Hair cell condition and auditory nerve response in normal and noise-damaged cochleas

Histological and physiological data are presented from cats born and raised in a low-noise environment and from cats with long-standing, noise-induced threshold shifts. Even after a 1 1/2-year survival from acoustic trauma, there were threshold elevations of roughly 40 dB in the response of single auditory-nerve fibers which could not be correlated with significant loss of hair cells. An attempt was made to correlate these threshold differences with differences in the condition of the sensory cells as seen in a light-microscopic examination of epon-embedded surface preparations. Of all the histological features evaluated, the orderlines of the stereocilia, on both inner and outer hair cells, showed the closest correlation with single-unit thresholds. In the final analysis, most of threshold shift in the noise-exposed ears could be accounted for by loss or damage to sensory cells clearly visible under the light microscope.