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Question #46364
The Question
A researcher experimentally severs the parasympathetic neural inputs (vagus nerve) to the sinoatrial (SA) node of a mammalian heart while leaving the sympathetic inputs intact. Which of the following best describes the immediate physiological consequence on cardiac function?
Options
- A.
The heart rate decreases because the SA node requires continuous neural stimulation to maintain its auto-excitability.
- B.
The resting heart rate increases because the tonic inhibitory effect of acetylcholine, which normally hyperpolarizes the SA node, is removed.
- C.
The heart rate remains unchanged at 70-75 beats per minute because the cardiac conducting system is inherently myogenic and independent of neural control.
- D.
The ventricles undergo fibrillation because the AV node immediately takes over as the primary pacemaker due to the loss of vagal control.
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Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Regulatory_Failure
Understand how NOT, EXCEPT, and logical constraints are used to manipulate exam results.
Concepts Behind This Question
Conducting System Sequence|Ipsilateral vs Contralateral Signaling|Vagus Nerve
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often confuse 'myogenic' (the ability to generate an impulse without neural input) with being entirely independent of neural regulation.
A common misconception is that the resting heart rate of 72 bpm is the SA node's unmodulated intrinsic rate.
In reality, the intrinsic rate is higher (~100 bpm), and the resting rate is a result of continuous parasympathetic 'braking' (vagal tone).
Option C acts as a 'Common Half-Truth' trap by stating a true fact (the heart is myogenic) to support a false conclusion.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Body Fluids and Circulation - Class 11 |
| Concept | Conducting System Sequence |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |