- In an experiment on healthy young men, the muscarinic receptor antagonist, atropine was administered to one group (Group A) while the P-adrenergic receptor antagonist, propranolol was administered to another group (Group B) in four increasing doses of equal concentration for both the drugs. The effects of these two drugs on the heart rate are shown below:
On the basis of these observations, an investigator proposed the following statements:
A. Atropine and propranolol block sympathetic and parasympathetic effects on the heart, respectively
B. As the change of heart rate is more in Group A than in Group B, the sympathetic tone usually
predominates in healthy resting individuals.
C. Atropine and propranolol block parasympathetic and sympathetic effects on the heart, respectively D. As substantial changes occur in the heart rate with atropine, the parasympathetic tone is predominant in healthy resting individuals.
Select the option with INCORRECT statement(s)
(1) Only A (2) A and B
(3) Only C (4) A and D
In pharmacological experiments studying heart rate regulation, atropine (a muscarinic receptor antagonist) and propranolol (a β-adrenergic receptor antagonist) are often used to block parasympathetic and sympathetic influences, respectively.
Summary of Drug Actions on Heart Rate
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Atropine blocks parasympathetic (vagal) effects, leading to heart rate increase by preventing acetylcholine’s slowing influence on the SA node.
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Propranolol blocks sympathetic β-receptors, reducing heart rate by inhibiting norepinephrine effects on the heart.
Evaluating the Investigator’s Statements
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A. Atropine and propranolol block sympathetic and parasympathetic effects on the heart, respectively.
Incorrect. Atropine blocks parasympathetic effects, propranolol blocks sympathetic effects (statements reversed). -
B. The change of heart rate is more in Group A (atropine) than in Group B (propranolol), so sympathetic tone predominates in healthy resting individuals.
Incorrect. Larger heart rate increase with atropine indicates the parasympathetic tone predominates at rest, not sympathetic. -
C. Atropine and propranolol block parasympathetic and sympathetic effects on the heart, respectively.
Correct. This is factually accurate. -
D. Substantial heart rate changes with atropine indicate parasympathetic tone predominates at rest.
Correct. Robust heart rate increase after blocking parasympathetic input confirms vagal dominance in resting heart rate control.
Selecting the Incorrect Statement(s)
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(1) Only A — Incorrect (A is wrong)
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(2) A and B — Both incorrect
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(3) Only C — Correct (C is correct, so not incorrect)
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(4) A and D — D is correct, so only A is incorrect here
Hence, the correct choice for incorrect statements is:
(2) A and B
Explanation Based on Literature
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Studies consistently show parasympathetic (vagal) tone dominates resting heart rate in healthy humans, with atropine causing the most pronounced heart rate increase.pubmed.ncbi.nlm.nih+1
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Propranolol reduces heart rate but typically produces less striking changes at rest due to lower baseline sympathetic tone.pubmed.ncbi.nlm.nih+1
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Confusing the receptor specificity of atropine and propranolol leads to incorrect conclusions as in statement A.
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Misinterpretation of tone dominance leads to the error in statement B.
Summary Table
Statement Correct/Incorrect Explanation A Incorrect Atropine blocks parasympathetic, not sympathetic B Incorrect Parasympathetic tone dominates at rest C Correct Atropine blocks parasympathetic, propranolol blocks sympathetic D Correct Heart rate increase by atropine shows parasympathetic predominance
Conclusion
The option containing the incorrect statements is:
(2) A and B
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2 Comments
Varsha Tatla
September 17, 2025Done sir
Aakansha sharma Sharma
September 26, 2025Atropine blocks parasympathetic (vagal) effects, leading to heart rate increase by preventing acetylcholine’s slowing influence on the SA node.
Propranolol blocks sympathetic β-receptors, reducing heart rate by inhibiting norepinephrine effects on the heart.