 In this video, I will explain the clinical significance of drugs that mimic or inhibit parasympathetic or sympathetic effects. A drug that activates a receptor is known as an agonist for that receptor, whereas a drug that inhibits a receptor is known as an antagonist for that receptor. Nicotine is an example of a nicotinic acetylcholine receptor agonist. Because all of the postganglionic neurons express nicotinic acetylcholine receptors, nicotine has the effects of stimulating both the sympathetic and the parasympathetic efferent fibers, leading to mostly conflicting signals that cancel out. However, there are some organs that do not have dual innervation. For example, most of the blood vessels in the body are only innervated by the sympathetic efferent fibers. Therefore, one of the effects of nicotine in the body is to increase blood pressure by stimulating vasoconstriction in the blood vessels throughout the body. And for this reason, smoking or consuming nicotine in any form, whether it's vaping or chewing tobacco, can increase blood pressure and increase the risk of cardiovascular disease. A sympathomimetic drug is a chemical that stimulates a sympathetic response. So an adrenergic receptor agonist would be a sympathomimetic drug. Fennel efferent is an example of an adrenergic receptor agonist, which is used clinically to dilate the pupils. During an eye exam, a drop of fennel efferent can be used to cause dilation of the pupils, enabling an ophthalmologist to get a view of the internal anatomy of the eye. A sympathomimetic drug is a chemical that decreases sympathetic responses. Sympathomimetic drugs are commonly adrenergic receptor antagonists. For example, propanolol is a beta adrenergic receptor antagonist, which is often just abbreviated beta blocker. So all beta blockers are beta adrenergic receptor antagonists. And propanolol is just the most common example of a beta blocker medication. And so one of the clinical uses of propanololol is to decrease the heart rate as a way of lowering blood pressure and treating abnormal heart rhythms, arrhythmia. And so propanololol is one of the most commonly prescribed medications as it's commonly used to treat hypertension or high blood pressure. A parasympathomimetic drug is a chemical that stimulates a parasympathetic response. So typically a parasympathomimetic drug is a muscarinic acetylcholine receptor agonist. For example, pilocarpine is a muscarinic acetylcholine receptor agonist that's used clinically to constrict the pupils. So after visiting an ophthalmologist and having eye drops of phenyl ephrin used to dilate the pupils, the ophthalmologist may use drops of pilocarpine to reverse the effects of phenyl ephrin. Another example of a muscarinic receptor agonist is muscarin. And so you'll notice that the muscarinic acetylcholine receptor was named after muscarin. Muscarin is found in a variety of plants and also in some mushrooms, including the mushroom shown in the image here, the Amanita muscaria. While there is some muscarin in Amanita muscaria that it was initially thought that a muscarin toxicity was the reason that Amanita muscaria is a toxic mushroom, later we've learned that it's a different toxin that really causes the symptoms of Amanita muscaria poisoning, but nevertheless the mushroom got its name from the fact that it does contain this chemical muscarin. And that is another example of a muscarinic acetylcholine receptor agonist that can function as a parasympathomimetic drug having the effects of constricting the pupils. But in another system in the heart, these medications will cause a decrease in heart rate by activating the muscarinic acetylcholine receptors in the heart. So if you've picked up on the pattern of nomenclature, you might expect that a muscarinic acetylcholine receptor antagonist would be classified as a parasympathomimetic drug. While that term certainly could be used, the more common term for a chemical that decreases parasympathetic responses is an anticholinergic drug. Atropine is an example of an anticholinergic medication that is a muscarinic acetylcholine receptor antagonist. Atropine can be found in the plant shown in the image here, atropa belladona, got its name from the chemical atropine. And the last part of that name, belladona, means beautiful woman and comes from the fact that this plant was used in order to cause dilation of the pupils, a feature that was thought to be beautiful. Well, although that is actually quite dangerous to consume this plant because atropine can also have effects in the heart to increase the heart rate. And atropine is clinically used as a medication to stimulate an increase in the heart rate.