So we know it moves this way, but why doesn't it move backwards? Well, this is because the region of the membrane right before the depolarized section is in its absolute refractory stage. And that means no matter how large our stimulus actually is, no action potential will take place in that region. And this is because the sodium channels are inactivated. There's nothing we can do that will activate those inactivated channels. So we have to wait until they recover from that inactivation period.