Proximal atrioventricular bundle, atrioventricular node, and distal atrioventricular bundle are distinct anatomic structures with unique histological characteristics and innervation.

BACKGROUND Direct 3D analysis (ie, stereotaxic analysis of 3 planes) has shown that the atrioventricular (AV) node (AVN) is continuous with only specialized myocardium of the proximal AV bundle (PAVB) and distal AV bundle (DAVB) or His bundle. The purpose of the present study was to determine whether the PAVB, AVN, and DAVB possess histological features distinct from each other and from the ordinary myocardium. METHODS AND RESULTS A protocol that preserves the cytoplasmic and interstitial integrity of the tissue and permits serial sections of the AV junction region to be made in 3 orthogonal planes showed that the PAVB, AVN, and DAVB are characterized by myocardium aggregated into fascicles containing approximately 8 myofibers. Myofibers within the fascicles are coiled or spiraled about each other; and spiraling is most compact in the PAVB. Collagen encases individual fascicles and segregates primary fascicles into secondary fascicles. Fascicles, and not myofibers, are in parallel array in the PAVB, interwoven in the AVN, and parallel in the DAVB. Narrow junctions of parallel fascicles separate the AVN from the PAVB and DAVB. Myocytes, which are largest in DAVB, possess clear perinuclear regions; thin finger-like end processes, which are most numerous in the AVN; uniform, delicate cross-striations; and intercalated disks, which are broader in the PAVB and form short stacks in the AVN. Sheaves of nerve terminals are found, including boutons as in skeletal muscle [corrected]. CONCLUSIONS The PAVB, AVN, and DAVB have distinct histological features. Collagen septation of primary and secondary fascicles presents natural barriers within the tissues and to surrounding myocardium and structures. These findings confirm that the AV junction region contains a specialized conduction system that is anatomically isolated from ordinary myocardium.