Figure 3.

Schematic illustration of the spatial architecture of Ca signaling in an atrial myocyte. The basic unit of Ca signaling is the CRU, which is composed of six compartments distributed in the cell. Concentrations at each CRU are designated with a superscript ijk at position i, j, k in a three-dimensional grid representation of the cell. The compartments are the dyadic junction with concentration $c_p^{ijk}$, the submembrane space with concentration $c_s^{ijk}$, the local cytosol concentration $c_i^{ijk}$, the junctional SR (JSR) concentration $c_{jsr}^{ijk}$, and finally the local concentration in the network SR (NSR) $c_{nsr}^{ijk}$. To model an atrial myocyte, we distinguish junctional CRUs close to the cell membrane that (A) possess LCC and NCX channels, whereas nonjunctional CRUs (B) do not have these membrane channels. (C) Spatial architecture of the cell interior showing TT invaginations along z planes is given. 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.