Fig. 1.

The minimal whole cell model represents two bulk compartments: the sarcoplasmic reticulum (SR; c_sr) and the myoplasm (c_myo). The membrane of the SR includes multiple release sites, each of which includes a local Ca²⁺ domain on both sides of the membrane. The domain [Ca²⁺] (c_myo^d and c_sr^d) are influenced by fluxes that involve bulk [Ca²⁺] (myoplasmic and SR) and the state of the release site. Fluxes include diffusion from myoplasmic domains to the bulk myoplasm (J_myo), diffusion from the bulk SR to the luminal domains (J_sr), sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) pump flux that resequesters Ca²⁺ into the SR (J_pump), a passive leak from the SR to the myoplasm (J_leak), and fluxes across the plasma membrane (J_pm).