ABSTRACT
The surface membrane and the T-tubules establish specialized contacts with the sarcoplasmic reticulum (Fig. 7.2), that contain the two key components of the excitationcontraction (E-C) coupling apparatus (Franzini-Armstrong, 1986, 1994;
Fig. 7.2. Schematic representation of the junctions between the sarcoplasmic reticulum and either the plasma membrane or the T tubules. The long axis of the fiber is vertical. In adult skeletal muscle, transversely oriented T tubule networks and triads composed of a T tubule flanked by two sarcoplasmic reticulum cisternae are most common (three arrows), The center left of the image shows a longitudinal T tubule, a dyad composed by a T tubule and one sarcoplasmic reticulum cisterna (two arrows) and a peripheral coupling between the plasma membrane and the sarcoplasmic reticulum (single arrow). These components are most often found in developing skeletal muscle but also in some adult fibers. The lower right portion of the image shows peripheral couplings (single arrowhead), and dyads (double arrowheads), in cardiac muscle. In
addition, there is corbular or extended junctional SR (asterisk), which has feet but does not form a junction. Corbular SR may function as a calciumrelease unit, triggered by calcium. In some arthropods (lower left), feet extend beyond the area of junction with T tubules. Junctional SR contains calsequestrin and other calcium binding proteins. The specialized junctional domains of plasma membrane and T tubules contain voltage-sensitive Ca2+ channels that bind dihydropyride Ca2+ channel blockers (DHPRs) and face the feet. From C.FranziniArmstrong (1996). Soc. Gen. Physiol. Series 51:3-18.
