Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs Indian Medical PG Practice Questions and MCQs

Question 391: Which of the following is a directly acting skeletal muscle relaxant?

A. Dantrolene (Correct Answer)
B. Suxamethonium
C. Pancuronium
D. Atracurium

Explanation: **Explanation:** The question tests the classification of skeletal muscle relaxants based on their site of action. **1. Why Dantrolene is Correct:** Dantrolene is the only **directly acting** skeletal muscle relaxant among the options. Unlike others that act at the neuromuscular junction (NMJ), Dantrolene acts intracellularly within the muscle fiber. It binds to the **Ryanodine Receptor (RyR1)** on the sarcoplasmic reticulum, inhibiting the release of calcium ions into the sarcoplasm. Since calcium is essential for excitation-contraction coupling, its blockade leads to muscle relaxation. **2. Why Other Options are Incorrect:** * **Suxamethonium (Succinylcholine):** It is a **depolarizing neuromuscular blocker**. It acts on the nicotinic receptors ($N_m$) at the NMJ, causing persistent depolarization of the motor endplate. * **Pancuronium & Atracurium:** These are **non-depolarizing (competitive) neuromuscular blockers**. They act by competing with Acetylcholine for the $N_m$ receptors at the NMJ, preventing muscle contraction. **3. NEET-PG High-Yield Pearls:** * **Drug of Choice (DOC):** Dantrolene is the life-saving drug of choice for **Malignant Hyperthermia** (often triggered by Halothane or Suxamethonium) and **Neuroleptic Malignant Syndrome (NMS)**. * **Atracurium:** Notable for undergoing **Hofmann Elimination** (spontaneous non-enzymatic degradation), making it safe in patients with liver or kidney failure. * **Suxamethonium:** Associated with muscle fasciculations, hyperkalemia, and post-operative myalgia. It is metabolized by **pseudocholinesterase**.

Question 392: Oximes are not useful in the management of which type of poisoning?

A. Organophosphate poisoning (e.g., Parathion)
B. Carbamate poisoning (e.g., Carbaryl) (Correct Answer)
C. Organophosphate poisoning (e.g., Malathion)
D. Organophosphate poisoning (e.g., Phosmet)

Explanation: The primary mechanism of **Oximes** (e.g., Pralidoxime/PAM) is to reactivate the enzyme **Acetylcholinesterase (AChE)** that has been inactivated by phosphorylation [1]. **Why Carbamates are the correct answer:** In **Carbamate poisoning** (e.g., Carbaryl, Neostigmine), the enzyme AChE undergoes "carbamylation" rather than phosphorylation [2]. This carbamylation is **spontaneously reversible** and occurs rapidly [2]. Oximes are not useful here because: 1. The carbamylated enzyme does not respond to oximes. 2. Oximes themselves have weak anticholinesterase activity; adding them can potentially worsen the clinical blockade. *Note: An exception is Propanil/Carbaryl where oximes are strictly contraindicated, though in clinical practice, they are generally avoided for all carbamates.* **Why the other options are incorrect:** * **Options A, C, and D (Parathion, Malathion, Phosmet):** These are all **Organophosphates (OPs)** [3]. OPs cause irreversible inhibition of AChE by phosphorylation [1, 2]. Oximes work by dephosphorylating the enzyme, provided they are administered before "aging" (the permanent chemical bond formation) occurs [1]. Therefore, oximes are a standard part of the treatment protocol for these agents. **High-Yield Clinical Pearls for NEET-PG:** * **Atropine** is the drug of choice for both OP and Carbamate poisoning (it treats the muscarinic symptoms). * **Oximes** are specific for OP poisoning but must be given early (within 24–48 hours) before **"Aging"** of the enzyme occurs [1]. * **Soman** is an OP nerve gas that "ages" extremely rapidly (within minutes), making oximes clinically ineffective [2]. * **Signs of Atropinization:** Mydriasis, tachycardia, and cessation of secretions (dry skin/mouth).

Question 393: Ipratropium bromide is contraindicated in which of the following conditions?

