RyR activation is incomplete during a spark. (A) Histogram of the number of RyRs opened during a spark for various RyR calcium sensitivities. Even at the highest level, when sparks become metastable, not all RyRs open. (B) Ensemble averages of the number of open RyRs and JSR calcium during the spark. The JSR calcium pool depletes rapidly during the period when RyRs are still being recruited by local propagation of CICR in the couplon, causing the spark to terminate before full recruitment.