When parasympathetic nerve fibers are stimulated, what neurotransmitter is released?

When parasympathetic nerve fibers are stimulated, acetylcholine is the neurotransmitter that is released. This is a hallmark of the parasympathetic nervous system, which is part of the autonomic nervous system that manages the body's rest-and-digest functions. Acetylcholine acts on muscarinic receptors at target organs, leading to effects such as decreased heart rate, increased gastrointestinal motility, and enhanced secretory activity.

In contrast, serotonin, norepinephrine, and dopamine have roles in other pathways and systems within the body. Serotonin primarily functions as a neurotransmitter involved in mood regulation and is not associated with the parasympathetic system. Norepinephrine is the primary neurotransmitter of the sympathetic nervous system, which prepares the body for 'fight or flight' responses. Dopamine is involved in reward, movement, and various cognitive functions but is not the neurotransmitter associated with parasympathetic stimulation. Thus, recognizing the specific role of acetylcholine is critical in understanding the functions of the parasympathetic nervous system.