We use the FitzHugh-Nagumo excitable media model in the form, Winfree [25],
where D is a diffusion coefficient, 
is the Laplacian, F(t)
is an external forcing, and the non-linear terms are
In polar coordinates r and 
the Laplacian reads
The first equation in (1) describes the fast process of formation and propagation of the wave front; the second equation describes the local recovery of the properties of the medium. In the context of biological excitable tissues the variables u and v are commonly interpreted as the membrane potential of the cell, and the slow repolarizing currents through the membrane; the F(t) can be interpreted as, for instance, an external electric current.
