Schematic diagram of the geometry of the model. (A) A schematic diagram of a section of the cylindrical cell, showing subsurface JSR and couplons, with the interior consisting of cytosolic and FSR spaces. (B) The nonuniform coordinate grid defining the voxels used in the simulations. (C) Relationship of surface couplon and voxels. Each couplon consists of a square array of RyRs and a single JSR terminal compartment. Each voxel has cytosolic and FSR components. Calcium released to the cleft diffuses from the edges of the cleft to the cytosolic component of the neighboring voxels. The JSR refills from the FSR component of containing voxels.