Spark extinction times (5-ms bin width) assuming two values of the JSR refilling rate parameter D_up, with the RyR gating rate constants held fixed at k_o = 100 mM⁻¹ ms⁻¹ and k_om = 0.1. Increasing D_up to the value predicted from modeling the FSR as a diffuse, random network causes some sparks to “smolder” for a prolonged period rather than extinguishing promptly.