Unifying the various incarnations of active hair-bundle motility by the vertebrate hair cell
Jean-Yves Tinevez 1, Frank Jülicher 2 and Pascal Martin 3*
1 MPI-CBG
2 MPI-PKS Dresden
3 Institut Curie/ CNRS U.M.R. 168
* To whom correspondence should be addressed. E-mail: pascal.martin{at}curie.fr.
Submitted on March 9, 2007
Revised on April 23, 2007
Accepted on 29 June 2007
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Abstract |
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The dazzling sensitivity and frequency selectivity of the vertebrate ear rely on mechanical amplification of the hair cells' responsiveness to small stimuli. As revealed by spontaneous oscillations and forms of mechanical excitability in response to force steps, the hair bundle that adorns each hair cell is both a mechano-sensory antenna and a force generator that might participate in the amplificatory process. To study the various incarnations of active hair-bundle motility, we combined Ca2+ iontophoresis with mechanical stimulation of single hair bundles from the bullfrog's sacculus. We identified three classes of active hair-bundle movements: a hair bundle could be quiescent but display non-monotonic twitches in response to either excitatory or inhibitory force steps, or oscillate spontaneously. Extracellular Ca2+ changes could affect the kinetics of motion and, when large enough, evoke transitions between the three classes of motility. We found that the Ca2+-dependant location of a bundle's operating point within its force-displacement relation controlled the type of movements observed. In response to an iontophoretic pulse of Ca2+ or of a Ca2+ chelator, a hair bundle displayed a movement whose polarity could be reversed by applying a static bias to the bundle's position at rest. Moreover, such polarity reversal was accompanied by a tenfold change in the kinetics of the Ca2+-evoked hair-bundle movement. A unified theoretical description, in which mechanical activity stems solely from myosin-based adaptation, could account for the fast and slow manifestations of active hair-bundle motility observed in frog, as well as in auditory organs of the turtle and the rat.
Key Words:
adaptation, amplification, auditory system, calcium feedback, mechanoelectrical transduction, vestibular system
