Which structure depolarizes the fastest in the cardiac conduction system?

• A. AV node
• B. Purkinje fibers
• C. Sinoatrial Node
• D. Atrioventricular bundle

Answer: (C)

The key idea is automaticity—the ability of pacemaker cells to depolarize spontaneously. The structure that depolarizes fastest has the steepest phase 4 depolarization, which lets it reach threshold most quickly and fire action potentials most often. The sinoatrial node has the most rapid spontaneous depolarization among the conduction system components. Its pacemaker cells express a strong funny current (If) that brings the membrane toward threshold quickly during diastole, so they fire about 60–100 times per minute. That makes the SA node the natural pace-maker of the heart.

The other sites have slower automaticity. The atrioventricular node can pace the heart if needed but at a slower rate (roughly 40–60/min). Purkinje fibers can conduct impulses rapidly for ventricular activation, but their rate of spontaneous depolarization is slower and they are not the primary pacemaker. The bundle of His (AV bundle) mainly serves as a conduction pathway and does not set the pace.

So, the fastest depolarizing structure is the Sinoatrial node.