A simplified version of the Huxley-Hodgkin equations.

V,m evolve on a similar fast timescale.

n,h evolve on a similar slow timescale.

==> Lump together (V,m) into a single activation variable $V(t)$, also (n,h) into $n(t)$.

After non-dimensionalizing:

$\epsilon \frac{dV}{dt} = AV(\delta - V)(V-1)-n+I_a$

$\frac{dn}{dt}=V-\gamma n$

$A, \gamma = O(1)$. $0 <\epsilon\ll \delta \ll 1$

$I_a$ is applied current.

Can analyze in Phase plane