Final answer:
The sympathetic nervous system increases heart rate for fight or flight responses, while the parasympathetic nervous system slows it down during rest and digest periods. They both work together to maintain the body's homeostasis.
Step-by-step explanation:
The neurons of the sympathetic nervous system increase heart rate, and neurons of the parasympathetic nervous system slow it down. In general, the parasympathetic nervous system takes care of "rest-and-digest" activities, and the sympathetic nervous system helps support exercise or emergency actions-the so-called "fight or flight responses".
The parasympathetic nervous system predominantly controls the resting heart rate through acetylcholine (ACh) release, which binds to muscarinic receptors causing the heart rate to slow. Conversely, the sympathetic nervous system, through the release of norepinephrine onto adrenergic receptors, causes the cells to depolarize faster, resulting in an increased heart rate. These systems work together to maintain homeostasis within the body, adjusting the heart rate based on physical demands and stress levels.