Conduction system - AV Node

D. The atrioventricular node (AV Node) is located within the triangle of Koch. The anatomic landmarks or this triangle are: 1) the coronary sinus (CS) posteriorly; 2) the line of insertion of the septal leaflet of the tricuspid valve and 3) an oblique line joining these two landmarks. In this specimen parallel serial sections have been made which are perpendicular to the line of insertion of the septal leaflet of the tricuspid valve. The incisions extend from the atrial septum well into the interventricular septum, to sample the AV node. (SVC = superior vena cava, FO = foramen ovale, PV =pulmonic valve )
E. The morphology of the slices from the atrioventricular tissue block is fairly typical. The atrial septum, the tricuspid valve and the mitral valves can be easily recognized for orientation. The insertion of the mitral valve into the fibrous annulus is always more cephalad than that of the tricuspid valve. The most anterior slice should always include the fibrous portion of the interventricular septum close to the posterior cusp of the aortic valve.

F. Histologic section of slice no. 2 (illustrated in E) shows the proximal-central junction of the AV node as it penetrates the central fibrous body and becomes the His bundle. It courses along the base of the membranous interventricular septum. The site of attachment of the mitral valve to the central fibrous body is labeled MV (H&E 4X).
G. The trichrome stain demonstrates the AV node-His bundle tissue as it approaches the fibrous trigone (Masson trichrome 4X).
H. Histologic section of the AV node of an adult individual shows that the myocytes are thinner and paler than the working myocytes. Their organization resembles that of the SA node myocytes.

Now that you have learned about the basic cellular components of the cardiac muscle, is time to learn about other very important structures which are part of the heart such as the valves and other cellular components.