Sinoatrial Node

About the Sinoatrial Node

The sinoatrial node is a small cluster of specialized cardiac muscle cells in the upper wall of the right atrium that acts as the natural pacemaker of the heart. Its cells spontaneously depolarize, generating electrical impulses that spread across both atria, causing them to contract, and then reach the atrioventricular node, which relays them to the ventricles through the bundle of His and Purkinje fibers. The rate at which the node fires sets the normal heart rate, called sinus rhythm, and it is continually adjusted by the autonomic nervous system: sympathetic stimulation speeds it up and vagal stimulation slows it down. The node was described in 1907 by Arthur Keith and Martin Flack. Dysfunction of the node, called sick sinus syndrome, can cause abnormally slow heart rates, pauses, or alternating slow and fast rhythms, and it is a common reason for the implantation of an artificial pacemaker.