Figure 7.

Schematic diagram of the Ca²⁺ model. Calcium can enter the cytosol through IP₃ receptors (IPR), ryanodine receptors (RyR), or generic voltage-gated channels and leaks. Ca²⁺ is removed from the cytoplasm by ATPase pumps on the membrane of the SR and on the plasma cell membrane. IPR are activated by IP₃, which is produced as an end-product of agonist stimulation of cell-surface receptors. Calcium in the cytosol can diffuse, and is heavily buffered. The buffers are not shown explicitly as they are incorporated via an effective diffusion coefficient and effective reaction rates.