Fig. 1.

SV pools and ΔC_m in whole-cell recordings from frog saccular hair cells. In these and all other whole-cell experiments, the intracellular Ca²⁺ buffer was 1 mM EGTA. (a) The SB is surrounded by docked (green) and nondocked (purple) SVs. “Afferent” labels a postsynaptic terminal. (Scale bar, 200 nm.) (b) Representative C_m traces for depolarizations to −20 mV lasting 10 ms (small response) or 200 ms (large response). We did not attempt to interpret C_m measurements during the depolarization-induced changes in membrane conductance, which have been blanked during the interval from the onset of the depolarization until 30 ms after V_m was returned to −80 mV (dashed lines). (c) Boxed region from b. C_m(t) was averaged in 100-ms windows (red) surrounding the blanked interval, and the difference was used to compute ΔC_m. (d) ΔC_m plotted as a function of step duration. Each point is the average ΔC_m for the first depolarization applied to each cell (mean ± SEM; n shown in parentheses; each cell contributed one ΔC_m value at one duration only; total, n = 66 cells). There was no significant difference for any pairwise comparison of means at 10, 25, 30, and 50 ms (see Table 2, which is published as supporting information on the PNAS web site). Green and purple dashed lines indicate estimated numbers of docked SVs and docked plus undocked SVs, respectively, that are associated with synaptic ribbons. (e) Mean Ca²⁺ influx during the stimuli in d. Data are for whole-cell, voltage-clamp recordings. Evidence for the Ca²⁺ dependence of ΔC_m and the ensemble-averaged C_m(t) traces for the data in d are shown in Figs. 8, 9b, and 10, which are published as supporting information on the PNAS web site.