Question
Why does an action potential travel in only one direction along an axon even though the membrane is symmetric?
Solution — Step by Step
If a patch of membrane depolarises, current spreads in both directions. So in principle the impulse could go backwards. Why doesn’t it?
Just after firing, Na⁺ channels enter an inactivated state. They can’t reopen for about 1–2 ms, no matter how strong the stimulus.
The patch behind the advancing impulse is still refractory, so it cannot fire again. The patch ahead is rested and ready. The wave moves forward by elimination.
Refractory period + forward spread of local currents = unidirectional conduction.
Final answer: Unidirectional conduction is enforced by the refractory period of voltage-gated Na⁺ channels.
Why This Works
Na⁺ channels have three functional states: closed, open, inactivated. The inactivated state is a safety latch that stops the signal from reverberating and garbling the message.
Alternative Method
Think of a burning fuse: the already-burnt section cannot reignite. The flame moves forward because the unburnt side is the only option.
Most neural-system numericals come down to unit hygiene and remembering what each gate does at each phase. Draw the action potential graph before reading the question — it saves time.
Common Mistake
Saying ‘the signal goes forward because the axon hillock is at one end’ — geometry alone doesn’t explain it, you need the refractory period.
Do not confuse passive channels (follow the gradient) with active pumps (fight the gradient). This single distinction clears half of all neural-system doubts.