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1 Division of Neuroscience, Baylor College of Medicine, Houston, Texas, USA
2 The Bobby R. Alford Dept. of Otorhinolaryngology and Communicative Sciences, Baylor College of Medicine, Houston, Texas, USA; Division of Neuroscience, Baylor College of Medicine, Houston, Texas, USA
* To whom correspondence should be addressed. E-mail: eatock{at}bcm.tmc.edu.
Whole-cell transduction currents were recorded from hair cells in early postnatal mouse utricles in response to step deflections of the hair bundle. For displacement steps delivered by a stiff probe (1-ms rise time), half-maximal responses decayed monoexponentially with a mean time constant of 30 ms. Adaptation and other transduction properties did not vary systematically with hair cell type (I vs. II) or region (striola vs. extrastriola). Thus, regional variation in the phasic properties of utricular afferents arises through other mechanisms. When bundles were deflected by a fluid jet, which delivers force steps, transduction currents decayed about three-fold slower than during displacement steps. A simple model of myosin-mediated adaptation predicts such slowing through forward creep of the bundle during a force step. For a faster stiff probe (rise time 200 µs), step responses of both mouse utricular and frog saccular hair cells decayed with two exponential components, which may correspond to distinct feedback processes. For half-maximal responses, the two components had mean time constants of 5 and 45 ms (mouse) and 2 and 18 ms (frog). The fast and slow components dominated the decay of responses to small and large stimuli, respectively. Adaptation shifts the instantaneous operating range in the direction of the adapting step. In frog saccular hair cells, the operating range shift is a constant percentage of the displacement. In mouse utricular hair cells, the percentage shift increases for large displacements, extending the range of background stimuli over which adaptation can restore instantaneous sensitivity.
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