Excitation-contraction (ec) coupling depends on appropriate communication between the specialized, junctional domains of the sarcolemma/transverse tubules (jSUjT) and those of the sarcoplasmic reticulum (jSR). Essential events at the SR-T/SL junctions are depolarization of the surface membrane, its detection by voltage sensors, and the resultant release of calcium from jSR. In skeletal muscle, ec coupling is independent of extracellular calcium (7, 16, 18, 27, 42, 70, 71, 85, 92 for reviews). The nonjunctional components of the two membrane systems, free SUfree T, and free or longitudinal SR, are continuous with the junctional domains, but functionally, and structurally distinct from them.

In this chapter we review the structure and protein composition of the membranes involved in ec coupling and the structure and development of junctions formed by the interaction of jSR with either jT or jSL (ec units). Structural information provides the framework into which the function of and the interaction between the protein components of these structures must fit. The article is written from the perspective of immunolabeling and electron microscopy (EM). The two morphological approaches complement each other. Immunolabeling approaches assign specific locations within the cell