Figure 3: Diagram of the components and fluxes in a subspace model that includes calcium-regulated calcium influx. A single calcium channel (with two calcium binding sites) is associated with a subspace of volume . The calcium influx rate is and when calcium is unbound and bound, respectively, and the transition rates between these states are and , where is the subspace calcium concentration. Subspace calcium is passively coupled at rate to the bulk cytosol with constant concentration .