with probability $\nu$ (stochastic connections, the transverse direction). This construction is a simplified computational implementation of the real myocardium, but it preserves the essential myocardial architecture which ensure that electrical impulses travel preferentially along muscle fibres rather than laterally between fibres. The strong coupling in the longitudinal direction represents individual muscle cells forming a single, long muscle fibre. The reduced coupling in the transverse direction represents the reduced electrical conductivity between neighbouring muscle fibres. The parameter $\nu$ controls the strength of the transverse coupling. This is manifested in the anisotropy of electrical conduction velocities. There are periodic boundary conditions in the transverse direction (across different muscle fibres) and open boundaries in the longitudinal direction (along a single muscle fibre) representing a simplified cylindrical geometry of the atria [@christensen2015]. The cells along the left boundary are the pacemaker cells which are excited every $T$ time steps. In the CMP model, the electrical cycle of a given cell is as follows: Cells are updated in discrete time steps. Initially a cell is at rest. In this state, the resting cell can become excited at the next time step by a neighbouring excited cell. The cell is in the excited state for one 