Nerve and muscle: smooth muscle latch state: 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: smooth muscle latch state

Sample item

A blood vessel wall can maintain tone for long periods using very little ATP and low calcium. Which feature of smooth muscle is responsible?

ASlow cycling of dephosphorylated myosin heads that remain attached to actinBRapid cross-bridge cycling at a high rateCConstant release of calcium through ryanodine receptorsDCalcium binding to troponin C

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

The answer and the reasoning

A. Slow cycling of dephosphorylated myosin heads that remain attached to actin

In the latch state, myosin heads that have been dephosphorylated stay attached to actin and detach very slowly, so tension is held with little ATP use. Smooth muscle uses calmodulin and myosin light chain kinase and has no troponin.

Option by option

A. Slow cycling of dephosphorylated myosin heads that remain attached to actin (correct)
Correct. Slowly cycling attached cross-bridges hold tension economically.
B. Rapid cross-bridge cycling at a high rate
Rapid cycling would use more ATP, which is the opposite of the latch state.
C. Constant release of calcium through ryanodine receptors
Smooth muscle relies mostly on calcium entry and IP3-mediated release, not skeletal-type ryanodine coupling.
D. Calcium binding to troponin C
Smooth muscle has no troponin; calmodulin is the calcium sensor.

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 A, Slow cycling of dephosphorylated myosin heads that remain attached to actin. In the latch state, myosin heads that have been dephosphorylated stay attached to actin and detach very slowly, so tension is held with little ATP use.

Why is "Rapid cross-bridge cycling at a high rate" not the answer?

Rapid cycling would use more ATP, which is the opposite of the latch state.

Why is "Constant release of calcium through ryanodine receptors" not the answer?

Smooth muscle relies mostly on calcium entry and IP3-mediated release, not skeletal-type ryanodine coupling.

Why is "Calcium binding to troponin C" not the answer?

Smooth muscle has no troponin; calmodulin is the calcium sensor.

What concept does this question test?

It tests Nerve and muscle: smooth muscle latch state in Physiology. Smooth muscle uses calmodulin and myosin light chain kinase and has no troponin.

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: smooth muscle latch state?

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.