Why is it that an action potential only travels in one direction?

(draw/picture of nerve cell with its ion channels and after learning about the action potential trace and how it's derived) After each action potential, see on the trace there's a refractory period where there's a deep dip - this indicates it can't be excited again immediately. This deep dip essentially makes it extremely hard to bring it back up to the threshold to trigger depolarisation, hence since the threshold can't be reached during that period another action potential can't be triggered in the section of the nerve cell that was just depolarised. It can only go forward.