Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs Indian Medical PG Practice Questions and MCQs

Question 101: Propranolol is useful in all conditions listed, except:

A. Migraine
B. Thyrotoxicosis
C. Parkinsonian tremor (Correct Answer)
D. Esophageal varices

Explanation: **Explanation:** The correct answer is **Parkinsonian tremor** because Propranolol, a non-selective beta-blocker, has no therapeutic role in treating the tremors associated with Parkinson’s disease. Parkinsonian tremors are "resting tremors" caused by a dopamine-acetylcholine imbalance in the basal ganglia. In contrast, Propranolol is the drug of choice for **Essential tremors** (action tremors), which are mediated by peripheral $\beta_2$ receptors. **Analysis of Options:** * **Migraine (A):** Propranolol is a first-line agent for the **prophylaxis** of migraine. It works by preventing vasodilation and modulating cortical excitability, though it is not used for acute attacks. * **Thyrotoxicosis (B):** It is used to control symptoms like tachycardia, palpitations, and tremors. Additionally, it inhibits the peripheral conversion of $T_4$ to the more active $T_3$. * **Esophageal Varices (D):** Propranolol is used for primary and secondary prophylaxis of variceal bleeding. It reduces portal pressure by causing splanchnic vasoconstriction (via $\beta_2$ blockade) and reducing cardiac output (via $\beta_1$ blockade). **NEET-PG High-Yield Pearls:** * **Drug of Choice (DOC):** Propranolol is the DOC for **Essential Tremor**, **Performance Anxiety** (Stage fright), and **Infantile Hemangioma**. * **Contraindications:** Avoid in Bronchial Asthma (causes bronchospasm), Insulin-dependent Diabetes (masks hypoglycemic tachycardia), and Prinzmetal Angina. * **Membrane Stabilizing Activity (MSA):** Propranolol possesses significant local anesthetic activity, making it unsuitable for topical ophthalmic use (risk of corneal anesthesia).

Question 102: Tizanidine has action on which receptor type?

A. Alpha adrenergic receptors (Correct Answer)
B. GABA A receptors
C. GABA B receptors
D. Beta adrenergic receptors

Explanation: **Explanation:** **Tizanidine** is a centrally acting skeletal muscle relaxant. Its primary mechanism of action is as a **selective alpha-2 ($\alpha_2$) adrenergic agonist**. It acts predominantly in the spinal cord, where it stimulates presynaptic $\alpha_2$ receptors. This stimulation inhibits the release of excitatory amino acids (like glutamate and aspartate) from spinal interneurons, thereby reducing the excitability of alpha motor neurons and decreasing muscle spasticity. **Analysis of Options:** * **Option A (Correct):** Tizanidine is a congener of clonidine. It binds to **alpha-2 adrenergic receptors**, making "Alpha adrenergic receptors" the correct categorical choice. * **Option B (Incorrect):** GABA-A receptors are the target for Benzodiazepines (like Diazepam), which also act as muscle relaxants but via a different pathway. * **Option C (Incorrect):** GABA-B receptors are the target for **Baclofen**, another common centrally acting spasmolytic. This is a frequent point of confusion in exams. * **Option D (Incorrect):** Beta-adrenergic receptors are involved in cardiac and pulmonary functions; beta-agonists (like Salbutamol) or blockers (like Propranolol) do not have a primary role in treating spasticity. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Use:** Used primarily for spasticity associated with Multiple Sclerosis, stroke, or spinal cord injury. * **Side Effects:** Compared to clonidine, Tizanidine has much less effect on blood pressure, but it can still cause **hypotension, xerostomia (dry mouth), and drowsiness**. * **Key Distinction:** Unlike Baclofen (which causes significant muscle weakness), Tizanidine reduces spasticity with **minimal effect on muscle strength**, making it preferable for patients who need to maintain motor power for ambulation.

Question 103: Which of the following is a selective Beta 2 antagonist?

