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1 Bioengineering, University of Utah, Salt Lake City, UT, USA; Marine Biological Laboratory, Woods Hole, MA, USA
2 BioVIS Technology Center, NASA Ames, Moffett Field, CA, USA; Marine Biological Laboratory, Woods Hole, MA, USA
3 Neurology and Center for Anatomy and Functional Morphology, Mount Sinai School of Medicine, New York, NY, USA
4 Otolaryngology and Neurobiology, Washington University, St Louis, MO, USA; Marine Biological Laboratory, Woods Hole, MA, USA
* To whom correspondence should be addressed. E-mail: r.rabbitt{at}utah.edu.
The time course and extent of adaptation in semicircular canal hair cells was compared to adaptation in primary afferent neurons for physiological stimuli in vivo in order to study the origins of the neural code transmitted to the brain. The oyster toadfish, Opsanus tau, was used as the experimental model. Afferent firing-rate adaptation followed a double-exponential time course in response to step cupula displacements. The dominant adaptation time-constant varied considerably between afferent fibers and spanned six orders of magnitude for the population (~1ms - >1000s). For sinusoidal stimuli (0.1 - 20 Hz), the rapidly-adapting afferents exhibited a 90° phase lead and frequency-dependent gain, while slowly-adapting afferents exhibited a flat gain and no phase lead. Hair cell voltage and current modulations were similar to the slowly-adapting afferents and exhibited a relatively flat gain with very little phase lead over the physiological bandwidth and dynamic range of tested. Semicircular canal microphonics also showed responses consistent with the slowly-adapting subset of afferents and with hair cells. The relatively broad diversity of afferent adaptation time constants and frequency-dependent discharge modulations relative to hair cell voltage implicate a subsequent site of adaptation that plays a major role in further shaping the temporal characteristics of semicircular canal afferent neural signals.
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