How does the sinoatrial node set the rhythm of a mammalian heart?

The sinoatrial (SA) node sets the rhythm of the mammalian heart through its specialized pacemaker cells, which possess the unique ability to generate spontaneous electrical impulses. Unlike typical atrial and ventricular muscle cells, which remain quiescent unless stimulated, the myocytes of the SA node exhibit a rhythmic, spontaneous upward drift in their resting membrane potential toward a critical threshold.1234

Once this threshold is reached, the cells trigger an action potential by opening ion channels that allow sodium and calcium to flow into the cell. Because the SA node has the fastest intrinsic rate of depolarization among all cardiac tissues, it acts as the primary pacemaker, setting the pace for the entire heart. This electrical signal then propagates through the heart’s conduction system—including the atrioventricular node and Purkinje cells—to coordinate the synchronized contraction of the atria and ventricles. This rhythm can be further modulated by the sympathetic nervous system and parasympathetic nervous system, which adjust the rate of diastolic depolarization to increase or decrease the heart rate.3412