Final answer:
The neurotransmitter released by the Vagus nerve is acetylcholine (ACh), which decreases heart rate.
Step-by-step explanation:
The neurotransmitter released by the Vagus nerve is acetylcholine (ACh). ACh slows down heart rate by opening chemical- or ligand-gated potassium ion channels to slow the rate of spontaneous depolarization. This extends repolarization and increases the time before the next spontaneous depolarization occurs, resulting in a decrease in heart rate.