<P> In the pacemaking cells of the heart (e.g., the sinoatrial node), the pacemaker potential (also called the pacemaker current) is the slow, positive increase in voltage across the cell's membrane (the membrane potential) that occurs between the end of one action potential and the beginning of the next action potential . This increase in membrane potential is what causes the cell membrane, which typically maintains a resting membrane potential of - 70 mV, to reach the threshold potential and consequently fire the next action potential; thus, the pacemaker potential is what drives the self - generated rhythmic firing (automaticity) of pacemaker cells, and the rate of change (i.e., the slope) of the pacemaker potential is what determines the timing of the next action potential and thus the intrinsic firing rate of the cell . In a healthy sinoatrial node (SAN, a complex tissue within the right atrium containing pacemaker cells that normally determine the intrinsic firing rate for the entire heart), the pacemaker potential is the main determinant of the heart rate . Because the pacemaker potential represents the non-contracting time between heart beats (diastole), it is also called the diastolic depolarization . 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 . </P> <P> In reality, the heart has several pacemakers known as autonomic foci, each which fires at its own intrinsic rate: </P>

Where are the group of cells that control the natural resting heart rate found