Figure 1.

Response of active hair bundles to stimulation by sinusoidal transepithelial current. (A) Before stimulation, a hair bundle oscillated spontaneously at ≈35 Hz. During negative stimulation at 1 Hz, the bundle displayed faster and smaller oscillations with occasional omissions; the positive stimulus phase slowed and eventually blocked oscillatory movements. Similar responses occurred during stimulation at 10 Hz. At 30 Hz, slightly below the natural frequency of the cell, the response comprised phase-locked movements adorned with complex spikes. At 100 and 300 Hz, the response was essentially out of phase with the stimulus. During a 300-Hz stimulation, spontaneous bundle oscillations resumed, superimposed on the rapid, phase-locked response. (B) The hair bundle of a second cell oscillated spontaneously at ≈8 Hz. Although electrical stimulation evoked responses similar to those in A, the transition from the low- to the intermediate-frequency regime of responsiveness occurred at a frequency <10 Hz. In this and the subsequent figures, each mechanical response is superimposed on a record of the electrical stimulus (red). Positive bundle movement is defined as that towards the kinocilium. Positive current is defined as that depolarizing the apical membrane of the hair cell. The top time calibration applies to the 1- and 10-Hz records, the middle calibration corresponds to the 30- and 100-Hz records, and the bottom calibration refers only to the 300-Hz records.