The heart conduction system of the dog was studied electron microscopically.
1. The nodal cells were classified into two types; typical nodal cell and transitional cell. Besides these two types of cells, the dark cell was observed as the third cell type in the atrio-ventricular node. The pseudotendon cells were also divided into two types: the first type was a clear Purkinje cell, and the second type cell was rich in myofibrils and did not have a T-system. The nodal cells were small and somewhat rounded, and were connected with each other in all directions forming a cellular network, while the A-V bundle and pseudotendon cells were long, wide, and arranged parallel to each other connecting mainly at the end.
2. Peripheral cisterns connected with the sarcoplasmic reticulum were observed in the specialized myocardial cells. However, these cisterns had morphological differences between the nodal cells and the A-V bundle and pseudotendon cells. In contrast with the nodal cistern with amorphous dense material, that of the A-V bundle and the pseudotendon was less dense and had a central membrane.
3. Tight junctions were numerous in the A-V bundle and the pseudotendon. This junction also occurred in the S-A node, more frequently, in the A-V node.
4. The nerve fibers were most frequently found in the S-A node, being followed in order by the A-V node, the pseudotendon, and the A-V bundle. Although in most cases a considerably wide space, (1, 000-3, 000Å or more) existed between the nerve fibers and the specialized cells, the neuro-muscular contacts were clearly observed in both the S-A and A-V nodes. In such cases, the neuro-muscular interspace was about 200Å and lacking the basement membrane. Moreover, the thicking of the plasma membrane which is usually seen at the typical synapse was not observed.