Schematic illustration of the spatial architecture of Ca signaling in an atrial myocyte. Ca signaling and release occurs within dyadic junctions distributed in the 3D volume of the cell. Junctional CRUs close to the cell membrane (A) possess LCCs and NCX channels, whereas nonjunctional CRUs (B) do not have these membrane channels. Here, the superscript ijk denotes the CRU at position i, j, k in a 3D grid representing the cell. (C) The spatial architecture of the cell interior showing TT invaginations along z-planes is displayed. All compartments in the outer boundary and along transverse TT invaginations are treated as junctional CRUs (red squares). To see this figure in color, go online.