Cardiac electrophysiology: vagal effect on SA node: 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 · Cardiac electrophysiology: vagal effect on SA node

Sample item

A trained 24-year-old athlete has a resting heart rate of 46 per minute from high vagal tone. How does acetylcholine slow the sinoatrial node?

AIt increases calcium influx and shortens the plateauBIt increases potassium conductance and reduces the slope of phase 4 depolarisationCIt blocks fast sodium channels in the nodal cellsDIt increases the funny sodium current and steepens phase 4

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

The answer and the reasoning

B. It increases potassium conductance and reduces the slope of phase 4 depolarisation

Acetylcholine acts on M2 receptors, opening potassium channels to hyperpolarise the nodal cell and lowering cAMP, which reduces the funny current. Phase 4 then rises more slowly from a more negative level, so threshold is reached later and the rate falls. Sympathetic stimulation does the reverse.

Option by option

A. It increases calcium influx and shortens the plateau
Increased calcium influx is a sympathetic effect and would quicken the rate.
B. It increases potassium conductance and reduces the slope of phase 4 depolarisation (correct)
Correct. Hyperpolarisation and a flatter phase 4 slow the pacemaker.
C. It blocks fast sodium channels in the nodal cells
Nodal depolarisation relies on calcium channels, not fast sodium channels.
D. It increases the funny sodium current and steepens phase 4
A greater funny current steepens phase 4 and is the effect of sympathetic stimulation.

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, It increases potassium conductance and reduces the slope of phase 4 depolarisation. Acetylcholine acts on M2 receptors, opening potassium channels to hyperpolarise the nodal cell and lowering cAMP, which reduces the funny current.

Why is "It increases calcium influx and shortens the plateau" not the answer?

Increased calcium influx is a sympathetic effect and would quicken the rate.

Why is "It blocks fast sodium channels in the nodal cells" not the answer?

Nodal depolarisation relies on calcium channels, not fast sodium channels.

Why is "It increases the funny sodium current and steepens phase 4" not the answer?

A greater funny current steepens phase 4 and is the effect of sympathetic stimulation.

What concept does this question test?

It tests Cardiac electrophysiology: vagal effect on SA node in Physiology. Phase 4 then rises more slowly from a more negative level, so threshold is reached later and the rate falls. Sympathetic stimulation does the reverse.

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 Cardiac electrophysiology: vagal effect on SA node?

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.