Dynamics of cytosolic, JSR, and FSR calcium of an isolated spark as in Fig. 2. (A) Cytosolic calcium as a function of time at the spark center. (B) Spatial profile of cytosolic calcium at three time points. (C) Time course of JSR calcium and FSR calcium at the center. (D) Spatial profile of the FSR blink. (E) Images of cytosolic and FSR calcium at three time points. (F and G) Overlay of multiple spark time and space profiles. (Inset) Time course of cytosolic calcium at a neighboring couplon 1.4 µm away. (H) The simulated fluorescence profile of the spark using 100 µM fluo-3 as it evolves in time (each millisecond): red, before peak (rise phase); green, at the peak; blue, after the peak (decay phase). At the peak of fluorescence it has a full width at half-maximum of 1.3 µm, comparable to other spark simulations (e.g., Smith et al., 1998). The spark has a high peak cytosolic [Ca²⁺] of tens of micromolar, but this decays rapidly with distance, so that at neighboring couplons it is insufficient to trigger another spark with high probability, although sufficiently above background to be visible with sensitive fluorescent dyes. The JSR “blink” has a deep nadir, and then recovers slowly, as found in previous simulations (Sobie et al., 2002; Ramay et al., 2011; Laver et al., 2013; Stern et al., 2013). FSR calcium near the center of the spark shows a shallower blink, but in the periphery there is an overshoot caused by uptake by SERCA of cytosolic calcium diffusing from the spark. An overlay of multiple sparks shows the presence of a long stochastic tail of release consistent with the fact that these sparks are on the border of metastability.