A. Esmolol
B. Betaxolol
C. Butoxamine (Correct Answer)
D. Celiprolol

Explanation: **Explanation:** **Correct Option: C. Butoxamine** Butoxamine is a specific, competitive **selective Beta-2 ($eta_2$) antagonist**. Unlike non-selective blockers (like Propranolol), it specifically blocks $eta_2$ receptors found in bronchial smooth muscle, skeletal muscle, and the liver. While it has no significant clinical therapeutic use, it is a vital **pharmacological tool** used in research to differentiate between $eta_1$ and $eta_2$ mediated effects. **Analysis of Incorrect Options:** * **A. Esmolol:** An ultra-short-acting **selective $eta_1$ blocker** (cardioselective). It is administered intravenously and is the drug of choice for intraoperative tachycardia and aortic dissection due to its rapid metabolism by RBC esterases (half-life ~9 minutes) [1]. * **B. Betaxolol:** A **selective $eta_1$ blocker** [1]. It is commonly used topically in glaucoma because it reduces aqueous humor production and has a long duration of action. * **C. Celiprolol:** A unique **third-generation $eta$ blocker**. It is a selective $eta_1$ antagonist but also possesses **$eta_2$ agonist** activity (Intrinsic Sympathomimetic Activity) and weak $oldsymbol{\alpha}_2$ blocking properties, making it useful in hypertensive patients with asthma [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for $\beta_1$ Selectives (Cardioselective):** *"**A**ll **N**ew **B**eta **B**lockers **E**xert **M**ainly **C**ardioselective **A**ctions"* (**A**tenolol, **N**ebivolol, **B**isoprolol, **B**etaxolol, **E**smolol, **M**etoprolol, **C**eliprolol, **A**cebutolol). * **Butoxamine** can provoke bronchospasm and inhibit glycogenolysis; hence, it is contraindicated in clinical practice. * **Nebivolol** is the most highly selective $\beta_1$ blocker and also produces vasodilation via Nitric Oxide (NO) release.

Question 104: Apart from the release of EDRF, what is another mechanism of the vasodilatory action of acetylcholine?

A. Direct action causing vascular smooth muscle relaxation
B. Inhibition of norepinephrine release from adrenergic nerve endings (Correct Answer)
C. Stimulation of platelet aggregation
D. Stimulation of vascular beta-2 receptors

Explanation: ### Explanation The vasodilatory effect of Acetylcholine (ACh) is mediated through two primary mechanisms: 1. **Endothelium-Dependent (Major):** ACh stimulates **M3 receptors** on vascular endothelial cells, leading to the release of **Endothelium-Derived Relaxing Factor (EDRF)**, now known as **Nitric Oxide (NO)**. NO diffuses into the underlying smooth muscle, increasing cGMP and causing relaxation. 2. **Presynaptic Inhibition (Minor but Significant):** ACh acts on **presynaptic M2 receptors** located on sympathetic (adrenergic) nerve terminals. This activation inhibits the release of **Norepinephrine (NE)** [1]. Since NE normally maintains vascular tone via alpha-1 receptors, its inhibition results in indirect vasodilation. #### Analysis of Options: * **Option A (Incorrect):** ACh does **not** have a direct relaxant effect on vascular smooth muscle. In fact, if the endothelium is damaged (denuded), ACh acts directly on M3 receptors on the smooth muscle itself, causing **vasoconstriction**. * **Option C (Incorrect):** ACh does not stimulate platelet aggregation; NO (released by ACh) actually inhibits platelet aggregation. * **Option D (Incorrect):** ACh acts on Muscarinic receptors, not Beta-2 receptors. Beta-2 receptors are stimulated by Adrenaline or specific agonists (e.g., Salbutamol). #### NEET-PG High-Yield Pearls: * **Furchgott’s Phenomenon:** The observation that ACh causes relaxation only when the endothelium is intact; otherwise, it causes contraction. * **M3 Receptors:** Located on both endothelium (vasodilation via NO) and smooth muscle (vasoconstriction). The vasodilatory effect usually predominates in vivo. * **Atropine:** Blocks the vasodilatory effect of IV-injected ACh but has no effect on resting blood pressure because most blood vessels lack cholinergic innervation (though they possess cholinergic receptors).

Question 105: What is the primary use of phenoxybenzamine?

