The heart is a muscular organ responsible for continuously pumping blood throughout the body. It is composed of four chambers connected to a passageway of arteries and veins. The two upper chambers, the atria, and the two lower chambers, the ventricles receive and deliver blood to the circulatory system via the impulses generated by the cardiac contraction cycle.
SA Node Impulse Generation
The cardiac contraction cycle, a single heartbeat, is controlled and started by the electrical impulse generated from the sinoatrial node, or SA, located near the top of the right atria of the heart. This node is called the natural pacemaker of the heart as it determines heart rate by the number of impulses sent per minute. When the sinoatrial node releases an impulse, the electrical signal spreads throughout the atria and then down to the atrioventricular, or AV, bundles. The spreading out of the electrical impulse causes the blood-filled atria to contract and release blood through valves on down to the ventricles.
AV Node Impulse Conduction
Also referred to as the "gatekeeper" node, the atrioventricular, or AV, node is located between the atria and ventricles and receives electrical impulses from the SA node. Electrical impulse release from the AV node is slightly delayed to allow the atria to fully contract, thus allowing the atria too completely empty and the ventricles to fill. Once the ventricles have filled, the AV node releases an impulse that is spread throughout the bundle of His, a group of heart muscle fibers that conduct impulses to keep the heartbeat regular.
Bundle of His Impulse Conduction
The bundle of His receives electrical impulses from the AV node, the third step in the cardiac conduction cycle. The bundle of His branches into three bundle branches that run along the wall in the heart that separates the two ventricles. These bundle branches transmit electrical impulses to the final step in the contraction cycle, the Purkinje fibers.
Purkinje Fibers Impulse Conduction
The Purkinje fibers are spread out across the ventricles and connect with the contracting cells of the ventricles. These cells are responsible for conducting the impulse that generates ventricular contraction. Ventricular contraction pushes blood out of the ventricles and into the circulatory system. Blood returning from the body enters the two atrial chambers and the sinoatrial node generates another impulse to start the cardiac contraction cycle all over again.
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