The sinoatrial (SA) node is commonly known as what?

• A. Pacemaker
• B. Atrioventricular node
• C. Purkinje fibers
• D. Bundle of His

Answer: (A)

The sinoatrial node is the heart's natural pacemaker because its cells generate electrical impulses automatically at the highest intrinsic rate, setting the baseline rhythm for the heart. These nodal cells spontaneously depolarize (pacemaker potential) and reach threshold to fire, producing regular impulses without external input. The impulse then travels through the conduction system to coordinate atrial and ventricular contractions, with the SA node dictating pace under normal conditions. If the SA node fails, other parts like the atrioventricular node can take over as a slower backup pacemaker, but they do not normally set the pace. The other structures—atrio­ventricular node, Purkinje fibers, and Bundle of His—primarily conduct and distribute the impulse rather than set the rate.