Which neurotransmitter is the only one used by the parasympathetic nervous system?

The correct answer is acetylcholine, which is the only neurotransmitter used by the parasympathetic nervous system. This neurotransmitter plays a critical role in facilitating communication between nerve cells and target organs, particularly in actions associated with resting and digesting functions. In the parasympathetic nervous system, acetylcholine is released from preganglionic neurons and binds to receptors on postganglionic neurons, leading to effects such as decreased heart rate, increased digestive activity, and relaxation of various muscles.

Understanding the function of acetylcholine in this system is essential, as it is fundamentally linked to bodily maintenance and energy conservation processes. This contrasts with other neurotransmitters mentioned, which may not be exclusive to the parasympathetic nervous system and are involved in different physiological pathways or have various functions within the central and peripheral nervous systems. For instance, norepinephrine is primarily associated with the sympathetic nervous system and is involved in the fight-or-flight response, while serotonin and dopamine are neurotransmitters that play roles in mood regulation and reward pathways, respectively.