The cardiac conduction system is a specialized network of cells that generates and transmits electrical impulses throughout the heart. This electrical activity coordinates the contraction of the heart chambers, ensuring efficient pumping of blood. The system consists of several key components: the sinoatrial node, atrioventricular node, bundle of His, bundle branches, and Purkinje fibers. Each component plays a crucial role in the proper functioning of the heart.
The cardiac conduction system begins with the sinoatrial node, or SA node, located in the upper wall of the right atrium. The SA node is known as the heart's natural pacemaker because it generates electrical impulses at a regular rate, typically between 60 and 100 beats per minute. These impulses initiate each heartbeat. The SA node's activity is regulated by the autonomic nervous system, with sympathetic stimulation increasing the heart rate and parasympathetic stimulation decreasing it. From the SA node, electrical impulses spread across both atria, causing them to contract simultaneously.
After spreading through the atria, the electrical impulse reaches the atrioventricular node, or AV node, located at the junction between the atria and ventricles. The AV node serves a crucial function by delaying the electrical impulse by approximately 0.1 seconds. This delay allows the atria to complete their contraction and fully empty their blood into the ventricles before ventricular contraction begins. From the AV node, the impulse travels to the bundle of His, which is the only electrical connection between the atria and ventricles. The bundle of His then divides into left and right bundle branches that extend toward the respective ventricles.
The bundle branches further divide into a network of specialized fibers called Purkinje fibers. These fibers rapidly conduct the electrical impulse throughout the ventricular myocardium, ensuring synchronized contraction of the ventricles. The Purkinje fibers have a very high conduction velocity, allowing the electrical signal to reach all parts of the ventricles almost simultaneously. This coordinated electrical activation causes the ventricles to contract from the apex, or bottom of the heart, upward toward the base. This efficient contraction pattern forces blood out of the ventricles into the pulmonary artery and aorta. After contraction, the cardiac muscle enters a refractory period during which it cannot be stimulated again, allowing the heart to relax and refill with blood before the next cycle begins.
Let's review the complete cardiac conduction cycle. The sinoatrial node, or SA node, serves as the heart's natural pacemaker, initiating electrical impulses that spread across the atria, causing them to contract. This atrial depolarization appears as the P wave on an electrocardiogram. The impulse then reaches the atrioventricular node, or AV node, which delays the signal briefly to allow the atria to complete their contraction. From the AV node, the impulse travels through the bundle of His and its branches to reach the Purkinje fibers. These specialized fibers rapidly distribute the impulse throughout the ventricular muscle, causing ventricular depolarization, which appears as the QRS complex on the ECG. Finally, ventricular repolarization occurs, represented by the T wave, as the heart muscle relaxes and prepares for the next cycle. This sophisticated conduction system ensures the coordinated contraction of the heart chambers, allowing for efficient blood pumping throughout the body.