Internodal conduction pathways: revisiting a century-long debate on their existence, morphology, and location in the context of 2023 best science.

Internodal conduction pathways are the communication apparatus of the cardiac conduction system conveying sinus node action potentials (APs) to the atrioventricular node and atrial working myocytes. In 1910, at the Deutschen Pathologischen Gesellschaft held in Erlangen, Charles Thörel related his discovery of an internodal bundle structured with Purkinje-like cells, which was rejected by the participants, who endorsed the then-leading doctrine that atrial contractile cardiomyocytes operate as internodal routes. Starting in 1963 and for five subsequent decades, two groups have revisited this issue. The first group, led by Thomas James, defended the histological existence of uninsulated, high-speed cordlike internodal conduction tracts. Although not supported by robust experimental data, this hypothesis achieved the status of a physiological creed in the scientific community. The second group, led by Robert Anderson, systematically refuted this stance and adopted anisotropy as an innovative internodal conduction mechanism operating via spindle-shaped atrial contractile myocytes structured in anisotropic muscular bundles relaying sinoatrial node (SAN) APs to their atrial destinations at physiologically required (fit for purpose) velocities to enable electromechanical synchrony of atrial systoles. Modern imaging and electrophysiological techniques can now clearly visualize muscular internodal and interatrial tracts and SAN depolarization electrical waves, confirming the existence of atrial conduction paths consisting of anisotropically arranged contractile cardiomyocytes. Mastery of the current best-science anatomy and physiology of the human atria is required for electrophysiologists to safely perform atrial radiofrequency ablation interventions to restore sinus rhythm in patients distressed by supraventricular arrhythmias. NEW & NOTEWORTHY This report reexamines disparate historical views on internodal pathway existence and nature. The conflicting research, ongoing since 1963, regarding these issues, is appraised. The discovery of myocytes anisotropically structured in atrial bundles relaying sinoatrial node (SAN) action potentials (APs) to atrioventricular node (AVN) and atrial wall destinations is emphasized, since it is still a relatively unfamiliar subject in physiology teaching. Modern imaging technologies can now visualize internodal pathways as muscular bundles displaying SAN electrical waves traversing the atrial walls.