/*FitzHugh-Nagumo equations at sluggish parameters familiar in Tucson lab simulations*/

      for(i=1;i<=NDIM;i++) {
      for(j=1;j<=NDIM;j++) {
        u_thresh =  v[i][j];
        v[i][j] += (dt*0.3)*(u[i][j]+0.7-v[i][j]/2);  
        u[i][j] +=  dt/0.3 *(u[i][j] -u_thresh-u[i][j]*u[i][j]*u[i][j]/3)    
                   +U_DIFFUSION;
}}
u = scaled membrane potential;  uo = -1.2 (the rest potential)
v = scaled refractoriness; vo = -0.6 (fully recovered)
NDIM=60 gridpoints about 1 mm apart along the square box edge representing around 6-10 cm