The sinoatrial node or sinus node (SA node) is the small mass of cells located in the upper portion of the upper right chamber of the heart (right atrium). The main purpose of the SA node is to generate the fastest intrinsic beat in the form of electrical impulse. This electrical impulse causes the wall of the atrium to contract and push the blood into the lower chambers of the heart (ventricles). As the ventricles become fully filled with the blood, the secondary pacemaker called atrioventricular node (AV node) transmits the electrical impulse to contract the walls of the ventricles. This pushes the blood out of the heart into the arteries.
The cells of the SA node and AV nodes are difficult to identify from other heart muscle cells based on physical appearance. The pacemaker cells are different from the cardiomyocytes (heart muscle cells) anatomically and functionally. The pacemaker cells have no organized sarcomeres which release calcium ions for the cellular contraction. This is because the SA and AV node cells do not contract and only transmit the electrical impulse for the contraction of the heart muscles. This from the base of the heart beat.
The cells of the pacemakers (sinus node and atrioventricular node) are phenotypically similar to the other cardiomyocytes. Anatomically they differ from the other heart muscle cells as they possess no organized sarcomeres and thus do not contract like cardiomyocytes.