Hair cell mechanotransduction contributes to spontaneous activity in dimorphic terminals. (A) A schematic diagram illustrating a slice through the sensory epithelium and our stimulation/recording paradigm. Data were recorded from calyx terminals while stimulating the hair bundle of the innervated type I hair cell. Individual hair bundles were deflected with a stiff probe coupled to the kinocilium. (B) Representative current-clamp trace showing modulation of the firing rate in a dimorphic calyx terminal using the hair bundle deflection protocol shown below. (C) Change in spike rate, relative to the spike rate at rest, plotted as a function of bundle deflection (toward the kinocilium is positive). Positive deflections increased the firing rate, whereas negative deflections decreased the firing rate. ***, P < 0.001. (D and E) Firing properties were also examined after treatment with EGTA to eliminate mechanotransduction. The percentage of spontaneously active calyx terminals was slightly reduced (D), but the firing rate was significantly lower in the absence of mechanotransduction (E). **, P < 0.01. The number of samples is shown inside the bars for each condition. Error bars indicate mean ± SEM.