A. Hypertension
B. Pheochromocytoma
C. Angiography
D. All the above (Correct Answer)

Explanation: **Phenoxybenzamine** is a non-selective, irreversible alpha-adrenergic blocker. It forms a covalent bond with both $\alpha_1$ and $\alpha_2$ receptors, leading to a long-lasting blockade that can only be overcome by the synthesis of new receptors. **Explanation of Options:** * **Pheochromocytoma (Option B):** This is the **classic primary indication**. Phenoxybenzamine is used pre-operatively to control hypertension and prevent "hypertensive crises" during surgical manipulation of the tumor. It ensures a stable blood pressure by blocking the effects of massive catecholamine release. * **Hypertension (Option A):** While not a first-line agent for essential hypertension due to reflex tachycardia, it is specifically indicated for hypertensive emergencies associated with **excess catecholamines**, such as those seen in pheochromocytoma or clonidine withdrawal. * **Angiography (Option C):** Phenoxybenzamine is sometimes used to prevent or treat **vasospasm** during diagnostic procedures like angiography or in peripheral vascular diseases (e.g., Raynaud’s phenomenon), as it promotes vasodilation by blocking $\alpha_1$-mediated vasoconstriction. **Why "All the above" is correct:** Although pheochromocytoma is the most common clinical association, the drug’s pharmacological profile allows for its use in any condition where potent, long-term alpha-blockade is required to manage high blood pressure or vasospasm. **High-Yield Clinical Pearls for NEET-PG:** 1. **Irreversible Blockade:** It is the only clinically used irreversible alpha-blocker (non-competitive). 2. **Side Effects:** Postural hypotension and **reflex tachycardia** (due to $\alpha_2$ blockade increasing NE release) are common. 3. **Epinephrine Reversal:** Administration of epinephrine after phenoxybenzamine leads to a **fall** in blood pressure (Dale’s Vasomotor Reversal) because the $\alpha$-constrictor effect is blocked, leaving the $\beta_2$-dilator effect unopposed. 4. **Preparation Rule:** In pheochromocytoma, always start the **Alpha-blocker before the Beta-blocker** to avoid a hypertensive crisis from unopposed alpha-stimulation.

Question 106: Which beta-blocker lacks local anesthetic (membrane stabilizing) effect?

A. Metoprolol
B. Pindolol
C. Propranolol
D. Timolol (Correct Answer)

Explanation: The **Membrane Stabilizing Activity (MSA)**, also known as a local anesthetic effect, refers to the ability of certain beta-blockers to inhibit the initiation and propagation of action potentials by blocking voltage-gated sodium channels [1]. This property is independent of beta-adrenoceptor blockade. **Why Timolol is the Correct Answer:** **Timolol** is a potent, non-selective beta-blocker that **lacks MSA**. Because it does not have a local anesthetic effect, it does not cause corneal anesthesia when applied topically. This makes it the drug of choice for the chronic treatment of **Glaucoma**, as it preserves the protective corneal reflex. **Analysis of Incorrect Options:** * **Propranolol:** This is the prototype non-selective beta-blocker and possesses the **highest degree of MSA** [1]. While this contributes to its anti-arrhythmic properties, it makes it unsuitable for ophthalmic use as it would numb the cornea. * **Pindolol:** A non-selective beta-blocker that possesses both MSA and **Intrinsic Sympathomimetic Activity (ISA)** [1]. * **Metoprolol:** A cardioselective ($\beta_1$) blocker that possesses **weak but present MSA** [1]. **NEET-PG High-Yield Pearls:** * **Beta-blockers with MSA:** Propranolol (Max), Acebutolol, Metoprolol, Pindolol, Labetalol. (Mnemonic: **P**alms **A**re **M**ostly **P**ink **L**ately). * **Beta-blockers without MSA:** Timolol, Atenolol, Sotalol, Nadolol. * **Clinical Significance:** Drugs with MSA are generally avoided in patients with cardiac conduction defects but are useful in preventing certain arrhythmias. * **Glaucoma Tip:** Timolol is preferred over Propranolol for the eye specifically because it lacks MSA.

