Final answer:
Acetylcholine is released by all preganglionic fibers and postganglionic parasympathetic fibers in the autonomic nervous system. It activates ganglionic neurons through nicotinic receptors and affects target cells through muscarinic receptors. Postganglionic sympathetic fibers mostly release norepinephrine, with some exceptions.
Step-by-step explanation:
The neurotransmitter acetylcholine (ACh) has a critical role in both the sympathetic and parasympathetic divisions of the autonomic nervous system. All preganglionic fibers, whether sympathetic or parasympathetic, secrete ACh, which then interacts with nicotinic receptors on ganglionic neurons. This leads to the depolarization of the postsynaptic membrane. Further, postganglionic parasympathetic fibers release ACh that targets muscarinic receptors, known for their more complex influence on target cells.
On the other hand, postganglionic sympathetic fibers generally secrete norepinephrine, except in cases where they innervate sweat glands and certain blood vessels where ACh is the neurotransmitter used. Preganglionic sympathetic fibers are short and myelinated because of their proximity to the spinal column, whereas postganglionic sympathetic fibers tend to be long and unmyelinated due to the distance they cover to reach effector organs.