
and Abstract: The auditory system's ability to interpret sounds over a wide range of
amplitudes rests on the nonlinear responsiveness of the ear. Whether measured
by basilar-membrane vibration, nerve-fiber activity, or perceived loudness,
the ear is most sensitive to small signals and grows progressively less
responsive as stimulation becomes stronger. Seeking a correlate of this
behavior at the level of mechanoelectrical transduction, we examined the
responses of hair bundles to direct mechanical stimulation. As reported
by the motion of an attached glass fiber, an active hair bundle from the
bullfrog's sacculus oscillates spontaneously. Sinusoidal movement of
the fiber's base by as little as +/-1 nm, corresponding to the application
at the bundle's top of a force of +/-0.3 pN, causes detectable phase-locking
of the bundle's oscillations to the stimulus. Although entrainment increases
as the stimulus grows, the amplitude of the hair-bundle movement does
not rise until phase-locking is nearly complete. A bundle is most sensitive
to stimulation at its frequency of spontaneous oscillation. Far from
that frequency, the sensitivity of an active hair bundle resembles that
of a passive bundle. Over most of its range, an active hair bundle's response
grows as the one-third power of the stimulus amplitude; the bundle's sensitivity
declines accordingly in proportion to the negative two-thirds power of
the excitation. This scaling behavior, also found in the response of the
mammalian basilar membrane to sound, signals the operation of an amplificatory
process at the brink of an oscillatory instability, a Hopf bifurcation. Comment: Martin and Hudspeth observed that a hair bundle's response at threshold was not an increase in oscillation amplitude, but an increase in phase locking to the stimulus. It remains to be seen if the same behavior can be observed in mammalian cochlear hair cells. If yes, we would have a definite proof that spontaneous oscillations of tuned hair cells are functional for our low threshold of hearing. (Comment Martin Braun) |