Arrangement of individual RyRs in a dyad. (A) Arrangement derived from a dyad in a rat cardiomyocyte obtained by electron tomography as shown in Fig. 1 D of Asghari et al. (2014). (B) Derived mask used to numerically calculate diffusion of Ca²⁺ in the purple areas. The rates of diffusion in either the vertical or horizontal directions were both reduced by a factor of 2 compared with diffusion when the yellow areas were absent.