the amount of net inward current required to move the cell membrane potential during the pacemaker phase is extremely small, in the order of few pas, but this net flux arises from time to time changing contribution of several currents that flow with different voltage and time dependence. evidence in support of the active presence of k, ca, na channels and na/k exchanger during the pacemaker phase have been variously reported in the literature, but several indications point to the “funny”(i) current as one of the most important.(see funny current). there is now substantial evidence that also sarcoplasmic reticulum (sr) ca-transients participate to the generation of the diastolic depolarization via a process involving the na–ca exchanger.