Figure 1.

Schematic of experimental fiber model. The model includes four physical spaces, the myoplasm, SR, JS, and t-tubule. In the SR, Ca²⁺ is in equilibrium with buffers (B), which includes Csq. In the myoplasm and JS, Ca²⁺ is in equilibrium with EGTA. Ca²⁺ movements were constrained as follows: (1) Ca²⁺ enters the SR only from the myoplasm via SERCA (labeled S); (2) Ca²⁺ enters the t-tubule only from the JS and via PMCA (P) and NCX (N); (3) Ca²⁺ enters the JS by leak from the SR (through the RYR [R]), potentially from NCX working in reverse and from dissociation of Ca²⁺ bound to EGTA; (4) Ca²⁺ can enter the myoplasm by leaking from the SR or the t-system and by diffusive exchange with the JS. The relative sizes of the spaces in the diagram do not reflect the actual relative volumes in a fiber.