What neurotransmitter is released by postganglionic nerves in the parasympathetic nervous system?

• A. Norepinephrine
• B. Glutamate
• C. GABA
• D. Acetylcholine

Answer: (D)

In the parasympathetic nervous system, postganglionic nerves primarily release acetylcholine as their neurotransmitter. This release occurs at target organs, where acetylcholine binds to muscarinic receptors on the effector cells, leading to a variety of physiological responses consistent with the parasympathetic system's role in promoting "rest and digest" functions.

Acetylcholine's action in this system is essential, as it slows down heart rate, increases gastrointestinal activity, and facilitates other bodily functions that conserve energy and promote maintenance. This is in stark contrast to other neurotransmitters like norepinephrine, which is predominantly associated with the sympathetic nervous system, or glutamate and GABA, which function primarily as neurotransmitters in the central nervous system impacting various neuronal signaling pathways. Thus, the unique role of acetylcholine in the parasympathetic nervous system highlights its importance in mediating autonomic responses.