A. Asthma
B. Urinary retention (Correct Answer)
C. Hypertension
D. Peptic ulcer

Explanation: **Explanation:** **Ipratropium bromide** is a short-acting muscarinic antagonist (SAMA). It works by blocking M3 receptors on smooth muscles, leading to bronchodilation and decreased secretions [3]. **Why Urinary Retention is the Correct Answer:** Muscarinic antagonists exert systemic anticholinergic effects. In the bladder, M3 receptors are responsible for the contraction of the **detrusor muscle** and relaxation of the sphincter to facilitate voiding. By blocking these receptors, Ipratropium can cause detrusor relaxation and increased sphincter tone, significantly worsening pre-existing **urinary retention** (especially in patients with Benign Prostatic Hyperplasia) [2], [4]. While Ipratropium is administered via inhalation to limit systemic absorption, enough can be absorbed to trigger these adverse effects in susceptible individuals. **Analysis of Incorrect Options:** * **A. Asthma:** This is a primary **indication**, not a contraindication [1]. Ipratropium is used as an adjunctive bronchodilator in acute asthma exacerbations. * **C. Hypertension:** Anticholinergics generally do not have a significant impact on blood pressure; they are more likely to cause tachycardia. It is not a contraindication. * **D. Peptic Ulcer:** Anticholinergics actually reduce gastric acid secretion (M1 blockade). While not a primary treatment for PUD today, they are certainly not contraindicated. **NEET-PG High-Yield Pearls:** * **Chemistry:** Ipratropium is a **quaternary ammonium compound**, meaning it is highly polar, does not cross the blood-brain barrier, and has poor systemic absorption compared to atropine [4]. * **Glaucoma Warning:** Use with caution in **narrow-angle glaucoma**, as it can increase intraocular pressure if the drug accidentally sprays into the eyes [2]. * **Drug of Choice:** Ipratropium/Tiotropium are the drugs of choice for **COPD** (more effective than beta-2 agonists in these patients) [1].

Question 394: What is the mechanism of action of Silodosin?

A. Alpha antagonist (Correct Answer)
B. Beta antagonist
C. Anticholinergic
D. PDE5 inhibitor

Explanation: **Explanation:** **Silodosin** is a highly selective **alpha-1A (α1A) adrenergic receptor antagonist**. These receptors are primarily located in the smooth muscles of the prostate, bladder neck, and prostatic urethra. By blocking these receptors, Silodosin induces smooth muscle relaxation, which reduces resistance to urine flow and alleviates the dynamic component of urinary obstruction in patients with **Benign Prostatic Hyperplasia (BPH)**. **Analysis of Options:** * **Option A (Correct):** Silodosin belongs to the class of uroselective alpha-blockers. Unlike older agents like Doxazosin, Silodosin is highly specific for the α1A subtype, minimizing systemic side effects like hypotension. * **Option B:** Beta-antagonists (Beta-blockers) are used for hypertension, arrhythmias, and heart failure; they have no role in relaxing prostatic smooth muscle. * **Option C:** Anticholinergics (e.g., Oxybutynin) are used for overactive bladder. In BPH, they are generally avoided as monotherapy because they can decrease detrusor contraction and precipitate acute urinary retention. * **Option D:** PDE5 inhibitors (e.g., Tadalafil) can be used for BPH, but their mechanism involves increasing cGMP levels to relax smooth muscle, not alpha-receptor blockade. **NEET-PG High-Yield Pearls:** 1. **Uroselectivity:** Silodosin is more selective for α1A than Tamsulosin, leading to the lowest incidence of orthostatic hypotension among alpha-blockers. 2. **Side Effect:** The most characteristic side effect of Silodosin is **retrograde ejaculation** (due to relaxation of the vas deferens and ejaculatory duct). 3. **IFIS:** Like Tamsulosin, Silodosin is associated with **Intraoperative Floppy Iris Syndrome (IFIS)** during cataract surgery. Patients should be screened before surgery.

