Cardiovascular system: overdrive suppression of the sinus 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 · Cardiovascular system: overdrive suppression of the sinus node

Sample item

An atrial tachycardia at 180 per minute stops abruptly in a patient with sinus node disease, and there is a pause of about three seconds before the first sinus beat. Which mechanism contributes most to the delay?

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

The answer and the reasoning

D. Enhanced Na+/K+ pump activity during fast firing hyperpolarises the pacemaker cells

A pacemaker cell that has been driven at a fast rate loads with sodium, and the extra Na+/K+ ATPase activity hyperpolarises it. Spontaneous phase 4 depolarisation then needs longer to reach threshold, which gives a pause after the drive stops. This overdrive suppression is exaggerated in sick sinus syndrome.

Option by option

  • A. Persistent depolarisation of nodal cells by extracellular potassium

    Depolarisation would speed up firing rather than delay it.

  • B. Complete depletion of intracellular calcium in nodal cells

    Calcium is not depleted by fast firing, and its entry still drives the nodal upstroke.

  • C. Sustained vagal stimulation by the sudden fall in heart rate

    The vagus is not the explanation; the pause occurs in the isolated nodal tissue itself.

  • D. Enhanced Na+/K+ pump activity during fast firing hyperpolarises the pacemaker cells (correct)

    Correct. Sodium loading increases the electrogenic pump current and holds the cell hyperpolarised for a time.

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.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is D, Enhanced Na+/K+ pump activity during fast firing hyperpolarises the pacemaker cells. A pacemaker cell that has been driven at a fast rate loads with sodium, and the extra Na+/K+ ATPase activity hyperpolarises it.

Why is "Persistent depolarisation of nodal cells by extracellular potassium" not the answer?

Depolarisation would speed up firing rather than delay it.

Why is "Complete depletion of intracellular calcium in nodal cells" not the answer?

Calcium is not depleted by fast firing, and its entry still drives the nodal upstroke.

Why is "Sustained vagal stimulation by the sudden fall in heart rate" not the answer?

The vagus is not the explanation; the pause occurs in the isolated nodal tissue itself.

What concept does this question test?

It tests Cardiovascular system: overdrive suppression of the sinus node in Physiology. Spontaneous phase 4 depolarisation then needs longer to reach threshold, which gives a pause after the drive stops. This overdrive suppression is exaggerated in sick sinus syndrome.

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 Cardiovascular system: overdrive suppression of the sinus 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.