Comparison of stochastic and hybrid models of Ca ²⁺ release. (A) The time course of [Ca²⁺] in the dyad cleft and TSR from Ca²⁺ spark simulations. The fully stochastic model for Ca²⁺ sparks used by Cannell et al. (2013); Laver et al. (2013) is compared with the hybrid model used here for clusters of 25 RyR2s. (B) Stochastic and hybrid simulations of dyad [Ca²⁺] for RyR2 cluster sizes of 9 and 36. (C) Time course of fluorescence during a Ca²⁺ spark using our hybrid model (black line). Colored lines show the time course of a second Ca²⁺ spark triggered at various delays after the first spark at the same site.