Calcium fluxes during spontaneous and stimulated activity. (A) Simulated spontaneous APs followed by simulated electrically evoked APs (bottom) with corresponding near-SR and near-SL calcium signals (top). (B) The difference in the I_NCX during spontaneous and electrically evoked activity. Arrows show the prominent inward current during spontaneous activity, which is absent during electrically evoked APs. (C) The net Ca²⁺ fluxes from the SR to cytosol (top) and from the extracellular space to cytosol (bottom). The left column shows the fluxes during spontaneous activity, and the right column shows them during the electrically evoked activity. The integrals of the positive and negative phases are indicated with arrows. The concentrations are in the volume of the cytosol.