function dy = func_HHapprox(t,y) 

global vna vk vl gna gk gl c I 

v = y(1); 
n = y(2); 

dv = (I - gna*(m_inf(v))^3*(0.8-n)*(v-vna) - gk*n^4*(v-vk)-gl*(v-vl))/c; 
dn = an(v)*(1-n)-bn(v)*n; 

dy = [dv;dn]; 
