Sinoatrial (SA) node

The sinoatrial node (SA or SAN, also spelled sinuatrial node, also called the sinus node) is a collection of impulse-generating (pacemaker) cells located in the right atrium of the heart. This node is responsible for generating a normal sinus rhythm.

The SA node is positioned on the anterior wall of the right atrium near the entrance of the superior vena cava. The cells of the SA node are specialized cardiomyocytes. Though they possess some contractile filaments, they do not contract robustly.

The SA node is richly innervated by parasympathetic fibers (CN X: Vagus Nerve) and by sympathetic fibers (T1-4, Spinal Nerves). This unique anatomical arrangement makes the SA node susceptible to both types of autonomic influence.

Although all of the heart's cells have the ability to generate the electrical impulses (or action potentials) that trigger cardiac contraction, the SA node normally initiates it, simply because it generates impulses slightly faster than the other cells with pacemaker potential. Cardiomyocytes, like all muscle cells, have refractory periods following contraction during which additional contractions cannot be triggered. Their pacemaker potential is overridden by the SA or atrioventricular nodes.

In the absence of extrinsic neural and hormonal control, cells in the SA node will naturally discharge (create action potentials) upwards of 100 beats/minute. Because the SA node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker.