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Journal of Neuroscience, Vol 12, 1906-1916, Copyright © 1992 by Society for Neuroscience
On the frequency limit and phase of outer hair cell motility: effects of the membrane filter
J Santos-Sacchi
Section of Otolaryngology, Yale University School of Medicine, New Haven, Connecticut 06510.
Whole-cell voltage clamp and displacement-measuring photodiode techniques
were used to study electrophysiological and mechanical properties of the
guinea pig outer hair cell (OHC). OHCs demonstrate a voltage-mechanical
response (V-M) function that can be fit by a two state Boltzmann relation,
where the cell normally rests near the hyperpolarizing saturation region
(-70 to -90 mV). The voltage at half- maximal length change (Vh) is
depolarized relative to the resting potential, and this ensures that for
symmetrical sinusoidal voltage stimulation about the resting potential, AC
and DC mechanical responses will be generated. Analysis of OHC motility
using pure tone voltage bursts from 11 to 3200 Hz demonstrates both AC and
DC mechanical responses. By exploiting the frequency-dependent
current-voltage phase separation that is characteristic of an RC-dominated
system under voltage clamp, it is demonstrated that OHC motility follows
the phase of AC transmembrane voltage and not that of current. For voltage
stimulation across frequencies in the acoustic range, the motility cutoff
frequency corresponds to the cutoff frequency of the imposed transmembrane
voltage. Frequency cutoffs approaching 1 kHz have been measured but are
clamp time constant limited. These observations are congruent with the
voltage dependency hypothesis of OHC motility. In addition, the DC
component of the mechanical response is shown to be frequency independent,
but to decrease in magnitude disproportionately compared to the AC
component as the magnitude of the driving voltage decreases. This is
predicted from the form of the V-M function, whose level dependent DC
nonlinearity is a consequence of the resting potential being displaced from
Vh. The net effect is that the mechanical DC: AC ratio approaches zero for
small AC voltages. Taken together, these findings question the ability of
the OHC mechanical response to influence organ of Corti micromechanics at
high acoustic frequencies where a tuned amplification of basilar membrane
motion is hypothesized.
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