Final answer:
The increased heart rate during inspiration is due to decreased vagal nerve activity to the SA node, which allows the heart rate to temporarily increase as part of the normal respiratory sinus arrhythmia.
Step-by-step explanation:
The underlying cause of the increased heart rate during inspiration is decreased vagal nerve activity to the SA node. During inspiration, there is a natural decrease in parasympathetic stimulation due to a reduction of signals sent through the vagus nerve. This reduces the release of the neurotransmitter acetylcholine, which is responsible for slowing the heart rate by extending repolarization. As parasympathetic activity decreases, the heart rate is allowed to increase slightly, which is a part of the normal respiratory sinus arrhythmia.
On the other side, sympathetic nerve activity to the SA node, mediated by neurotransmitters like norepinephrine, can also result in an increase in heart rate by shortening the repolarization period, leading to a faster depolarization and contraction. However, this is mostly associated with stress or physical activity, rather than the respiratory cycle.
The correct answer to the question of what is the underlying cause of the increased heart rate during inspiration is: Option 2 - Decreased vagal nerve activity to the SA node.