time step after which the cell is in the refractory state for the next $\tau$ time steps. In the refractory state, a cell is unexcitable after which the cell re-enters the resting state. This cycle is indicative of the action potential of real cardiomyocytes. The beating of the atria is simulated by exciting all the cells along the left boundary of the tissue. This signal can then propagate across the tissue until it is dissipated at the opposite open boundary. The pacemaker cells are activated every $T$ time steps. The full excitation rules of the CMP model are summarised in the box below.

1.  All cells are connected longitudinally forming a muscle fibre.

2.  A fraction $\nu$ of the transverse connections between cells are filled linking two muscle fibres.

3.  A resting (excitable) cell connected to an excited cell at time step $t$ will become excited at time step $t+1$.

4.  A refractory (unexcited) cell connected to an excited cell at time step $t$ will not become excited at time step $t+1$.

5.  A cell remains refractory for a duration of $\tau$ time steps before returning to the resting state.

6.  Every $T$ time steps, the pacemaker cells along one 