CHANGES IN THE MECHANICAL TUNING CHARACTERISTICS OF THE HEARING ORGANFOLLOWING ACOUSTIC OVERSTIMULATION

An in vitro preparation of the guinea-pig temporal bone was used to st udy the effects of acoustic overstimulation on the mechanical tuning c haracteristics of the inner ear. Using laser heterodyne interferometry , the vibratory responses of selected sensory and supporting cells wit hin the hearing organ were measured in response to acoustic signals ap plied to the ear to obtain mechanical tuning curves before and after a pplying acoustic overstimulation. Following overstimulation the freque ncy at which the maximal vibration response occurred moved towards low er frequencies, the vibration amplitude generally increased and the sh ape of the mechanical tuning curves became considerably flatter. These effects were seen within minutes of overstimulation. The micromechani cal changes were accompanied by distinct morphological changes mainly affecting the first row of outer hair cells, which were swollen and sh ortened. Hensen bodies and swelling of the subsurface cisternae were o bserved in the affected cells. Apart from this, most of the shortened cells looked structurally intact, had undamaged sensory hair bundles a nd made synaptic contacts to both afferent and efferent nerve fibres. The results demonstrate that the outer hair cells play a key role in d etermining the tuning of the hearing organ.