Question 395: Pilocarpine is a:

A. Muscarinic cholinergic drug (Correct Answer)
B. Nicotinic cholinergic drug
C. Alpha-adrenergic drug
D. Beta-adrenergic drug

Explanation: **Explanation:** **Pilocarpine** is a naturally occurring alkaloid derived from the leaves of *Pilocarpus microphyllus*. It acts as a **direct-acting cholinomimetic** that primarily stimulates **muscarinic (M1, M2, M3) receptors**. 1. **Why Option A is Correct:** Pilocarpine has a dominant effect on muscarinic receptors, particularly the **M3 receptors** found in exocrine glands and smooth muscles. When applied to the eye, it causes contraction of the sphincter pupillae (miosis) and the ciliary muscle (accommodation), which facilitates the drainage of aqueous humor. 2. **Why Other Options are Incorrect:** * **Option B:** While some cholinergic drugs (like Nicotine or Varenicline) target nicotinic receptors at the ganglia or neuromuscular junction, Pilocarpine has negligible action on nicotinic receptors at therapeutic doses. * **Options C & D:** Alpha and Beta-adrenergic drugs belong to the Sympathetic Nervous System (e.g., Adrenaline, Phenylephrine). Pilocarpine belongs to the Parasympathetic (Cholinergic) system and does not interact with adrenergic receptors. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Pilocarpine is the DOC for **Acute Angle-Closure Glaucoma** (administered as 0.5%–4% drops) to rapidly lower intraocular pressure. * **Sjögren’s Syndrome/Xerostomia:** Oral pilocarpine is used to increase salivation and lacrimation. * **Ciliary Muscle Contraction:** Unlike sympathomimetics, pilocarpine causes "spasm of accommodation" (false myopia) because it contracts the ciliary muscle. * **Sweat Test:** It is used via iontophoresis to induce sweating for the diagnosis of **Cystic Fibrosis**.

Question 396: Dicyclomine is a

A. Cholinergic drug
B. Adrenergic drug
C. Anticholinergic drug (Correct Answer)
D. Anti-adrenergic drug

Explanation: **Explanation:** **Dicyclomine** is a synthetic tertiary amine compound that acts as a **competitive antagonist at muscarinic (M3) receptors**. By blocking these receptors, it inhibits the action of acetylcholine on smooth muscles, leading to its classification as an **Anticholinergic drug** (specifically an antispasmodic). * **Why Option C is Correct:** Dicyclomine exerts direct relaxant effects on the smooth muscles of the gastrointestinal tract. It reduces GI motility and secretions, making it a first-line agent for treating functional bowel disorders like **Irritable Bowel Syndrome (IBS)**. * **Why Option A is Incorrect:** Cholinergic drugs (parasympathomimetics) mimic acetylcholine. These would increase GI motility and cramping, which is the opposite of dicyclomine’s effect. * **Why Options B & D are Incorrect:** Adrenergic and anti-adrenergic drugs act on the sympathetic nervous system (alpha and beta receptors). Dicyclomine has no significant affinity for these receptors. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mechanism:** It possesses dual action—antimuscarinic activity and direct non-specific smooth muscle relaxation (musculotropic action). 2. **Clinical Use:** Primarily used in **IBS** and **spastic colon**. 3. **Contraindications:** Like all anticholinergics, it is contraindicated in patients with **Glaucoma** (increases intraocular pressure), **Prostatic Hypertrophy** (causes urinary retention), and **Myasthenia Gravis**. 4. **Side Effects:** Common "atropine-like" effects include dry mouth, blurred vision, and tachycardia.

Question 397: Dale's vasomotor reversal is due to which of the following?

A. Alpha blocker (Correct Answer)
B. Beta blocker
C. Acetylcholine inhibitor
D. All of the above

Explanation: **Explanation:** **Dale’s Vasomotor Reversal** (or the Dale Phenomenon) is a classic pharmacological observation involving the biphasic response of blood pressure to Adrenaline (Epinephrine). 1. **Mechanism of the Correct Answer (Alpha Blocker):** Adrenaline acts on both **α-receptors** (causing vasoconstriction and rise in BP) and **β2-receptors** (causing vasodilation and fall in BP). Normally, the α-effect is dominant, leading to a net rise in blood pressure. However, if an **α-blocker** (e.g., Phentolamine or Ergot alkaloids) is administered beforehand, the α-mediated vasoconstriction is abolished. This "unmasks" the pure β2-mediated vasodilation, causing the blood pressure to fall instead of rise. This reversal of the pressor response to a depressor response is Dale's Vasomotor Reversal. 2. **Why Other Options are Incorrect:** * **Beta blockers:** If a β-blocker is given, the β2-mediated vasodilation is blocked. This leads to an "exaggerated" pressor response (higher rise in BP) because the α-effect is now unopposed. * **Acetylcholine inhibitor:** Dale’s phenomenon is specific to adrenergic receptors and the sympathetic nervous system; it does not involve the cholinergic system or Acetylcholine. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Dale’s reversal is most commonly demonstrated using **Phentolamine** (non-selective α-blocker). * **Selective α1-blockers:** Drugs like Prazosin also produce this effect. * **Noradrenaline:** Does **not** show Dale’s reversal because it has negligible action on β2 receptors; it only causes a rise in BP or a return to baseline. * **Clinical Significance:** This explains why α-blockers are used first in Pheochromocytoma to prevent a hypertensive crisis from unopposed α-stimulation.

