Figure 1. Typical geometrical framework in three dimensions (left) with a 2D section in the x–y plane (right).

The model consists of two intracellular domains labelled Ω_{i, k} (k ∈ {1,2}), corresponding to two cells. In the default model, two cylindrical cells of equal size are juxtaposed along the x‐axis and the cleft between them is centred at x = 0. The intracellular space is surrounded by a rectangular cuboid, representing the extracellular space Ω_e. Intra‐ and extracellular spaces are separated by the membrane Г _k . A, explicit cleft model (note that the cleft width is greatly exaggerated). The cleft width is varied by changing the relative position of both cylinders to each other. B, Dirichlet (red) and/or Neumann (blue) boundary conditions applied as explained in the text. The boundaries of the extracellular domain perpendicular to the x‐axis are grounded (Dirichlet condition with V _e = 0). On the circular/spherical patches inside the intracellular space a specific voltage or current clamp protocol is applied (corresponding to a Dirichlet or a Neumann condition, respectively). C, collapsed cleft model, in which the pre‐ and post‐junctional membranes as well as the cleft are collapsed onto a 2D surface Г_ID. D, top: mesh of the entire collapsed cleft framework. Bottom: meshes of the two cylindrical cells (cylinder length L, radius R _ID) interconnected by the 2D mesh on the surface Г_ID (with a uniform finite element density in this example).