STIFFNESS CHANGES IN CHICK HAIR BUNDLES FOLLOWING IN-VITRO OVERSTIMULATION

1. The in vitro effect of intense stimulation on the micromechanical s tiffness of hair cell sensory hair bundles was studied at three locati ons on the chick basilar papilla. Threshold levels of hair bundle moti on, produced by a water jet stimulus, were examined before and after e xposure to a 300 Hz water jet stimulus set at 25 dB above the pre-expo sure threshold level. 2. Threshold levels of motion were systematicall y examined in 8 unexposed control cells. The level of water jet stimul us needed to achieve the detection threshold of motion remained consta nt in these cells when periodically tested over a 36.5-min interval. 3 . Post-exposure changes in the motion detection threshold of hair bund les were examined in 82 hair bundles, and a number of effects were ide ntified: 2.4% of the hair bundles showed no threshold change; 31.7% of the hair bundles had threshold shifts which indicated an increase in stiffness; 18.3% exhibited a threshold shift that indicated a decrease in hair bundle stiffness, but with no recovery; and 47.6% had thresho lds that indicated a decrease in hair bundle stiffness with recovery t o pre-exposure levels within 16-18 min. 4. The results suggest that ch ick hair bundles exhibit complex and varied responses to overstimulati on which are very different from that seen in the mammal.