Growth of threshold shift in hair-cell stereocilia following overstimulation.

Sensory hair bundle micromechanics were measured from all four hair-cell rows in the isolated guinea pig cochlea before and during overstimulation. The stereocilia bundle was stimulated by an oscillating water jet, and stroboscopic illumination slightly offset from the frequency of the stimulus revealed their motion. The intensity of the water jet could be varied in decibel steps and a 'visual detection level' threshold of stereocilia movement served as the criterion response. Pre-exposure thresholds provided a reference and were compared to thresholds sampled at 1-min intervals during a 10-min exposure period. The exposure stimulus consisted of the water jet adjusted to either 8, 13, or 18 dB above the visual detection level. Stereocilia on hair cells in each of the four rows showed a loss in stiffness which systematically increased during the first 5 to 6 min of exposure. Between 6 and 10 min of overstimulation the threshold shift exhibited a plateau whose magnitude was proportional to the exposure level. There were also differences in the magnitude of threshold shift between the hair-cell rows. The results clearly showed that overstimulation changed the micromechanical behavior of the stereocilia bundle, and that there was a complex interaction between exposure duration, level, and hair-cell row.