The effects of mechanically gated ion channels of the inner ear on thermal sensitivity of spontaneous otoacoustic emissions

Abstract

In quiet environments, the inner ears of vertebrates can produce low intensity sounds that are detectable in the ear canal, termed spontaneous otoacoustic emissions (SOAEs). This background activity of the inner ear has been regarded as an epiphenomenon of the active processes performed by hair cells – the sensory receptors of the auditory system. Experimental measurements of SOAEs from the ears of tokay geckos reveal a linear increase of SOAE frequency with body temperature, with the emission at higher frequencies displaying greater thermal sensitivity. In this work, we elucidated the cellular mechanism underlying the thermal sensitivity of SOAE frequency using a mathematical model of hair cell’s transduction process performed by mechanically gated ion channels. We employed the previously proposed gating-spring model which described an individual ion channel by a two-state system, whose activation energy associated with channel gating depended on the level of temperature. Our results from numerical simulations revealed that a rise in temperature elicited an increase in the frequency of spontaneous oscillations displayed by a single hair cell. The magnitude of the frequency shift increased with the oscillation frequency following a quadratic polynomial, a characteristic observed in the physiological recordings of SOAEs from tokay geckos. The model further suggested that the intrinsic energy difference between the open and closed state of the ion channels greatly controlled the temperature sensitivity of a hair bundle.