Which structure in the heart contains additional myocardial conductive fibers that spread the impulse to ventricular contractile cells?

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

Answer: (D)

Specialized conduction fibers in the ventricles spread the electrical impulse quickly to all ventricular muscle cells so they contract together. Purkinje fibers form a subendocardial network that carries the impulse from the bundle branches into the ventricular myocardium, distributing it rapidly and almost simultaneously to contractile cells.

The impulse travels from the AV node to the bundle of His, then down the left and right bundle branches, and finally into Purkinje fibers. This final network is designed for speed and distribution, with large-diameter fibers and extensive gap junctions, ensuring synchronized ventricular contraction. In contrast, the sinoatrial node starts the heartbeat, and the AV node provides a delay to allow atrial contraction before the ventricles respond. The bundle of His acts as a conduit to the ventricles, but the actual widespread spread to ventricular cells is accomplished by the Purkinje system.