Conduction System of the Heart

Autorhythmicity is the heart's innate ability to initiate electrical signals and trigger muscle contractions without external stimulation. This self-regulating property depends on pacemaker cells in the SA and AV nodes and specialized conducting cells throughout the myocardium. Autorhythmicity ensures the heart maintains a consistent rhythm adapted to the body's changing needs during rest and activity.

The SA node pacemaker cells spontaneously depolarize, triggering an action potential that spreads to the atria. The impulse reaches the AV node via internodal pathways, pauses briefly to allow complete atrial contraction, then progresses through the bundle of His into left and right bundle branches. Finally, it reaches the Purkinje fibers, initiating ventricular contraction.

The SA node contains pacemaker cells that spontaneously depolarize to generate the heart's intrinsic rhythm at approximately 100 contractions per minute. These cells initiate the action potential that triggers atrial contraction and sets the pace for the entire cardiac cycle. The SA node functions as the heart's primary pacemaker.

The parasympathetic division of the ANS slows SA node pacing to around 75 contractions per minute at rest, while the sympathetic division elevates the rate during activity. Hormones also adjust both the timing and strength of each heartbeat. This dual control allows the heart to adapt its output to meet the body's metabolic demands.

The AV node receives the electrical impulse from the SA node via internodal pathways and causes a momentary pause in conduction. This delay allows the atria to complete their contraction before the impulse proceeds to the ventricles through the bundle of His. The AV node ensures coordinated, sequential contraction of the heart chambers.

Purkinje fibers are specialized conducting cells that receive the electrical impulse from the bundle branches and distribute it throughout the ventricular myocardium. They initiate rapid, coordinated ventricular contraction, ensuring efficient pumping of blood into systemic and pulmonary circulation. Purkinje fibers are the final component of the intrinsic conduction pathway.

Pacemaker cells in the SA and AV nodes possess the unique ability to spontaneously depolarize and generate action potentials without external stimulation. Unlike typical cardiac muscle cells that only respond to electrical signals, pacemaker cells are self-excitable and establish the heart's intrinsic rhythm. This autorhythmicity distinguishes them as specialized conducting cells.