Question 398: How would a drug that competes with acetylcholine (ACh) for receptors at the motor end plate affect skeletal muscle?

A. Produce uncontrolled muscle spasms
B. Cause the muscles to contract and be unable to relax
C. Cause muscles to relax and be unable to contract (Correct Answer)
D. Make the muscles more excitable

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The drug described is a **competitive (non-depolarizing) neuromuscular blocker**, such as **d-Tubocurarine** or **Vecuronium** [1]. These drugs act as antagonists at the **Nicotinic-M ($N_M$) receptors** located on the motor end plate of skeletal muscles [2]. By competing with Acetylcholine (ACh) for these receptor sites, they prevent ACh from binding. Since ACh cannot bind, the sodium channels do not open, the end-plate potential is not generated, and the muscle remains in a state of **flaccid paralysis** (relaxed and unable to contract) [3]. **2. Why the Incorrect Options are Wrong:** * **Option A & B:** These describe the effects of excessive cholinergic stimulation or **depolarizing blockers** (like Succinylcholine) in their initial phase [5]. If a drug were an *agonist* or an *Acetylcholinesterase inhibitor* (like Neostigmine), it would cause persistent depolarization, leading to spasms or "spastic" paralysis [1]. * **Option D:** A competitive antagonist *decreases* excitability by raising the threshold required for a nerve impulse to trigger a contraction. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Reversibility:** The blockade caused by competitive blockers can be reversed by increasing the concentration of ACh. This is clinically achieved using **Acetylcholinesterase inhibitors** (e.g., **Neostigmine**) [1]. * **Sequence of Paralysis:** Small, rapidly moving muscles (eyes, fingers) are paralyzed first, followed by limbs, trunk, and finally the **diaphragm**. Recovery occurs in the reverse order. * **Drug of Choice:** **Rocuronium** is often preferred for rapid sequence intubation when Succinylcholine is contraindicated [4]. * **Sugammadex:** A novel reversal agent specifically for Rocuronium and Vecuronium that encapsulates the drug molecules, rendering them inactive.

Question 399: Which of the following statements regarding 5-HT derivatives is true?

A. Cisapride is a selective 5-HT4 antagonist
B. Ondansetron is a 5-HT3 agonist
C. Cyproheptadine is a 5-HT2A agonist
D. Sumatriptan's action is mediated by agonistic 5-HT 1B/1D receptor activity (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Sumatriptan's action is mediated by agonistic 5-HT 1B/1D receptor activity** **1. Why Option D is Correct:** Sumatriptan is the prototype of the "Triptan" class used for acute migraine attacks. Its mechanism involves selective agonism at **5-HT 1B** and **5-HT 1D** receptors. * **5-HT 1B activation** causes vasoconstriction of dilated cranial extracerebral blood vessels. * **5-HT 1D activation** acts presynaptically on trigeminal nerve terminals to inhibit the release of pro-inflammatory neuropeptides (like CGRP and Substance P), thereby suppressing neurogenic inflammation. **2. Why Other Options are Incorrect:** * **Option A:** **Cisapride** is a 5-HT4 **agonist**, not an antagonist. It acts as a prokinetic agent by increasing acetylcholine release in the myenteric plexus. (Note: It is largely restricted due to QT interval prolongation). * **Option B:** **Ondansetron** is a potent 5-HT3 **antagonist**. It is used as a first-line antiemetic, especially for chemotherapy-induced nausea and vomiting (CINV), by blocking receptors in the Chemoreceptor Trigger Zone (CTZ) and the gut. * **Option C:** **Cyproheptadine** is a 5-HT2A **antagonist** (with additional H1 blocking and anticholinergic properties). It is used in the management of Serotonin Syndrome and as an appetite stimulant. **3. NEET-PG High-Yield Pearls:** * **Drug of Choice (DOC) for Acute Migraine:** Triptans (Sumatriptan). * **DOC for Serotonin Syndrome:** Cyproheptadine. * **5-HT3 Antagonists (Setrons):** Only class of 5-HT ligands that act on **ionotropic receptors** (ligand-gated ion channels); all other 5-HT receptors are G-protein coupled (GPCRs). * **Tegaserod/Prucalopride:** 5-HT4 agonists used for chronic constipation/IBS-C.

