Activation of muscle contraction is a rapid event that is initiated by electrical activity in the surface membrane and transverse (T) tubules. This is followed by release of calcium from the inner membrane system, the sarcoplasmic reticulum (SR). Using electron microscopy (EM), K. R. Porter and his laboratory defined the SR, the unique junctions between SR and T tubules, and the continuity between T tubules and surface membrane. Current research in this area centers on the interaction between T tubules and SR. This is mediated by 2 well-identified calcium channels: the dihydropyridine receptors (DHPRs) that act as voltage sensors in the T tubules, and the ryanodine receptors (RyRs) or calcium release channels of the SR. The relative positions of these 2 molecules differ significantly in skeletal and cardiac muscle, and this correlates well with known functional differences in the control of contraction. Molecular biology experiments combined with EM indicate that DHPRs are linked to RyRs in skeletal but probably not in cardiac muscle.