Muscle physiology: length-tension relation: 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: length-tension relation

Sample item

In an isolated skeletal muscle experiment, the muscle is progressively stretched before stimulation. Active force is greatest at a sarcomere length of about 2.2 micrometres and falls to almost zero beyond about 3.6 micrometres. What explains the near-zero force at 3.6 micrometres?

AThe thin filaments collide in the centre of the sarcomereBAll myosin heads are detached because ATP is exhaustedCCalcium release from the sarcoplasmic reticulum is abolished by stretchDThick and thin filaments no longer overlap, so no cross bridges can form

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

The answer and the reasoning

D. Thick and thin filaments no longer overlap, so no cross bridges can form

Active tension depends on the number of cross bridges between overlapping thick and thin filaments. At optimal length, overlap is maximal without interference. At very long lengths the filaments are pulled apart, so cross bridge formation is minimal, whereas at short lengths thin filaments overlap one another and force falls too.

Option by option

A. The thin filaments collide in the centre of the sarcomere
Collision of thin filaments reduces force at short lengths, not at extreme stretch.
B. All myosin heads are detached because ATP is exhausted
ATP is not exhausted in the experiment; the force loss is geometric.
C. Calcium release from the sarcoplasmic reticulum is abolished by stretch
Calcium release is unaffected by moderate muscle stretch, which acts mechanically on filament overlap.
D. Thick and thin filaments no longer overlap, so no cross bridges can form (correct)
At 3.6 micrometres there is virtually no overlap, so force is almost zero.

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, Thick and thin filaments no longer overlap, so no cross bridges can form. Active tension depends on the number of cross bridges between overlapping thick and thin filaments.

Why is "The thin filaments collide in the centre of the sarcomere" not the answer?

Collision of thin filaments reduces force at short lengths, not at extreme stretch.

Why is "All myosin heads are detached because ATP is exhausted" not the answer?

ATP is not exhausted in the experiment; the force loss is geometric.

Why is "Calcium release from the sarcoplasmic reticulum is abolished by stret…" not the answer?

Calcium release is unaffected by moderate muscle stretch, which acts mechanically on filament overlap.

What concept does this question test?

It tests Muscle physiology: length-tension relation in Physiology. At optimal length, overlap is maximal without interference. At very long lengths the filaments are pulled apart, so cross bridge formation is minimal, whereas at short lengths thin filaments overlap one another and force falls too.

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: length-tension relation?

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.