Final answer:
Acetylcholine (ACh) affects the parasympathetic system by binding to muscarinic receptors in target cells such as heart muscle cells, where it leads to hyperpolarization and a reduced heart rate, as part of the body's rest-and-digest response.
Step-by-step explanation:
When acetylcholine (ACh) affects parasympathetic regulation in the heart muscle cell, it binds to muscarinic receptors, which leads to a slowed heartbeat. However, ACh can affect both heart muscle cells and salivary gland cells through muscarinic receptors, eliciting parasympathetic responses such as slowing of the heart rate and stimulation of saliva production respectively. The receptors on the target cells determine the effect of the neurotransmitter.
In the heart, ACh binding to muscarinic receptors causes the cells to hyperpolarize, making it less likely to reach threshold and thereby slowing the heart rate. This is part of the rest-and-digest response facilitated by the parasympathetic nervous system. Conversely, norepinephrine causes heart cells to depolarize faster, increasing heart rate via adrenergic receptors during sympathetic activation. The parasympathetic system regulates the heart to a resting rate of 60-80 beats per minute, as opposed to the 100 beats per minute it would operate at in the absence of parasympathetic input.