Changing Relationships between Structure and Function in the Cochlea during Recovery from Intense Sound Exposure

Abstract

This study examines the relationship between the morphological and electrophysiological changes in the guinea pig cochlea at intervals following exposure to intense sound (5 kHz, 125 dB SPL, 30 minutes). The cochlear compound action potential (N₁) audiogram (2–30 kHz) was determined within one hour or 1, 7, 14, or 28 days after exposure. When these organs of Corti were examined by scanning electron microscopy, most (92%) showed damage to hair cells and supporting cells around the 5-kHz region. The number of damaged hair cells increased significantly (p<0.01) in the first 24 hours following exposure, but there was no significant change thereafter. In contrast, the initial extensive loss of function (maximum at 8–12 kHz) recovered partially during the first 24 hours but also showed no significant further change. All morphological lesions occurred within regions corresponding tonotopically to the N₁ threshold losses, but only the larger lesions, which were observed seven or more days after exposure, fully reflected the extent of the loss of N₁ thresholds. This indicates that functionally important damage to the cochlea is usually more extensive than indicated by topographical abnormalities in the organ of Corti.