Final answer:
The primary neurotransmitter released by preganglionic sympathetic and both preganglionic and postganglionic parasympathetic fibers is Acetylcholine (ACh).
Step-by-step explanation:
The primary neurotransmitter released by the preganglionic fibers of the sympathetic nervous system and the preganglionic and postganglionic fibers of the parasympathetic nervous system is Acetylcholine (ACh).
All preganglionic fibers, both sympathetic and parasympathetic, release ACh. Once released, ACh acts on nicotinic receptors located on ganglionic neurons, which are ligand-gated ion channels leading to depolarization. Additionally, the postganglionic parasympathetic fibers also release ACh, but they target muscarinic receptors, which are G protein-coupled receptors that do not solely lead to depolarization.
By contrast, postganglionic sympathetic fibers generally release norepinephrine, except for those projecting to sweat glands and blood vessels associated with skeletal muscles, which release ACh. Understanding the role of these neurotransmitters in the autonomic nervous system highlights the different actions and effects that these systems have on various organs and tissues throughout the body.