Final answer:
Acetylcholine released onto heart muscle cells causes them to hyperpolarize and slows down the heart rate.
Step-by-step explanation:
Acetylcholine (ACh) has an effect on parasympathetic regulation, specifically on heart muscle cells. When ACh is released onto heart muscle cells, it binds to muscarinic receptors, causing the cells to hyperpolarize. This makes it more difficult for the cells to reach threshold, slowing down the heart rate.
Without the parasympathetic input, the heart would work at a rate of approximately 100 beats per minute (bpm), but the parasympathetic system slows it down to a resting heart rate of 60-80 bpm.