Question

A travelling nerve impulse does not depolarize the area behind it, because

• A. It is hyperpolarized
• B. It is refractory
• C. It is not self propagating
• D. The condition is always orthodromic

Hint:

The membrane behind an advancing impulse is refractory, its sodium channels cannot reopen right away.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks why a nerve impulse, once it has passed through a point on the axon, does not turn around and depolarize that same point again.

Step 2: Key Formula or Approach:
Right after an action potential fires at a point on the membrane, that point enters a refractory period, made of an absolute refractory period, when the sodium channels are inactivated and cannot open no matter how strong the stimulus, followed by a relative refractory period when a much stronger than normal stimulus is needed.

Step 3: Detailed Explanation:
As the action potential moves forward, the area just behind it has sodium channels sitting in the inactivated state, they need the membrane to repolarize back to resting potential before they can reset and be ready to open again. Because these channels cannot reopen so soon, that area cannot be depolarized a second time immediately, which forces the impulse to move only forward, this is what keeps conduction one directional.
Hyperpolarization does occur briefly after an action potential, but it is not the main reason backward depolarization is blocked, hyperpolarization actually makes the membrane further from threshold, while the true block comes from sodium channels being unable to open, the refractory state.
It is not self propagating is factually wrong, a nerve impulse is by definition self propagating, that is exactly how it travels along the whole length of the axon.
The condition is always orthodromic describes the normal forward direction of conduction, it is a description of the result, not the reason, so it does not answer why the area behind stays undepolarized.

Step 4: Final Answer:
The area just behind the moving impulse is in its refractory period, so the correct option is (B).