Final answer:
All preganglionic axons of the autonomic nervous system release acetylcholine (ACh), which activates nicotinic receptors on ganglionic neurons, leading to depolarization. Postganglionic parasympathetic fibers also release ACh, but they act on muscarinic receptors. Majority of postganglionic sympathetic fibers release norepinephrine, except for specific cases such as sweat glands and blood vessels in skeletal muscles.
Step-by-step explanation:
All preganglionic axons of the autonomic nervous system release acetylcholine (ACh). This is consistent for both the sympathetic and parasympathetic systems. Within the autonomic system, all ganglionic neurons-the targets of these preganglionic axons-have nicotinic receptors, which are ligand-gated cation channels. Activation of these receptors by ACh results in depolarization of the postsynaptic membrane.
Furthermore, it is interesting to note that postganglionic parasympathetic fibers continue to release ACh, but they act on different receptors, known as muscarinic receptors, which are G protein-coupled receptors. However, the majority of postganglionic sympathetic fibers release norepinephrine, with exceptions including sympathetic fibers that project to sweat glands and to blood vessels related to skeletal muscles, which release ACh instead.