Which structure causes the contraction of the ventricles in the heart?

The Purkinje fibers are specialized conducting fibers that play a critical role in the heart's electrical conduction system, particularly in the contraction of the ventricles. They are located in the inner ventricular walls and branch from the bundle of His, allowing for rapid conduction of electrical impulses throughout the ventricles. When the electrical signal reaches the Purkinje fibers, it induces a strong and coordinated contraction of the ventricular myocardium, which is essential for efficiently pumping blood out of the heart to the lungs and the rest of the body.

In contrast, the sinoatrial node, often referred to as the heart's natural pacemaker, is responsible for initiating the electrical impulse that triggers the heartbeat but is primarily related to atrial contraction rather than ventricular contraction. The atrioventricular node serves as a relay point for the electrical impulse from the atria to the ventricles but does not directly cause ventricular contraction. The coronary arteries, on the other hand, are responsible for supplying blood to the heart muscle itself and do not have a role in the conduction or contraction mechanism.

Overall, the Purkinje fibers are vital for ensuring that the ventricles contract timely and forcefully, providing effective blood circulation.