RAPID, ACTIVE HAIR BUNDLE MOVEMENTS IN HAIR-CELLS FROM THE BULLFROGS SACCULUS

Hair bundles, the mechanically sensitive organelles of hair cells in t he auditory and vestibular systems, are elastic structures that are de flected by sound or acceleration. To examine rapid mechanical events a ssociated with mechanoelectrical transduction, we stimulated individua l hair bundles with flexible glass fibers and measured their responses with a temporal resolution of 400 mu sec. When a hair bundle from the bullfrog's sacculus was abruptly deflected in the positive direction, the bundle's motion in the direction of stimulation was interrupted w ithin the initial few milliseconds by an active movement, or twitch. T his response was biphasic, with an initial component in the direction of the stimulus and a second component in the opposite direction. The amplitude and duration of the twitch depended on the bundle's initial position and the size and rise time of the stimulus; the twitch was la rgest over the range of bundle deflections in which transduction was m ost sensitive. Under displacement clamp conditions, in which a hair bu ndle's position was changed and then held constant with negative feedb ack, the twitch manifested itself as a biphasic force exerted by the b undle. Some hair bundles produced twitches in response to negatively d irected stimuli, exhibited stimulus-evoked damped oscillations, or twi tched spontaneously. The hair bundle's ability to perform work against an external load and to oscillate in response to stimulation indicate s that the bundle could supply feedback for mechanical amplification i n vertebrate auditory organs.