24/02/2026
The Sinoatrial (SA) Node, known as the natural pacemaker of the heart, is a specialized cluster of pacemaker cells located in the right atrium at the junction of the superior vena cava and near the crista terminalis. It has the unique property of automaticity, allowing it to generate electrical impulses spontaneously without any external stimulation. These impulses initiate atrial depolarization, represented by the P wave on ECG, and establish the normal rhythm of the heart.
The SA node generates impulses at an intrinsic rate of 60–100 beats per minute, making it the fastest pacemaker and the primary controller of heart rate. Its pacemaker action potential is characterized by spontaneous Phase 4 depolarization due to funny sodium (If) currents and calcium influx, rapid depolarization via L-type calcium channels, and repolarization through potassium efflux. Unlike ventricular muscle cells, depolarization in SA nodal cells depends mainly on calcium ions rather than fast sodium channels.
Once generated, the impulse spreads through the atrial myocardium to the AV node, Bundle of His, bundle branches, and Purkinje fibers, ensuring coordinated atrial and ventricular contraction. The activity of the SA node is precisely regulated by the autonomic nervous system—sympathetic stimulation increases heart rate, while parasympathetic (vagal) stimulation decreases it. The SA node is primarily supplied by the right coronary artery in most individuals.
Any dysfunction of the SA node can lead to rhythm disturbances such as sick sinus syndrome, bradycardia, or SA nodal block, and in such cases, secondary pacemakers like the AV node may take over at a slower rate.
