Fig. 1. Structure of ventricular, atrial and neonatal rat cardiac myocytes. Panels A and B depict portions of a ventricular and an atrial myocyte, respectively. They illustrate the relative positioning of some of the key elements involved in EC coupling. The network SR elements wrap around both the myofibrils and mitochondria. For additional representations of these cells see Franzini-Armstrong et al. and others (Franzini-Armstrong et al., 2005; Sommer and Jennings, 1992; Yamasaki et al., 1997). Panel C shows the topology of a myofibril relative to the ventricular myocyte section above it in A. The junctional couplings involving the peripheral sarcolemma in the middle of the A-band of ventricular myocytes has clearly been visualised in some studies (Chen-Izu et al., 2006) but are not always evident (Fig. 3Aii). In atrial cells, it appears that junctional coupling exists at the Z-lines and also in the middle of the A-band (Fig. 3Bii).