Question 107: What effect do muscarinic cholinergic agonists have?

A. Decrease production of IP3
B. Activate inhibitory G-proteins (Gi) (Correct Answer)
C. Decrease release of intracellular calcium
D. Inhibit the activity of phospholipase C

Explanation: **Explanation:** Muscarinic cholinergic receptors (M1–M5) are G-protein coupled receptors (GPCRs). To understand the correct answer, we must categorize these receptors based on their signaling pathways: * **M1, M3, M5:** Coupled with **Gq** proteins. They activate Phospholipase C (PLC), leading to increased IP3/DAG and increased intracellular calcium. * **M2, M4:** Coupled with **Gi** (inhibitory) proteins. **Why Option B is Correct:** Muscarinic agonists (like Acetylcholine, Pilocarpine, or Bethanechol) act on M2 receptors (primarily in the heart) and M4 receptors. Activation of the **Gi protein** inhibits Adenylyl Cyclase, leading to a decrease in cAMP levels. In the heart, this results in the opening of K+ channels and hyperpolarization, causing bradycardia. Since "muscarinic agonists" encompass effects on all subtypes, activating Gi is a primary mechanism for the M2/M4 subset. **Why Other Options are Incorrect:** * **Options A, C, and D:** These describe the *inhibition* of the Gq pathway. In reality, muscarinic agonists (acting via M1, M3, M5) **increase** the activity of Phospholipase C, **increase** the production of IP3, and **increase** the release of intracellular calcium (leading to smooth muscle contraction and gland secretion). **High-Yield NEET-PG Pearls:** * **M2 Receptors:** Located in the SA and AV nodes; activation causes negative chronotropy and dromotropy. * **M3 Receptors:** Located in smooth muscles (bronchoconstriction, bladder contraction) and glands (sweating, salivation). They also cause vasodilation via **Nitric Oxide (NO)** release from vascular endothelium. * **Mnemonic:** "Q-I-Q-I-Q" for M1 through M5 (Gq, Gi, Gq, Gi, Gq).

Question 108: Mechanism of action of clonidine is mediated by which of the following receptors?

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

Explanation: **Explanation:** **Clonidine** is a centrally acting sympatholytic drug. Its primary mechanism of action is the **selective stimulation of presynaptic Alpha-2 ($\alpha_2$) receptors** in the nucleus tractus solitarius (NTS) of the medulla. 1. **Why Alpha-2 is correct:** $\alpha_2$ receptors are primarily inhibitory G-protein coupled receptors ($G_i$). When clonidine stimulates these receptors in the vasomotor center of the brain, it inhibits the release of norepinephrine. This leads to a decrease in sympathetic outflow from the CNS to the peripheral tissues, resulting in a reduction in blood pressure and heart rate. 2. **Why other options are incorrect:** * **Alpha-1 ($\alpha_1$):** These are excitatory receptors found on vascular smooth muscle. Stimulation causes vasoconstriction and increases blood pressure (e.g., Phenylephrine). * **Beta-1 ($\beta_1$):** Located mainly in the heart; stimulation increases heart rate and contractility (e.g., Dobutamine). * **Beta-2 ($\beta_2$):** Located in bronchial and vascular smooth muscle; stimulation causes bronchodilation and vasodilation (e.g., Salbutamol). **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Uses:** Hypertension (not first-line), opioid withdrawal, ADHD, and prophylaxis for migraine. * **Adverse Effects:** Sedation, mental depression, and significant **dry mouth (xerostomia)**. * **Rebound Hypertension:** Sudden withdrawal of clonidine can cause a hypertensive crisis due to a massive surge in catecholamines. * **Diagnostic Test:** The **Clonidine Suppression Test** is used to differentiate between essential hypertension and Pheochromocytoma (norepinephrine levels fail to drop in Pheochromocytoma).

Question 109: In organophosphorus compound poisoning, atropine can reverse all the following signs except?

