Journal of Neuroscience, Vol 13, 1767-1783, Copyright © 1993 by Society for Neuroscience
Electrical resonance of isolated hair cells does not account for acoustic tuning in the free-standing region of the alligator lizard's cochlea
RA Eatock, M Saeki and MJ Hutzler
Physiology Department, University of Rochester, New York 14642-8642.
The cochlea of the alligator lizard is divided into two morphologically and
physiologically distinct regions. In the "tectorial region," hair bundles
of hair cells are draped by a tectorial membrane, whereas in the
"free-standing region," hair bundles are said to be free-standing because
there are no overlying tectorial structures. The acoustic tuning of the
free-standing region depends at least in part on mechanical resonances of
the hair bundles. In the turtle cochlea, in contrast, acoustic tuning
depends in large part upon the electrical properties of the hair cells. We
have investigated the electrical properties of hair cells isolated from the
free-standing region of the alligator lizard's cochlea. When injected with
steps of depolarizing current, these "free-standing hair cells" exhibited
electrical resonances that were comparable in frequency and quality to
electrical resonances in cochlear hair cells from turtles, chicks, and
alligators, and in saccular hair cells from frogs and fish. In the lizard's
free- standing hair cells, however, the electrical resonance frequencies
(< 300 Hz) were a decade below the cells' acoustic characteristic
frequencies (between 1 and 4 kHz), showing that the electrical resonance is
not likely to contribute to acoustic tuning. The electrical resonances were
not apparent at rest. The cells' resting potentials were significantly more
negative than the activation voltage (approximately -40 mV) of the
Ca(2+)-dependent K+ current upon which the electrical resonance has been
shown to depend in other hair cells. At potentials more negative than -50
mV, an inwardly rectifying K+ conductance dominated. Because we observed no
electrical tuning above 300 Hz, our results indirectly support a mechanical
origin for acoustic tuning in the free-standing region of the alligator
lizard cochlea. These results further show that acoustic tuning cannot be
inferred solely from the electrical resonances of isolated hair cells.
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