Figure 1.

Ca²⁺ sparks in frog skeletal muscle. Sparks were recorded during voltage-clamp depolarizations to +40 mV after repriming fibers for 0.3–2 s at −90 mV. Each panel consists of five identified sparks obtained during depolarizations of 15 ms (A) or 3 ms (B). Individual sparks were identified by using selection criteria described in ref. 7 and are displayed as strip images of ΔF/F (below) obtained from high-speed confocal line-scan images, and the time course of ΔF/F averaged over 5 pixels (1.1 μm) at the center of the identified triad (black line, above). Superimposed on each ΔF/F time course is a fit of an empirical function (red line) to the data (12). The fitting procedure provided estimates of the latency, rise time, peak amplitude, and time constant of decline of ΔF/F for each identified spark. The relative timing of the depolarization is shown diagrammatically at the bottom; the repriming interval that preceded each test pulse is not shown.