Figure 1. The multiscale nature of Ca²⁺ regulation in heart.

The figure illustrates schematically the different spatial scales at which the effects of cardiac SR Ca²⁺ release must be considered. Considerable research has been performed to investigate electrophysiology and Ca²⁺ release at the level of the individual cardiac myocyte (middle). The physiological effects of changes to cellular Ca²⁺ release, however, occur at the tissue level, when cells are connected to one another through gap junctions (left). At a smaller spatial scale, thousand of individual units of SR Ca²⁺ release, Ca²⁺ sparks, are triggered so-called Ca²⁺ release units (right). These are locations where junctional SR, containing clusters of RyRs, sits in close proximity to T-tubular membranes.