The relationship between the transverse tubular system and other tubules at the Z disc levels of myocardial cells in the ferret

Apart from cytoplasmic microtubules, which exist in cardiac muscle as in most other tissues, there are two tubular systems in mammalian myocardial cells — the transverse (T) tubules which are invaginations of the sarcolemma (i.e., plasma membrane and basement membrane) and the finer tubules which appear to be separate from the transverse system and which have hitherto been known as longitudinal (L) tubules. Because so many of these finer tubules are transversely orientated it seems better to call them simply “sarcotubules.” The sarcotubular system is best envisaged as a three‐dimensional network extending throughout the cell, with specialized areas of flattened sarcotubular sacs at points of contact with the plasma membrane or its extensions in the transverse tubular system. A particularly constant element of the sarcotubular system is found encircling the myofibrils at the Z regions.