Final answer:
The sympathetic and parasympathetic nervous systems have opposing effects on heart rate; sympathetic stimulation increases heart rate by releasing norepinephrine, while parasympathetic stimulation decreases heart rate by releasing acetylcholine.
Step-by-step explanation:
The sympathetic and parasympathetic stimulation affect heart rate through different mechanisms. When sympathetic stimulation occurs, norepinephrine (NE) is released onto cells in the heart which then bind to adrenergic receptors. This action causes the cells to depolarize faster, leading to an increase in heart rate. During exercise, for instance, the heart rate can increase to 120-140 beats per minute (bpm). On the other hand, the parasympathetic stimulation, mainly through the vagus nerve, releases acetylcholine (ACh) onto cells where it binds to muscarinic receptors, resulting in hyperpolarization of the cells. This makes it harder for the cells to reach threshold and thus, the heart rate slows down to a resting rate of approximately 60-80 bpm. Without parasympathetic input, the heart's pacemaker cells would set a rhythm of about 100 bpm. The balance between these two autonomic branches allows precise regulation of heart function.