Question 400: Glycogenolysis is mediated by which receptors?

A. Alpha 1 receptor
B. Alpha 1 and Beta 2 receptors (Correct Answer)
C. Beta 2 receptor
D. Alpha 2 and Beta 1 receptors

Explanation: ### Explanation **Concept Overview** Glycogenolysis (the breakdown of glycogen into glucose) is a critical metabolic response to sympathetic stimulation, ensuring the body has adequate energy during "fight or flight" situations. In the liver, this process is unique because it is mediated by **both Alpha-1 ($\alpha_1$) and Beta-2 ($\beta_2$) receptors** [1]. **Why Option B is Correct** * **Beta-2 ($\beta_2$) Receptors:** These are the primary mediators of glycogenolysis in humans [1], [2]. Activation of $\beta_2$ receptors increases intracellular **cAMP**, which activates protein kinase A, leading to the activation of phosphorylase (the enzyme that breaks down glycogen). * **Alpha-1 ($\alpha_1$) Receptors:** In the liver, $\alpha_1$ stimulation increases intracellular **Calcium ($Ca^{2+}$)** and Inositol triphosphate ($IP_3$). This rise in calcium also activates phosphorylase kinase, contributing significantly to glycogenolysis. * While $\beta_2$ is often emphasized, the synergistic action of both receptors is the physiologically accurate mechanism for hepatic glucose release. Adrenaline-induced hyperglycaemia in humans is blocked completely by a combination of $\alpha$ and $\beta$ antagonists but not by either on its own [1]. **Analysis of Incorrect Options** * **Option A ($\alpha_1$ only):** While $\alpha_1$ contributes via the $IP_3/DAG$ pathway, it is not the sole mediator; the $\beta_2$ pathway is equally, if not more, significant. * **Option C ($\beta_2$ only):** This is a common distractor. While $\beta_2$ mediates glycogenolysis in both **liver and skeletal muscle**, the liver specifically utilizes both $\alpha_1$ and $\beta_2$. * **Option D ($\alpha_2$ and $\beta_1$):** $\alpha_2$ receptors generally inhibit insulin release (pancreas) [1], and $\beta_1$ receptors primarily affect the heart (inotropy/chronotropy) and renin release (kidney). **NEET-PG High-Yield Pearls** 1. **Muscle vs. Liver:** In **skeletal muscle**, glycogenolysis is mediated **only by $\beta_2$ receptors** [1]. $\alpha_1$ receptors do not play a role in muscle glycogenolysis. 2. **Hyperglycemia:** Adrenaline causes hyperglycemia via hepatic glycogenolysis ($\alpha_1 + \beta_2$) and by inhibiting insulin release ($\alpha_2$) [1]. 3. **Beta-blockers:** Non-selective beta-blockers (like Propranolol) can mask hypoglycemic symptoms and delay recovery from hypoglycemia in diabetics by blocking $\beta_2$-mediated glycogenolysis.

Practice by Chapter

Cholinergic Agonists

Practice Questions

Cholinergic Antagonists

Practice Questions

Adrenergic Agonists

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Adrenergic Antagonists

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Ganglionic Agents

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Neuromuscular Blocking Agents

Practice Questions

Autonomic Drugs in Ophthalmology

Practice Questions

Autonomic Drugs in Cardiovascular Disease

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Autonomic Drugs in Respiratory Disease

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Autonomic Drugs in Urological Disorders

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