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The Purkinje fibers are the component of the heart's conduction system that directly lead to the contraction of the ventricular muscle, thereby playing a crucial role in the heartbeat during depolarization. They are specialized fibers that rapidly conduct electrical impulses throughout the ventricles, allowing for a coordinated and efficient contraction. When the action potential reaches the Purkinje fibers, it triggers the contraction of the myocardial cells, resulting in the pumping action of the heart.

The SA node, while pivotal in initiating the electrical impulse and setting the pace for the heartbeat, primarily facilitates atrial contraction. The AV node serves as a relay point, delaying the impulse to ensure that the atria have enough time to empty completely before the ventricles contract. The internodal pathways assist in the transmission of impulses from the SA node to the AV node but do not directly cause ventricular contraction. Thus, while all components of the conduction system are essential for the overall cardiac cycle, the Purkinje fibers are directly responsible for the mechanical aspect of the heartbeat during the contraction phase.