“Coupling(s)” (Fig. 1) are structure complexes in cardiac muscle consisting of junctional SR (JSR) tightly apposed to the cytoplasmic side of the plasmalemma. They are either peripheral (PC; when on the cell surface) or interior (IC; when on transverse tubule(s), TT). The JSR is a store for calcium (Ca)-release through Ca channels/ryanodine receptors in junctional processe(s) (JP) on the JSR surfaces. Action potential(s) (AP) for excitation-contraction coupling translate, as mediated by L-type channels, into Ca-entry into a ˜ 20 nm junctional gap (JG) containing JP between plasmalemma and the JSR's junctional membrane proper. In mammalian cardiac myocyte(s) (CM), where IC are ubiquitously distributed along TT at Z-bands, an AP quickly effects almost simultaneous global contraction, even in the center of cells of large diameter, whence CICR can be viewed as a one-step event between Ca-entry and JSR Ca-release from couplings. Avian CM, however, have no TT and, thus, only PC.