Question:medium
A travelling nerve impulse does not depolarize the area behind it, because
A travelling nerve impulse does not depolarize the area behind it, because
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The membrane behind an advancing impulse is refractory, its sodium channels cannot reopen right away.
Updated On: Jul 8, 2026
- It is hyperpolarized
- It is refractory
- It is not self propagating
- The condition is always orthodromic
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The Correct Option is B
Solution and Explanation
Step 1: Picture the moving impulse.
An action potential travels along the axon like a wave, and behind that wave the membrane has just finished firing.
Step 2: Look at the channels behind the wave.
Sodium channels that just opened and closed enter an inactivated state. In this state they will not open again, even with a strong stimulus, until the membrane potential returns close to resting level.
Step 3: Name this state.
This period where the channels refuse to reopen is called the refractory period, split into an absolute part where no stimulus works and a relative part where only an unusually strong stimulus works.
Step 4: Rule out the distractors.
The membrane is briefly hyperpolarized after firing, but that only makes it harder to reach threshold, it is not the direct reason the sodium channels refuse to reopen. Saying the impulse is not self propagating is simply false, since propagation is the defining feature of a nerve impulse. Calling the direction orthodromic just names the forward direction, it does not explain the blockade.
Step 5: Conclude.
The impulse cannot go backward because the membrane just behind it is refractory.
\[ \boxed{\text{Refractory state}} \]
An action potential travels along the axon like a wave, and behind that wave the membrane has just finished firing.
Step 2: Look at the channels behind the wave.
Sodium channels that just opened and closed enter an inactivated state. In this state they will not open again, even with a strong stimulus, until the membrane potential returns close to resting level.
Step 3: Name this state.
This period where the channels refuse to reopen is called the refractory period, split into an absolute part where no stimulus works and a relative part where only an unusually strong stimulus works.
Step 4: Rule out the distractors.
The membrane is briefly hyperpolarized after firing, but that only makes it harder to reach threshold, it is not the direct reason the sodium channels refuse to reopen. Saying the impulse is not self propagating is simply false, since propagation is the defining feature of a nerve impulse. Calling the direction orthodromic just names the forward direction, it does not explain the blockade.
Step 5: Conclude.
The impulse cannot go backward because the membrane just behind it is refractory.
\[ \boxed{\text{Refractory state}} \]
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