Information processing in the mammalian auditory periphery
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Inner hair cells (IHC) are the primary sensory cells of the mammalian cochlea. They transduce sound energy into a changing receptor potential which stimulates electrical activity in the Type I spiral ganglion cells of the auditory nerve. The auditory information thus encoded leads to the sensation of hearing. This thesis comprises my attempts to elucidate some of the biophysical mechanisms operating in the cochlea by analyzing intracellular recordings from guinea pigs, and to investigate the role these mechanisms play in auditory information processing via conceptual and computational models. Noise in the IHC receptor potential sets limits on the performance of a single cell. The magnitude of the intracellular noise averages 0.3 m V rms. A single IHC will be limited by this noise to: (i) a minimum detectable receptor potential of 0.3 mV (corresponding to about 0 dB SPL), (ii) a channel capacity of 5100 bits/sec, and (iii) a temporal resolution of 42 JLS. I compare these single cell limits to auditory performance as observed in published behavioral studies. The IHC receptor potential is shaped by at least two nonlinear processes: nonlinear transduction and a voltage dependent membrane conductance. I characterized the nonlinear conductance by analyzing recordings made during intracellular current injection. A simple model containing a two-state voltage-gated channel was sufficient to replicate the current-voltage characteristic found in these cells. I investigated the information transfer from inner hair cells to the auditory nerve by comparing the growth of the de receptor potential to the average firing rate in spiral ganglion cells. This comparison suggests that neural units with different thresholds encode different portions of the IHC dynamic range; at conditions well above threshold, low threshold units may be carrying predominantly temporal information while high threshold units may encode the absolute sound level. To help understand the complex behavior of the IHC receptor potential, I developed a computational model for its generation. The model contains gated ion channel descriptions of the nonlinear transducer and membrane conductance. Analysis of the model suggests a possible role for the voltage dependent conductance: efficiently trading sensitivity for temporal resolution as stimulus level increases.
Thesis (Ph.D.)--Boston UniversityPLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at firstname.lastname@example.org. Thank you.