Nerve and muscle: end-plate reversal potential: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Nerve and muscle: end-plate reversal potential

Sample item

The acetylcholine-gated channel at the motor end plate is about equally permeable to sodium and potassium. Towards what membrane potential does the end plate move during a maximal end-plate potential?

AAbout 0 mVBAbout -90 mVCAbout +60 mVDAbout -70 mV

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. About 0 mV

With roughly equal permeability to Na+ and K+, the channel drives the membrane towards a value midway between the equilibrium potentials, about 0 mV, which is the reversal potential of the end-plate potential. This large depolarisation easily exceeds the threshold of the neighbouring muscle membrane.

Option by option

A. About 0 mV (correct)
Correct. Equal Na+ and K+ permeability gives a reversal potential near 0 mV.
B. About -90 mV
About -90 mV is the potassium equilibrium potential, which would apply only if it were selective for potassium.
C. About +60 mV
About +60 mV is the sodium equilibrium potential, which would apply only if the channel were selective for sodium.
D. About -70 mV
About -70 mV is a typical neuronal resting potential, not the end-plate reversal potential.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, About 0 mV. With roughly equal permeability to Na+ and K+, the channel drives the membrane towards a value midway between the equilibrium potentials, about 0 mV, which is the reversal potential of the end-plate potential.

Why is "About -90 mV" not the answer?

About -90 mV is the potassium equilibrium potential, which would apply only if it were selective for potassium.

Why is "About +60 mV" not the answer?

About +60 mV is the sodium equilibrium potential, which would apply only if the channel were selective for sodium.

Why is "About -70 mV" not the answer?

About -70 mV is a typical neuronal resting potential, not the end-plate reversal potential.

What concept does this question test?

It tests Nerve and muscle: end-plate reversal potential in Physiology. This large depolarisation easily exceeds the threshold of the neighbouring muscle membrane.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Nerve and muscle: end-plate reversal potential?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.