Muscle physiology: motor unit recruitment: 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: motor unit recruitment

Sample item

A person gradually increases the force of voluntary contraction of the biceps from a very light grip to a maximal effort. Which sequence of motor unit recruitment occurs?

ALarge motor neurons with fast fatigable fibres are recruited first, then smaller unitsBAll motor units are recruited together at the start of contractionCUnits are recruited in a random order without regard to sizeDSmall motor neurons with slow fatigue-resistant fibres are recruited first, then larger units

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

The answer and the reasoning

D. Small motor neurons with slow fatigue-resistant fibres are recruited first, then larger units

According to the Henneman size principle, small motor neurons have lower thresholds because of their higher input resistance, so they fire first. These supply small slow-twitch, fatigue-resistant units that handle posture and light tasks. Larger neurons supplying fast fatigable units join as more force is needed, and the process is orderly rather than random.

Option by option

A. Large motor neurons with fast fatigable fibres are recruited first, then smaller units
The reverse order would waste the large fatigable units on light tasks.
B. All motor units are recruited together at the start of contraction
Recruitment is graded, so units are added progressively as force rises.
C. Units are recruited in a random order without regard to size
Recruitment is orderly and reproducible, not random.
D. Small motor neurons with slow fatigue-resistant fibres are recruited first, then larger units (correct)
Recruitment follows motor neuron size, starting with small units.

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 D, Small motor neurons with slow fatigue-resistant fibres are recruited first, then larger units. According to the Henneman size principle, small motor neurons have lower thresholds because of their higher input resistance, so they fire first.

Why is "Large motor neurons with fast fatigable fibres are recruited first, t…" not the answer?

The reverse order would waste the large fatigable units on light tasks.

Why is "All motor units are recruited together at the start of contraction" not the answer?

Recruitment is graded, so units are added progressively as force rises.

Why is "Units are recruited in a random order without regard to size" not the answer?

Recruitment is orderly and reproducible, not random.

What concept does this question test?

It tests Muscle physiology: motor unit recruitment in Physiology. These supply small slow-twitch, fatigue-resistant units that handle posture and light tasks. Larger neurons supplying fast fatigable units join as more force is needed, and the process is orderly rather than random.

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 Muscle physiology: motor unit recruitment?

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.