Cardiac excitation-contraction coupling (EC-coupling) is initiated by depolarisation of the sarcolemma as an action potential sweeps over a cell. The change in membrane potential activates voltage-operated Ca2+ channels (VOCs). This leads to a trigger Ca2+ signal that evokes a more substantial Ca2+ release from closely apposed RyR clusters on the SR. For ventricular myocytes, both channels are expressed with regular spacing throughout the cells. Consequently, action potential evoked Ca2+ signals in ventricular myocytes physiologically take the form of homogenenous global increases.The situation in atrial myocytes is different. Atrial cells lack the well-developed T-tubule invaginations of the sarcolemma found in ventricular myocytes, and therefore express VOCs only on the plasma membrane surrounding the cells. The distribution of RyRs in atrial cells is similar to that in ventricular myocytes, but with the important exception that only a small fraction of the RyRs (the ‘junctional RyRs’) are positioned to respond to the opening of the VOCs. Ca2+ signals in atrial myocytes therefore originate around the periphery of the cells and are locally amplified by the junctional RyRs. In many species, the subsarcolemmal Ca2+ signal does not propagate fully, or at all, into the centre of an atrial cell. This means that at the peak of the response, substantial Ca2+ gradients can be observed. This is surprising given that clusters of RyRs are expressed as a regular 3-dimensional lattice with a spacing of ~2 micrometres. It would be expected that RyRs would convey the subsarcolemmal Ca2+ signal deep into the cell via Ca2+-induced Ca2+ release (CICR). However, the non-junctional RyRs in the centre of atrial myocytes can be largely non-responsive. The mechanisms that inhibit inward propagation of the Ca2+ signal are unknown. Furthermore, the function of the non-junctional RyRs, which form the majority of intracellular Ca2+ release channels, is unclear.We examined how cellular organelles and Ca2+ transport mechanisms, such as SERCA pumps, ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (InsP3Rs), impact on the properties of physiological Ca2+ signals in adult rat atrial myocytes. The cellular ultrastructure of atrial myocytes is highly organised, providing an ideal system to examine the contribution of individual components of the Ca2+ signalling ‘toolkit’ in shaping the signals that occur during EC-coupling. We demonstrate that to stimulate significant contraction of atrial myocytes, Ca2+ signals need to reach the contractile machinery in the cell centre. Under control conditions, Ca2+ signals are limited to the periphery atrial cells by a functional ‘firewall’ formed by mitochondria and SERCA pumps. Consequently, the twitch of cells under control conditions is modest. An increase in cellular contraction is achieved by either reducing the potency of the firewall or boosting the triggers for CICR. The latter effect is utilised by physiological inotropic agents that modulate cardiac contractility. The non-junctional RyRs therefore constitute an inotropic reserve that is recruited under conditions where greater contractility is required.