The cardiac conduction system: A narrative review

The cardiac conduction system (CCS) is a hierarchically organized yet structurally heterogeneous network of specialized tissues that initiates and coordinates cardiac electrical activation. The CCS arises from region-specific cellular phenotypes, graded structural transitions, and spatially heterogeneous intercellular coupling that together enable robust cardiac excitation. This review synthesizes anatomical, histological, electrophysiological, and imaging evidence to provide a multiscale description of the human CCS. We describe the sinoatrial node as a protected pacemaker complex stabilized by fibrous insulation, cellular heterogeneity, and discrete exit pathways that regulate atrial activation. Interatrial conduction occurs through aligned atrial myocardium, with the interatrial bundle and posterior connections supporting rapid activation without strict insulation. The atrioventricular node enables physiological delay through slow, yet safe conduction shaped by specialized cellular architecture, connexin gradients, and dual-pathway organization. Distal to the node, the atrioventricular bundle and its branches - collectively known as the His-Purkinje system - deliver rapid, synchronous ventricular activation through fibrous-insulated pathways, with the Purkinje-ventricular junction acting as an inherent source-sink discontinuity that supports reliable excitation but also predisposes to conduction delay and ectopic activity. Such an integrated understanding of the CCS is essential for applications including optimization of cardiac pacing therapies, as well as for the development of physiologically accurate cardiac digital twins for patient-specific modeling, thereby supporting both clinical decision-making and mechanistic representation of cardiac activation. By consolidating dispersed experimental and conceptual insights into a single framework, this review is intended as a reference for clinicians, researchers, and engineers working at the interface of cardiac electrophysiology, clinical application, and computational modeling.