Electron Microscope Observations of the S.A and A.V Nodes and Purkinje Fibers of the Rabbit

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The S-A and A-V nodes, under strict microscopic identification, and the Purkinje fibers from the false bands of rabbits were examined by electron microscope. The entire cardiac conduction system is cellular in nature ; each cell is completely enclosed by plasma membrane and intimately. connected with the adjacent cells by apposition of their membranes. The plasma membranes bordering on the interstitium are commonly covered by a basement membrane which extends from cell to cell. Where the cell junctions interrupt the myofibrillar axis, dense material is constantly seen ; their structures are occasionally free of any relation to the myofibrils at the cell junctions. The S-A and A-V nodes are characterized by a polyhedral connection of small and irregularly shaped muscle cells, and abundant unmyelinated nerve fibers in the interstitium. Although the myofibrils of the nodal cells appear to be identical in fine structure to those of ordinary cardiac muscle cells, they are very sparse and run in various directions. The nucleus occupies a relatively large part of the small nodal cell. The sarcosomes are scattered throughout the sarcoplasm often without intimate topographic relation to the myofibrils. The smooth-and occasional rough-surfaced profiles of the sarcoplasmic reticulum, circular membrane profiles containing small vesicles and dense bodies often lie in intimate topographic relation with each other. The nerve axons in the vicinity of the nodal cells contain small vesicles and small mitochondria as in the presynaptic nerve axons. No specific neuromuscular junction, however, could be observed. Occasionally axons contain additional small dense granules (0.05-0.1μ) enclosed by a single membrane. Although the Purkinje cells appear to be rather similar to ordinary cardiac muscle cells, their sarcolemma indentated more irregularly, the myofibrils are relatively scanty, the mitochondria are considerably smaller and often aggregate in the wide perinuclear or subsarcolemmic sarcoplasm, and small dense granules may occur more frequently in the sarcoplasm following lead hydroxide staining. In the interstitium of the purkinje tissue, nerve fibers are rather sparse.