If the vagal nerves to the heart were cut, the result would be that
parasympathetic stimulation would increase, causing a decrease in heart rate
the atrioventricular (AV) ?node would become the pacemaker of the heart
the heart would stop, since-the vagal nerves trigger the heart to contract
the heart rate would increase by about 25 ?beats per minute
The correct answer and explanation is :
The correct answer is:
“The heart rate would increase by about 25 beats per minute.”
Explanation:
The vagus nerve (cranial nerve X) is a major component of the parasympathetic nervous system and plays a crucial role in regulating heart rate. It primarily innervates the sinoatrial (SA) node, which is the natural pacemaker of the heart. The vagus nerve releases acetylcholine (ACh), which binds to muscarinic receptors in the heart, leading to a decrease in heart rate (a process known as vagal tone).
Effects of Cutting the Vagal Nerve
If the vagal nerves to the heart were severed, parasympathetic input to the SA node would be lost, and the heart would be left under the influence of the sympathetic nervous system. Since the vagus nerve normally slows down the heart, its removal would increase the resting heart rate.
In a healthy adult, the intrinsic firing rate of the SA node (without parasympathetic control) is about 100 beats per minute (bpm). However, due to normal vagal tone, the resting heart rate is usually around 75 bpm. When the vagal nerve is cut, the heart rate increases by approximately 25 bpm, bringing it closer to the intrinsic rate of the SA node.
Why the Other Options Are Incorrect:
- “Parasympathetic stimulation would increase” – This is false because cutting the vagus nerve eliminates parasympathetic stimulation, not increasing it.
- “The AV node would become the pacemaker” – The SA node remains the pacemaker unless it is damaged. The vagus nerve does not change the hierarchy of pacemakers.
- “The heart would stop” – The heart has intrinsic pacemaker activity, so it does not depend on the vagus nerve for contraction.
Thus, cutting the vagus nerve results in an increase in heart rate by about 25 bpm.