Generation of hair cell receptor potentials and basilar membrane tuning

Abstract

Inner hair cell (IHC) receptor potentials recorded from the first turn of the guinea pig cochlea phase-lead the extracellularly recorded cochlear microphonic indicating a response during basilar membrane velocity towards scala vestibuli. This phase-lead was reduced for higher frequencies, faster than could be expected from the effect of the membrane time constant alone suggesting that the IHCs changed from responding to velocity, to displacement. In order to resolve this question period histograms were generated from single units recorded in the spiral ganglion to frequencies between 40 and 500 Hz. Cells responded at low intensities during displacement towards scala tympani and at higher intensities this response changed to scala vestibuli velocity. We interpret this as being the effect of cochlear microphonic generated by the outer hair cells on the IHC synapse and the IHC receptor potential respectively and conclude that IHCs respond to basilar membrane velocity at least up to 500 Hz. Using the Mossbauer technique we have measured basilar membrane vibration at low sound pressures. The measurements were performed on guinea pigs at about the 18-kHz point in a fluid-filled scala tympani. The results show strong nonlinearities and sharp tuning, both of which are very vulnerable. There is a direct relationship between basilar membrane velocity and neural thresholds over a 60-dB range. This suggests that only minor processing takes place between basilar membrane vibration and hair cell stimulation.