Acoustic injury of the inner ear: Morphological and physiological perspectives

Abstract

Of the many approaches to the study of acoustic injury, one of the most fruitful has been to view the functional changes in the response properties of single auditory‐nerve fibers. Since each cochlear afferent contacts only one inner hair cell, single‐fiber responses provide a functional window onto a restricted portion of the cochlear duct. Through the use of intracellular labeling, which allows the cochlear origin of selected afferents to be precisely specified, correlations between structural and functional changes have been made on the single‐cell level. Correlations so obtained have revealed the functionally important structural changes underlying acoustic injury of the inner ear. Morphological data from both the light‐ and electron‐microscopic levels suggest that most sound‐induced permanent threshold shifts can be accounted for quantitatively based on the loss of sensory cells and/or the damage to their stereocilia. Of all the structures in the inner ear, the stereocilia, and especially the “rootlets” that anchor them to the tops of the hair cells, appear to be the most vulnerable to permanent damage from acoustic overstimulation. The structural changes underlying reversible threshold shifts are clearly different from those underlying permanent loss, but exactly which structural changes underly the temporary losses are much less clear. Structure‐function correlations in damaged ears have also helped define the contributions of inner versus outer hair cells to the response properties of auditory‐nerve fibers. Damage to inner versus outer hair cells results in strikingly different abnormalities in neural tuning, spontaneous activity, and maximal discharge rates. The nature of these differences suggests that auditory perceptual abilities would be very different given threshold shifts of identical severity but differing balance of damage to the two classes of sensory cells.