Final answer:
Postganglionic parasympathetic fibers release acetylcholine (ACh), which binds to muscarinic receptors on target cells to facilitate rest-and-digest functions, in contrast to postganglionic sympathetic fibers that usually release norepinephrine.
Step-by-step explanation:
The postganglionic parasympathetic fibers release a neurotransmitter known as acetylcholine (ACh). This ACh binds to muscarinic receptors on the target cells within organs to promote rest-and-digest activities. While all preganglionic neurons in both the sympathetic and parasympathetic divisions release ACh onto nicotinic receptors to excite the ganglionic neurons, it is important to distinguish that postganglionic sympathetic fibers typically release norepinephrine, with the exception of fibers projecting to sweat glands and blood vessels associated with skeletal muscles, which release ACh as well.