Sinoatrial (SA) Node

The sinoatrial (SA) node is the heart's natural pacemaker, the first component of the cardiac conduction system. It is a small, crescent-shaped cluster of specialized cardiac muscle cells (pacemaker cells) that spontaneously generate the electrical impulses that trigger each heartbeat.

Location and Structure

The SA node sits in the wall of the right atrium, near the junction where the superior vena cava meets the atrium, at the upper end of a muscular ridge called the crista terminalis. It measures only a few millimeters and is supplied in most people by a branch of the right coronary artery. Its cells are richly connected to the surrounding atrial muscle so that impulses can spread quickly.

Function

Unlike ordinary heart muscle, SA node cells do not hold a stable resting voltage. Instead they slowly and automatically depolarize until they reach threshold and fire, a property called automaticity. At rest this produces an intrinsic rate of roughly 60 to 100 beats per minute, which is why the SA node normally controls the heart rhythm and sets the pace for all other tissues. Each impulse spreads across both atria, causing them to contract, and then travels onward to the atrioventricular node. The autonomic nervous system fine-tunes this rate moment to moment: sympathetic ("fight-or-flight") signals speed the SA node up, while parasympathetic (vagal) signals slow it down, allowing the heart to match demands such as exercise or rest.

Clinical Relevance

Dysfunction of the SA node is called sick sinus syndrome and can cause an abnormally slow heart rate (sinus bradycardia), pauses, or alternating slow and fast rhythms. Symptoms may include fatigue, dizziness, or fainting, and some patients require an artificial pacemaker. Conversely, the SA node can fire too quickly (sinus tachycardia) in response to fever, stress, or other triggers. This is educational information and not medical advice.