Changes in the mechanical tuning characteristics of the hearing organ following acoustic overstimulation.

An in vitro preparation of the guinea-pig temporal bone was used to study the effects of acoustic overstimulation on the mechanical tuning characteristics of the inner ear. Using laser heterodyne interferometry, the vibratory responses of selected sensory and supporting cells within the hearing organ were measured in response to acoustic signals applied to the ear to obtain mechanical tuning curves before and after applying acoustic overstimulation. Following overstimulation the frequency at which the maximal vibration response occurred moved towards lower frequencies, the vibration amplitude generally increased and the shape of the mechanical tuning curves became considerably flatter. These effects were seen within minutes of overstimulation. The micromechanical changes were accompanied by distinct morphological changes mainly affecting the first row of outer hair cells, which were swollen and shortened. Hensen bodies and swelling of the subsurface cisternae were observed in the affected cells. Apart from this, most of the shortened cells looked structurally intact, had undamaged sensory hair bundles and made synaptic contacts to both afferent and efferent nerve fibres. The results demonstrate that the outer hair cells play a key role in determining the tuning of the hearing organ.