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The electrical relay that delays the impulse, then routes it through the bundle of His and Purkinje fibers to the ventricles.
Medically reviewed & updated
After the sinoatrial node fires and the atria contract, the electrical impulse must reach the ventricles in a precisely timed way. The atrioventricular (AV) node and the His-Purkinje system form this critical relay, ensuring the lower chambers contract a fraction of a second after the upper chambers.
The AV node is a small knot of specialized cells located in the lower part of the interatrial septum within an anatomical region called the triangle of Koch, near the opening of the coronary sinus and the tricuspid valve. It is normally the only electrical pathway between the atria and the ventricles. Its defining feature is conduction delay: the impulse slows as it passes through, giving the atria time to finish emptying blood into the ventricles before the ventricles contract. The AV node also acts as a gatekeeper, limiting how many rapid atrial impulses reach the ventricles, and it can serve as a backup pacemaker at an intrinsic rate of about 40 to 60 beats per minute if the SA node fails.
From the AV node, the impulse enters the bundle of His, which travels into the interventricular septum and splits into the right and left bundle branches. These branches carry the signal down toward the apex of the heart and then fan out into a fine network of Purkinje fibers that rapidly distribute the impulse throughout the ventricular muscle. This high-speed wiring lets both ventricles contract almost simultaneously and forcefully, efficiently ejecting blood to the lungs and body.
Disease of this system produces conduction blocks. Slowed or blocked conduction at the AV node causes the various degrees of "heart block," which can lead to dangerously slow rhythms requiring a pacemaker. Abnormal reentry circuits involving the AV node cause a common fast rhythm called AV nodal reentrant tachycardia (AVNRT). Damage to a bundle branch produces a "bundle branch block" seen on an electrocardiogram. This content is educational and not a substitute for medical care.