Auditory nerve activity and cochlear morphology after noise exposure

Four chinchillas were exposed for 5 days to an octave band of noise centered at 4 kHz and having an SPL of 86 dB. After a recovery period of approximately 6 months, behavioral audiograms were obtained and auditory nerve fiber activity was recorded. The animals were killed and the cochleas embedded in plastic to obtain a surface preparation and 1 μ radial sections of the organ of Corti. Behavioral threshold shifts ranged from 5 to 20 dB at frequencies between 4 and 11 kHz. Auditory nerve fiber thresholds were elevated up to 70 dB for units with characteristic frequencies between 4 and 14 kHz. Units with higher and lower characteristic frequencies had normal thresholds. Cochleagrams showed narrow lesions of inner and/or outer hair cells over approximately a 1 mm distance. A comparison of the three realms of data revealed the following: (1) The greatest threshold shifts from the noise exposure were seen in the single nerve fiber thresholds while the smallest shifts were seen in the behavioral thresholds, (2) the greatest behavioral and neural threshold shifts and greatest cochlear damage occurred 1 octave above the center frequency of the noise exposure, and (3) based on the frequency-place map of the chinchilla cochlea, the range of fibers with elevated thresholds exceeded the extent of the OHC lesion. A number of anatomical changes were seen that effectively increased the extent of the damage found in the chochleagram. These changes included: distortions in the surface topography of the organ of Corti affecting the orientation of IHC; missing pillar cells in the presence of normal OHC and/or IHC and protrusion of the IHC cuticular plate into the subtectorial space.