The Age-related Strial Degeneration Inhibits the Inner Hair Cell- Auditory Synapse Complex: A Computational Investigation.
Abstract
The Stria Vascularis regulates the electrical potentials inside the mammalian cochlea that are vital for the optimal operation of the ‘cochlear amplifier’. The effects of the age-related deficits of the endocochlear potential (EP) on the outer hair cell (OHC) motile forces have been investigated both theoretically and experimentally. Nevertheless, it is much less known how the age-related EP reduction explicitly affects the sensory inner hair cells (IHCs). This study analyzes the effects of the EP reduction on the IHC membrane potential and thereby the auditory nerve (AN) neurotransmitter release rate. This numerical analysis is performed using a biophysical lumped-element model of the IHC [Lopez-Poveda and Eustaque-Martin, JARO.7, 218-235 (2006)] and a computational model of the human AN [Zilany et al., J. Acoust. Soc. Am.135, 283-286 (2014)] for frequencies from 0.25 to 6 kHz. The hearing threshold was defined as the minimum displacement of the IHC stereocilia which generates a firing on AN fibers with high spontaneous rate. The hearing thresholds were quantified at different frequencies for the healthy cochlea (EP =89 mV). These thresholds were compared to the ones estimated for conditions in which the EP has declined to 25% and 50% of its optimal value, to quantify the corresponding hearing threshold elevations. The results show that the hearing threshold elevations increase as a function of the frequency from 7 dB at 250 Hz to around 25 dB at 6 kHz, consistent with the high-frequency profile of the age-related hearing loss. The presented analysis can help better differentiate between the OHC-related versus the IHC-related hearing threshold elevation in age-related hearing loss.
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