Final answer:
Acetylcholine (ACh) causes a reduced frequency of action potentials in cardiac muscle contraction, which decreases heart rate.
Step-by-step explanation:
Acetylcholine (ACh) causes a reduced frequency of action potentials in cardiac muscle contraction, which decreases heart rate. When ACh is released, it binds to receptors on the cardiac muscle cells and inhibits their contraction, resulting in a slower heart rate. This is known as parasympathetic stimulation and is part of the autonomic nervous system control of heart rate and cardiac function.