A. Lacrimation
B. Diaphoresis
C. Diarrhea
D. Muscle weakness (Correct Answer)

Explanation: **Explanation:** Organophosphorus (OP) compounds inhibit the enzyme acetylcholinesterase, leading to an accumulation of acetylcholine (ACh) at both **muscarinic** and **nicotinic** receptors. **Why Muscle Weakness is the Correct Answer:** Muscle weakness and paralysis in OP poisoning are mediated by **nicotinic (N$_M$) receptors** at the neuromuscular junction [2]. Atropine is a competitive **muscarinic antagonist**; it has no effect on nicotinic receptors [2]. Therefore, while atropine can reverse life-threatening respiratory secretions, it cannot reverse skeletal muscle weakness or fasciculations. To treat nicotinic effects, oximes (like Pralidoxime) are required to reactivate the enzyme [1], [4]. **Why the other options are incorrect:** * **Lacrimation (A), Diaphoresis (B), and Diarrhea (C):** These are all classic **muscarinic (M$_3$) effects** of ACh excess [3]. Atropine effectively blocks muscarinic receptors in the lacrimal glands, sweat glands, and gastrointestinal smooth muscle, thereby reversing these symptoms [3]. Note: While sweat glands are innervated by sympathetic nerves, they utilize muscarinic receptors, making diaphoresis an atropine-reversible sign. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for OP Poisoning:** **DUMBELS** (Diarrhea, Urination, Miosis, Bradycardia/Bronchospasm, Emesis, Lacrimation, Salivation/Sweating). * **Atropine's Role:** It reverses all "DUMBELS" but does **not** treat muscle weakness or respiratory muscle paralysis [2]. * **Therapeutic Endpoint:** In OP poisoning, atropine is titrated until **"Atropinization"** occurs, characterized by the clearing of lung secretions (most important) and a heart rate >80 bpm. Mydriasis is a sign but not the primary endpoint. * **Oximes:** Must be given early (before "aging" of the enzyme occurs) to reverse nicotinic effects [1], [4].

Question 110: Clinical signs of atropine intoxication are all EXCEPT:

A. Decreased bowel sounds
B. Dry skin
C. Scarlet flushing of face
D. Increased bowel sounds (Correct Answer)

Explanation: **Explanation:** Atropine is a competitive antagonist of **muscarinic acetylcholine receptors**. The clinical signs of atropine intoxication (Anticholinergic Syndrome) result from the blockade of the parasympathetic nervous system, which normally governs "rest and digest" functions. **Why "Increased bowel sounds" is the correct answer:** Atropine blocks M3 receptors in the gastrointestinal tract, leading to decreased smooth muscle motility and relaxation. This results in **decreased or absent bowel sounds** (paralytic ileus) and constipation. Therefore, "Increased bowel sounds" is the physiological opposite of what occurs in atropine toxicity. **Analysis of incorrect options:** * **Decreased bowel sounds:** This is a classic sign of anticholinergic toxicity due to reduced peristalsis. * **Dry skin:** Atropine blocks M3 receptors on eccrine sweat glands (which are innervated by sympathetic cholinergic fibers). This leads to anhidrosis (suppression of sweating) and hyperthermia. * **Scarlet flushing of face:** Known as "Atropine flush," this occurs due to compensatory cutaneous vasodilation in the blush area to dissipate heat, as the body cannot sweat. **High-Yield Clinical Pearls for NEET-PG:** To remember the signs of Atropine poisoning, use the classic mnemonic: * **Red as a beet:** Cutaneous vasodilation/flushing. * **Dry as a bone:** Anhidrosis (dry skin) and dry mouth (xerostomia). * **Blind as a bat:** Mydriasis and cycloplegia (loss of accommodation). * **Mad as a hatter:** Delirium, hallucinations, and agitation. * **Hot as a hare:** Hyperthermia. * **Full as a flask:** Urinary retention. **Antidote of choice:** **Physostigmine** (a tertiary amine acetylcholinesterase inhibitor that crosses the blood-brain barrier).

Practice by Chapter

Cholinergic Agonists

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Cholinergic Antagonists

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Adrenergic Agonists

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Adrenergic Antagonists

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Ganglionic Agents

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Neuromuscular Blocking Agents

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Autonomic Drugs in Ophthalmology

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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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