Final answer:
Muscarinic agonists and antagonists influence the parasympathetic nervous system by mimicking or inhibiting the effects of acetylcholine, affecting organs and potentially used for treatments, with atropine being an example of an antagonist used in nerve agent poisoning.
Step-by-step explanation:
Muscarinic Receptor Agonism and Antagonism
Drugs that affect the parasympathetic system can either be muscarinic agonists, which enhance the action of acetylcholine (ACh), or muscarinic antagonists, which inhibit the action of ACh. Parasympathetic postganglionic fibers release ACh impacting muscarinic receptors of types M1-M5, which can cause different effects including depolarization or hyperpolarization. Muscarinic agonists, or parasympathomimetic drugs, mimic ACh released by the nerve, whereas anticholinergic drugs block the receptors, suppressing the parasympathetic influence over the organs. An example of anticholinergic drug is atropine which is used to manage symptoms of nerve agent poisoning by acting as an antagonist to muscarinic receptors. Moreover, drugs can have psychoactive effects as muscarinic receptor agonists or antagonists, influencing a patient's neurological state and potentially serving as treatments like dopamine agonists do in Parkinson's disease.