Inner hair cell and organ of Corti responses to very low frequency tones.

The timing of excitation observed in the auditory nerve exhibits a strong dependence on best frequency (BF). Fibers innervating the base of the cochlea respond to near threshold, low-frequency inputs approximately in phase with basilar membrane velocity to scala tympani while fibers innervating apical regions respond in phase with basilar membrane velocity to scala vestibuli [M. A. Ruggero and N. C. Rich, J. Neurophysiol. 58, 379–403 (1987)]. Although the latter is consistent with the velocity dependence of inner hair cells (IHC) and with the classical view of hair cell stimulation, the response phase of single units in the base of the cochlea has been difficult to explain. Consequently, IHC recordings from the second turn of the guinea pig cochlea (BF = 4000 Hz) were used to determine phases of depolarization relative to basilar membrane displacement. To better estimate synaptic drive, the organ of Corti response was subtracted from that recorded in the IHC since the voltage gradient that induces transmitter release can be influenced by extracellular responses that reflect receptor currents generated in nearby outer hair cells. Results indicate that IHC depolarization occurs near basilar membrane velocity to scala tympani consistent with single unit excitation in the base of the cochlea. [Work supported by Grant No. 5R01DC00089, National Institute on Deafness and Other Communication Disorders.]