Cardiac Conduction System
Explore each structure of the cardiac conduction system — anatomy, function, conduction velocity, and clinical relevance.
Sinoatrial Node
The sinoatrial node is the primary pacemaker of the heart, a small cluster of specialized autorhythmic cells located in the posterior wall of the right atrium near the superior vena cava. It spontaneously depolarizes at the highest intrinsic rate of any cardiac tissue, setting the heart rate under normal conditions.
Atrioventricular Node
The atrioventricular node is the sole normal electrical gateway between the atria and the ventricles. It introduces a critical physiological delay in conduction — producing the PR interval on ECG — to allow atrial contraction to fill the ventricles before they contract. The AV node is also capable of independent pacing at 40–60 bpm when the SA node fails.
Bundle of His
The Bundle of His is the only normal electrical connection between the AV node and the ventricular conduction system. It is a specialized strand of conduction tissue that pierces the fibrous skeleton of the heart and then divides into the right and left bundle branches. Conduction velocity through the Bundle of His is significantly faster than through the AV node.
Right Bundle Branch
The right bundle branch is a slender, cord-like structure that carries the electrical impulse from the Bundle of His down the right side of the interventricular septum to the right ventricle. It remains undivided for most of its course before fanning out into the right Purkinje fiber network. Its anatomical narrowness makes it vulnerable to block, producing right bundle branch block (RBBB).
Left Bundle Branch
The left bundle branch is a broad, fan-like structure that carries the electrical impulse from the Bundle of His down the left side of the interventricular septum. It rapidly divides into the left anterior fascicle and left posterior fascicle, ensuring synchronous activation of the larger, higher-pressure left ventricle. Left bundle branch block (LBBB) is always pathological and signals underlying cardiac disease.
Left Anterior Fascicle
The left anterior fascicle is the thin, superior division of the left bundle branch. It activates the anterior and superior walls of the left ventricle and runs to the anterolateral papillary muscle. Because it is long, thin, and has a single blood supply, it is the most frequently blocked of the three left-sided fascicles, producing left anterior fascicular block (LAFB), also called left anterior hemiblock.
Left Posterior Fascicle
The left posterior fascicle is the short, broad, inferior division of the left bundle branch. It activates the posterior and inferior walls of the left ventricle and runs toward the posterolateral papillary muscle. Because it has a dual blood supply and a broader structure, isolated block of the left posterior fascicle (LPFB) is uncommon and, when present, suggests extensive cardiac disease.
Purkinje Fibers
The Purkinje fibers are the terminal, highly specialized conduction fibers of the ventricular conduction system. Forming a dense subendocardial network in both ventricles, they conduct impulses at the fastest velocity in the heart — up to 4 m/s — enabling nearly simultaneous activation of the entire ventricular endocardium. Purkinje fibers also possess slow automaticity and can serve as a ventricular escape pacemaker at 20–40 bpm.