Ultrastructural changes to the cochlea resulting from impulse noise.

Following impulse noise trauma to chinchillas, observation of plastic-embedded surface preparations of the organ of Corti showed no consistent relationship between cochlear hair cell loss and permanent hearing loss (Hamernik et al. 1980). In some animals there was a loss of hearing when hair cells were present. The cochleas from that experiment were examined with transmission electron microscopy to determine at the ultrastructural level if there was damage to the sensory cells that would explain the change in threshold sensitivity. Ultrastructural changes in cochlear hair cells include an increase in lysosomes, multivesicular bodies, vacuolization of subsurface cisternae, and proliferation of Hensen bodies. These changes are observed in all experimented animals. Alterations to the ultrastructure of the stereocilia vary from animal to animal and on the outer hair cells, the changes include loosening of the stereocilia membranes, loss of stiffness, fusion of the stereocilia and disintegration of the rootlets. These changes are observed only in animals that have a permanent threshold shift after noise trauma.