Muscle physiology: smooth muscle: 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: smooth muscle

Sample item

In smooth muscle, how does a rise in intracellular calcium produce contraction?

ACalcium inhibits myosin phosphatase only, with no kinase activationBCalcium binds troponin C and moves tropomyosin off the actin binding sitesCCalcium binds calmodulin, which activates myosin light-chain kinase to phosphorylate myosinDCalcium directly binds actin to expose the myosin head

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

The answer and the reasoning

C. Calcium binds calmodulin, which activates myosin light-chain kinase to phosphorylate myosin

Smooth muscle has no troponin. Calcium binds calmodulin, and the complex activates myosin light-chain kinase, which phosphorylates the regulatory light chain of myosin and permits cross-bridge cycling. Myosin light-chain phosphatase reverses this and relaxes the muscle.

Option by option

A. Calcium inhibits myosin phosphatase only, with no kinase activation
Inhibition of the phosphatase may sustain contraction but the primary step is kinase activation.
B. Calcium binds troponin C and moves tropomyosin off the actin binding sites
This is the mechanism of skeletal and cardiac muscle, which have troponin.
C. Calcium binds calmodulin, which activates myosin light-chain kinase to phosphorylate myosin (correct)
Correct. The calcium-calmodulin complex activates myosin light-chain kinase.
D. Calcium directly binds actin to expose the myosin head
Calcium acts through regulatory proteins, not by binding actin directly.

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 C, Calcium binds calmodulin, which activates myosin light-chain kinase to phosphorylate myosin. Smooth muscle has no troponin.

Why is "Calcium inhibits myosin phosphatase only, with no kinase activation" not the answer?

Inhibition of the phosphatase may sustain contraction but the primary step is kinase activation.

Why is "Calcium binds troponin C and moves tropomyosin off the actin binding…" not the answer?

This is the mechanism of skeletal and cardiac muscle, which have troponin.

Why is "Calcium directly binds actin to expose the myosin head" not the answer?

Calcium acts through regulatory proteins, not by binding actin directly.

What concept does this question test?

It tests Muscle physiology: smooth muscle in Physiology. Calcium binds calmodulin, and the complex activates myosin light-chain kinase, which phosphorylates the regulatory light chain of myosin and permits cross-bridge cycling. Myosin light-chain phosphatase reverses this and relaxes the muscle.

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: smooth muscle?

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.