The for loop of my Matlab code only iterates once and I don't know how to fix it. Thank you.

clc; clear all; close all;

t = 0:0.01:15;

cm = 1*10^-6;

gna = 120;

Ena = 115;

gk = 36;

Ek = -12;

gl = 0.3;

El = 10.6;

v = 0;

am = ((25-v)/10)/(-1+exp((25-v)/10));

bm = 4*exp(-v/18);

m(1) = am/(am+bm);

ah = 0.07*exp(-v/20);

bh = 1/(1+exp((30-v)/10));

h(1) = ah/(ah+bh);

an = ((9.5-v)/100)/(-1+exp((9.5-v)/10));

bn = 0.125*exp(-v/80);

n(1) = an/(an+bn);

for i=1:length(t)

Ina(i) = gna.*m(i)^3.*h(i).*(v(i)-Ena);

Ik(i) = gk.*n(i)^4.*(v(i)-Ek);

Il(i) = gl.*(v(i)-El);

if t(i) >= 0.5 && t(i) <= 0.55

Istim = 200;

v(i) = -(Ina(i)+Ik(i)+Il(i)-Istim)/cm;

else

Istim = 0;

v(i) = -(Ina(i)+Ik(i)+Il(i)-Istim)/cm;

end

end

figure(1)

plot(t,v);