SA Node is the natural pacemaker of the heart.

• A. SA Node
• B. AV Node
• C. Bundle of His
• D. Purkinje fibers

Answer: (A)

The heart’s rhythm is set by automaticity in specialized conduction tissue, and the pace is determined by the tissue with the fastest intrinsic firing rate. The sinoatrial node sits in the right atrium and generates spontaneous action potentials at about 60–100 times per minute in a healthy adult. Its pacemaker cells slowly depolarize during diastole due to the funny current (If) and calcium influx, creating a pacemaker potential that reaches threshold quickly. Because this tissue has the highest intrinsic rate, it normally sets the heart’s pace and starts each heartbeat, initiating atrial contraction that then prompts ventricular activation.

The atrioventricular node can also generate impulses if the SA node fail, but its intrinsic rate is slower (roughly 40–60 per minute), so it can take over only at a slower pace. The Bundle of His and Purkinje fibers are fast-conducting pathways that rapidly spread the impulse into the ventricles; they do not typically generate impulses themselves. Thus, the SA node is the natural pacemaker because it provides the fastest, initiating rhythm that coordinates the sequence of atrial and ventral activation. Autonomic nerves can modulate this rate, speeding it up with sympathetic input and slowing it with parasympathetic input, but the SA node remains the primary pacemaker.