All of the heart's myocytes can generate the electrical impulses that trigger cardiac contraction, but it is the sinoatrial node that initiates it, as it generates impulses slightly faster than the other areas with pacemaker potential. Because cardiac myocytes, like all muscle cells, have refractory periods following contraction during which additional contractions cannot be triggered, their pacemaker potential is overridden by the sinoatrial node.
In the absence of extrinsic neural and hormonal control, cells in the sinoatrial node will naturally discharge at about 60-100 beats/minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker. The electrical impulses from the sinoatrial node triggers a sequence of electrical events in the heart to control the orderly sequence of muscle contractions that pump the blood out of the heart.
If the sinoatrial node stopped functioning, a group of cells further down the heart will become the heart's pacemaker. These cells form the atrioventricular node, which is an area between the atria and ventricles, within the atrial septum. The autonomic nervous system controls the firing of the sinoatrial node to trigger the initiation of the cardiac cycle. The autonomic nervous system can transmit a message quickly to the sinoatrial node so it can increase the heart rate to twice normal within only 3 to 5 seconds.