Primary (SA Node)
One percent of the Cardiomyocytes in the myocardium possess the ability to generate electrical impulses (or action potentials).
A specialized portion of the heart, called the sinoatrial node, is responsible for atrial propagation of this potential.
The sinoatrial node (SA node) is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava. These cells are modified cardiomyocyte. They possess rudimentary contractile filaments, but contract relatively weakly.
Cells in the SA node spontaneously depolarize, resulting in contraction, approximately 100 times per minute. This native rate is constantly modified by the activity of sympathetic and parasympathetic nerve fibers, so that the average resting cardiac rate in adult humans is about 70 beats per minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker.
Read more about this topic: Cardiac Pacemaker
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