Muscle physiology: denervation: 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 · Muscle physiology: denervation

Sample item

A 45-year-old man with a lumbar radiculopathy has had weakness of ankle dorsiflexion for six weeks. Needle electromyography at rest shows spontaneous fibrillation potentials in the tibialis anterior. What is the mechanism of these potentials?

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

The answer and the reasoning

C. Spread of acetylcholine receptors over the whole muscle fibre membrane, with spontaneous depolarisation of single denervated fibres

After denervation, the muscle fibre expresses acetylcholine receptors beyond the end plate and develops spontaneous membrane oscillations, causing random contraction of single fibres. These fibrillations are invisible through the skin but are seen on needle EMG from about two to three weeks after nerve injury. Fasciculations, in contrast, are visible twitches of whole motor units.

Option by option

  • A. Discharge of muscle spindles from stretch of the denervated muscle

    Spindle discharge does not generate muscle fibre potentials at rest.

  • B. Spontaneous firing of surviving motor units that have increased their territory

    Firing of enlarged surviving motor units gives fasciculations and large polyphasic potentials on voluntary activation.

  • C. Spread of acetylcholine receptors over the whole muscle fibre membrane, with spontaneous depolarisation of single denervated fibres (correct)

    Extrajunctional receptor expression and membrane instability produce single-fibre fibrillations.

  • D. Continuous acetylcholine release from regenerating axon terminals

    Regenerating terminals produce a different pattern, and the fibrillations appear before reinnervation.

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 C, Spread of acetylcholine receptors over the whole muscle fibre membrane, with spontaneous depolarisation of single denervated fibres. After denervation, the muscle fibre expresses acetylcholine receptors beyond the end plate and develops spontaneous membrane oscillations, causing random contraction of single fibres.

Why is "Discharge of muscle spindles from stretch of the denervated muscle" not the answer?

Spindle discharge does not generate muscle fibre potentials at rest.

Why is "Spontaneous firing of surviving motor units that have increased their…" not the answer?

Firing of enlarged surviving motor units gives fasciculations and large polyphasic potentials on voluntary activation.

Why is "Continuous acetylcholine release from regenerating axon terminals" not the answer?

Regenerating terminals produce a different pattern, and the fibrillations appear before reinnervation.

What concept does this question test?

It tests Muscle physiology: denervation in Physiology. These fibrillations are invisible through the skin but are seen on needle EMG from about two to three weeks after nerve injury. Fasciculations, in contrast, are visible twitches of whole motor units.

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 Muscle physiology: denervation?

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.