Nerve and muscle: safety factor at the end plate: 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: safety factor at the end plate

Sample item

Why does every motor nerve action potential normally produce a muscle action potential at the neuromuscular junction?

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

The answer and the reasoning

D. The end-plate potential far exceeds the threshold, giving a wide safety factor

A single nerve impulse releases much more acetylcholine than needed, so the end-plate potential is several times the threshold for the muscle action potential. Transmission therefore fails only when a large fraction of receptors is blocked, as in severe myasthenia or with neuromuscular blockers.

Option by option

  • A. Each muscle fibre is supplied by several motor neurones that summate

    Each skeletal muscle fibre has a single end plate from one motor neurone.

  • B. Muscle fibres have no threshold for firing

    Muscle fibres have a threshold, which is just exceeded many times over.

  • C. Acetylcholine stays in the cleft for several seconds

    Acetylcholine is broken down within a millisecond or so by acetylcholinesterase.

  • D. The end-plate potential far exceeds the threshold, giving a wide safety factor (correct)

    Correct. The end-plate potential is well above threshold, so transmission is reliable.

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.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is D, The end-plate potential far exceeds the threshold, giving a wide safety factor. A single nerve impulse releases much more acetylcholine than needed, so the end-plate potential is several times the threshold for the muscle action potential.

Why is "Each muscle fibre is supplied by several motor neurones that summate" not the answer?

Each skeletal muscle fibre has a single end plate from one motor neurone.

Why is "Muscle fibres have no threshold for firing" not the answer?

Muscle fibres have a threshold, which is just exceeded many times over.

Why is "Acetylcholine stays in the cleft for several seconds" not the answer?

Acetylcholine is broken down within a millisecond or so by acetylcholinesterase.

What concept does this question test?

It tests Nerve and muscle: safety factor at the end plate in Physiology. Transmission therefore fails only when a large fraction of receptors is blocked, as in severe myasthenia or with neuromuscular blockers.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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: safety factor at the end plate?

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.