Histograms of spark extinction time with and without the lumenal activation factor multiplying the RyR opening rate (Eq. 1.1). The lumenal activation factor provides about a 10-fold variation in opening rate over the range of JSR calcium concentrations during a spark, but its absence causes only a modest prolongation of RyR opening, and sparks still terminate based entirely on the direct effect of JSR depletion to reduce RyR flux and CICR loop gain.