Recovery of threshold shift in hair-cell stereocilia following exposure to intense stimulation.

The isolated cochlear coil preparation was used to study changes in hair-cell stereocilia stiffness before and after overstimulation. Results were obtained from inner and outer hair cells in the apical and middle turns of the guinea pig cochlea. The stereocilia bundles were stimulated with an oscillating water jet and their movements were identified with stroboscopic illumination in a differential interference contrast microscope. The intensity of the water jet could be varied in decibel steps and the attenuation needed to achieve a 'visual detection level' threshold of movement was the criterion response throughout. Pre-exposure thresholds were sampled, the stereocilia bundle was overstimulated, and thresholds were measured during a recovery interval. Sensory hair bundles on all hair-cell rows showed a loss in stiffness following overstimulation which was proportional to the impedance of the stereocilia bundle. During recovery, stiffness increased and often showed a return to the pre-exposure threshold level. The results demonstrated that the loss of sensory hair stiffness following overstimulation recovered with the passage of time. The magnitude of the initial threshold shift, however, was related to the exposure conditions, cochlear location, and the impedance of the sensory hair bundle. The rate of recovery appeared to be independent of cochlear location, hair-cell row, or exposure conditions.