Spinal reflexes: reciprocal inhibition: 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 · Spinal reflexes: reciprocal inhibition

Sample item

When the patellar tendon is tapped, the quadriceps contracts and the hamstrings relax at the same time. Which mechanism produces the hamstring relaxation?

ADirect inhibition of hamstring muscle spindles by gamma motor neuronsBIa afferent collaterals activating inhibitory interneurons that inhibit the antagonist motor neuronsCRenshaw cell inhibition of the quadriceps motor neuronsDIb afferent activation of excitatory interneurons to the hamstring motor neurons

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

The answer and the reasoning

B. Ia afferent collaterals activating inhibitory interneurons that inhibit the antagonist motor neurons

Stretch of the quadriceps spindle sends Ia afferents to the quadriceps alpha motor neurons (monosynaptic excitation) and, via collaterals, to Ia inhibitory interneurons that inhibit the antagonist hamstring motor neurons. This reciprocal innervation allows the agonist to contract without opposition. The same principle operates in many voluntary movements.

Option by option

A. Direct inhibition of hamstring muscle spindles by gamma motor neurons
Gamma motor neurons set spindle sensitivity and do not inhibit the antagonist muscle's spindles.
B. Ia afferent collaterals activating inhibitory interneurons that inhibit the antagonist motor neurons (correct)
Ia inhibitory interneurons mediate reciprocal inhibition of antagonists.
C. Renshaw cell inhibition of the quadriceps motor neurons
Renshaw cells give recurrent inhibition to the motor neurons that excited them and would depress quadriceps firing.
D. Ib afferent activation of excitatory interneurons to the hamstring motor neurons
Ib fibres from tendon organs inhibit rather than excite their own muscle, and are not responsible here.

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 B, Ia afferent collaterals activating inhibitory interneurons that inhibit the antagonist motor neurons. Stretch of the quadriceps spindle sends Ia afferents to the quadriceps alpha motor neurons (monosynaptic excitation) and, via collaterals, to Ia inhibitory interneurons that inhibit the antagonist hamstring motor neurons.

Why is "Direct inhibition of hamstring muscle spindles by gamma motor neurons" not the answer?

Gamma motor neurons set spindle sensitivity and do not inhibit the antagonist muscle's spindles.

Why is "Renshaw cell inhibition of the quadriceps motor neurons" not the answer?

Renshaw cells give recurrent inhibition to the motor neurons that excited them and would depress quadriceps firing.

Why is "Ib afferent activation of excitatory interneurons to the hamstring mo…" not the answer?

Ib fibres from tendon organs inhibit rather than excite their own muscle, and are not responsible here.

What concept does this question test?

It tests Spinal reflexes: reciprocal inhibition in Physiology. This reciprocal innervation allows the agonist to contract without opposition. The same principle operates in many voluntary movements.

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 Spinal reflexes: reciprocal inhibition?

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.