Morphological Classification of Atrial Muscle in the Atrioventricular Junctional Area

To trace anatomical structures that might be associated with the dual atrioventricular (AV) nodal pathway and to investigate the morphologic characteristics of the cells that form these pathways, we examined serial sections of the AV junctional area with a light microscope and reconstructed them 3-dimensionally with a computer. Twelve hearts were obtained at autopsy from patients who had not shown AV conduction disturbances or supraventricular tachycardia before death. The method of Lev et al was used to prepare serial sections. Fascicles of atrial muscle contiguous with the AV node were examined with a light microscope and were classified into 3 groups, on the basis of morphologic characteristics and myocyte diameter. A computer was used to reconstruct 3-dimensionally the course of the fascicles and surrounding structures. At the border of the AV node and bundle of His relatively large myocytes extended directly into the AV bundle from the anterosuperior interatrial septum. Morphologically, the course was considered to be consistent with the fast pathway. In contrast, small cells that entered the AV node from the inferoposterior interatrial septum resembled sinus node cells with few myofibrils and a winding shape. These cells extended from the coronary sinus ostium to the tricuspid valve annulus and are thought to make up the slow pathway.