Identification of the SN, AV rings, inferior nodal extensions, AVN, and penetrating bundle. Double labeling of HCN4 (green signal) and Cx43 (red signal) proteins in long‐axis sections at different levels through the heart is shown (distance in mm from the back to the front of the heart shown). A, Section at 3.15 mm showing that the right and left rings are continuous with the right and left nodal extensions. B, Section at 3.25 mm showing the transitional area between the nodal extension and the AVN. C, Section at 3.35 mm showing the SN, right and left rings, and AVN. D, Section at 3.45 mm showing the atrial component of the AVN. The left ring is continuous with the AVN. E, Section at 3.55 mm showing again that the left ring is continuous with the AVN. F, Section at 3.65 mm showing that the left ring is continuous with the penetrating bundle. AVN indicates atrioventricular node; INE, inferior nodal extension (right and left); IVS, interventricular septum; LA, left atrium; LAVR, left atrioventricular ring; LV, left ventricle; PB, penetrating bundle; RA, right atrium; RAVR, right atrioventricular ring; RV, right ventricle; SN, sinus node.