NERVE IMPULSES IN INDIVIDUAL AUDITORY NERVE FIBERS OF GUINEA PIG

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Abstract
A technique was developed to record single-fiber responses from the cochlear nerve in the modiolus of the guinea pig. Spontaneous discharges were recorded from many auditory and non-auditory nerve fibers in the 8th nerve. Spontaneous discharges in auditory nerve fibers were not inhibited by acoustic stimulation. In response to a strong 8000 cps. tone pip, single-fiber spikes started at, or slightly after, the peak of the whole-nerve response; double, sometimes triple, responses were frequently observed in stimulation by single high-frequency tone pips. The nerve fibers arising in the basal turn of the cochlea responded to tones between 500 and 10,000 cps. The fibers arising in the upper part of the cochlea responded only, to low-frequency tones. The response-area was mapped out for several single auditory fibers. The curve relating frequency and threshold intensity for an auditory nerve fiber in the modiolus consisted of 2 parts, one a gradual fall in threshold with increasing frequency and the other a sharp rise in threshold for frequencies above a certain "cut-off" frequency. The frequency at which the threshold showed a rough minimum was a function of the position of the nerve ending in the cochlea. With 290 and 500 cps. tones the nerve impulses were initiated early in the phase of the micro-phonic response, during which the hair cells were traversed by electric current flowing from scala vestibuli to scala tympani. This strengthens the view that the microphonic responses are direct electric stimuli for initiating impulses at the nerve endings.

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