Simulated cardiomyocyte spontaneous activity. (A) Near-SR and near-SL Ca²⁺ concentrations (top panel) from the simulated cytosolic calcium distribution (bottom panel). (B) The corresponding calcium fluxes through RyRs and IP₃Rs and the SR Ca²⁺ concentration. (C) The corresponding APs accompanied by central ion currents: I_f with I_NCX and I_CaL with I_Na.