Final answer:
Sympathetic postganglionic fibers mainly release norepinephrine, except for those innervating sweat glands and blood vessels in skeletal muscles, which release ACh. Postganglionic parasympathetic fibers release ACh. Both systems' preganglionic fibers release ACh onto nicotinic receptors.
Step-by-step explanation:
The question asks about the neurotransmitters released by the postganglionic fibers of the sympathetic and parasympathetic nervous systems. Postganglionic sympathetic fibers primarily release norepinephrine to act on adrenergic receptors of the target cells. However, there is an exception to this, as sympathetic fibers that project to sweat glands and to blood vessels associated with skeletal muscles release acetylcholine (ACh) instead. In contrast, postganglionic parasympathetic fibers release ACh, which binds to muscarinic receptors on their target cells, inducing rest-and-digest responses.
All preganglionic fibers, whether sympathetic or parasympathetic, release ACh onto nicotinic receptors at the ganglia, leading to depolarization. Therefore, the distinction between sympathetic and parasympathetic postganglionic fibers lies not in the neurotransmitter released by preganglionic neurons, but in the neurotransmitters released by postganglionic neurons, with the sympathetic division mostly utilizing norepinephrine, except in certain cases where it releases ACh.