Pathology of the inner ear after acoustic injury

We examined the impact of acoustic injury on the cochlea by assessing auditory brain-stem response (ABR) threshold shifts, outer hair cells, and the number of synaptic ribbons before and after noise exposure. The ABR threshold was elevated immediately after noise exposure, but recovered over time; 14 days after noise exposure, the ABR threshold was approximately 15 dB higher than before exposure. There were few damaged outer hair cells. The number of synapses directly under inner hair cells markedly decreased immediately after noise exposure, but then exhibited early signs of recovery. Twenty-four hours after noise exposure, the number of synapses was higher than before exposure. Subsequently, synapse number decreased. Seven days after noise exposure, synapes number was lower than before exposure. In this acoustic injury model, although the synapses were damaged, as described in the previous reports on the temporary threshold shift (TTS), the regeneration of synapses was promoted soon thereafter. There appears to be a point during the repair process when the number of synapses temporarily increases. This model is considered to be a partially reversible synaptic damage model. This study revealed an association between the intensity of noise exposure and the severity of functional and histological damage.