Summary
The cells of the sinuatrial ring bundle are smaller than the ordinary myocardial cells; they have a regular outline and a large content of myofibrils exhibiting distinct H-bands and M-lines. Rudimentary T-tubules are found. The cells are connected by frequent nexus junctions, desmosomes and regions of interfibrillar contact, both on the well developed intercalated discs and at the periphery of the cells.
The cells in the cauda of the sinus node, which extends alongside the crista terminalis together with the right branch of the sinuatrial ring bundle, are irregularly outlined and have a highly variable diameter (1–10 μm). They occur in clusters of closely packed cells. The content of myofibrils is sparse and the fibrils exhibit no M-lines and only weak H-bands. No T-tubules are found. The cells are not connected by intercalated discs and no nexus junctions are found.
Both tissues contain unmyelinated nerves and nerve fibres. Varicosities with mitochondria and vesicles are found in close apposition to the muscle cells.
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Tranum-Jensen, J., Bojsen-Møller, F. The ultrastructure of the sinuatrial ring bundle and of the caudal extension of the sinus node in the right atrium of the rabbit heart. Z.Zellforsch 138, 97–112 (1973). https://doi.org/10.1007/BF00307081
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DOI: https://doi.org/10.1007/BF00307081