Parasympathetic nervous system a healthy heart, the main pacemaker is a collection of cells on the border of the atria and vena cava called the sinoatrial node. Heart cells exhibit automaticity which is the ability to generate electrical activity independent of external stimulation. As a result, the cells of the node spontaneously generate electrical activity that is subsequently conducted throughout the heart, resulting in a regular heart rate. In absence of any external stimuli, sinoatrial pacing contributes to maintain the heart rate in the range of 60-100 beats per minute (bpm). At the same time, the two branches of the autonomic nervous system
Sinoatrial node hyperpolarisation-activated cyclic nucleotide–gated (HCN) channels, are activated. Activation of these channels at very negative membrane potential is an unusual property for ion channels, therefore the flow of sodium (Na) and some potassium (K) through the activated HCN channel is referred to as a "funny current" (I). This funny current causes the membrane potential of the cell to gradually increase, as the positive charge (Na and K) is flowing into the cell. Another mechanism involved in pacemaker potential is known as the calcium clock. Here, calcium is released spontaneously from the sarcoplasmic reticulum (a calcium store) into the cell, this is