function xprime = FHN(t,x)
global R L C E A I_0 
xprime=zeros(2,1);
V_C=x(1);I_L=x(2);
%
% choose one of the following to simulate
%
xprime(1)=-(F(E+V_C)+I_L+(-A+I_0))/C; % constant input current A+I_0 or -A+I_0
%xprime(1)=-(F(E+V_C)+I_L+Iin(t))/C; % sinusoid input 
%xprime(1)=-(F(E+V_C)+I_L+sqw(t))/C; % square wave input
xprime(2)=(V_C-R*I_L)/L;