Final answer:
The neurotransmitter released from parasympathetic postganglionic fibers is acetylcholine, which binds to muscarinic receptors on target cells.
Step-by-step explanation:
The neurotransmitter released from the parasympathetic postganglionic fibers is acetylcholine (ACh). Both the parasympathetic and sympathetic preganglionic fibers release ACh, which binds to nicotinic receptors on ganglionic neurons. However, the postganglionic parasympathetic fibers also release ACh, and the receptors present on their respective target cells are of the muscarinic type.
These muscarinic receptors are G protein-coupled receptors that can cause various effects on the postsynaptic membrane, not exclusively depolarization. Understanding these mechanisms is crucial for grasping how the autonomic nervous system regulates body functions, particularly the 'rest and digest' activity promoted by the parasympathetic nervous system.