Figure 1. Light-evoked hair-bundle movement.

(A) A schematic drawing of a hair bundle depicts a file of four actin-filled stereocilia; the kinocilium is not shown. At the tip of each stereocilium a single tip link inserts upon one or two mechanically sensitive ion channels. Application of a force (arrow) in the positive direction, toward the bundle’s tall edge, tenses the tip links and opens the channels. At the insertional plaque atop each tip link, myosin motors adjust the tension in the link and thus regulate the probability of channel opening. (B) Irradiation with ultraviolet light of three different hair cells from the bullfrog’s sacculus evoked hair-bundle movements with distinct timecourses. To record hair-bundle motion, the shadow of a hair bundle was projected onto a dual photodiode (see Methods). In this and subsequent figures, a gray or purple band denotes the duration of irradiation. Wavelength, 375 nm; power density, 152 MW·m⁻² (1 MW·m⁻² = 1 W·mm⁻²). (C) The peak amplitude of hair-bundle movement increased linearly with the power density of illumination. (D) Measurements of light-induced hair-bundle motion before (upper trace) and after (lower trace) iontophoretic application of EGTA demonstrated that the response required intact tip links. Data were obtained from hair bundles embedded in the saccular macula and irradiated from a direction orthogonal to the plane of the macula. See also Figures S1 and S2 and Movies S1 and S2.