Single-neuron labeling in the cat auditory nerve.

Single auditory nerve fibers in the cat were labeled intracellularly with horseradish peroxidase. The sample of fibers was selected to represent different response types over a wide range of characteristic frequencies. All 56 labeled neurons were found to be radial fibers innervating inner hair cells, suggesting that none of the single-unit data reported to date has been from the outer hair cell innervation. Differences in rates of spontaneous discharge and thresholds to tones among these labeled neurons were closely correlated with morphological differences in the caliber and location of their unmyelinated terminals on the body of the inner hair cell.