Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs Indian Medical PG Practice Questions and MCQs

Question 531: Bradycardia is seen with which of the following drugs?

A. Midazolam
B. Epinephrine
C. Succinylcholine (Correct Answer)
D. Dopamine

Explanation: ### Explanation **Correct Answer: C. Succinylcholine** **Mechanism of Action:** Succinylcholine is a depolarizing neuromuscular blocking agent. While its primary action is at the nicotinic receptors ($N_m$) of the neuromuscular junction, it also possesses structural similarities to acetylcholine. This allows it to stimulate **muscarinic ($M_2$) receptors** in the sinoatrial (SA) node of the heart. Stimulation of these receptors leads to negative chronotropy (bradycardia). This effect is particularly pronounced in children and can occur in adults upon administration of a second dose (repeat bolus). **Analysis of Incorrect Options:** * **A. Midazolam:** This is a benzodiazepine used for sedation. It typically has minimal effects on heart rate, though it may cause a slight decrease in systemic vascular resistance, occasionally leading to a mild compensatory *tachycardia* or no change at all. * **B. Epinephrine:** A potent catecholamine that stimulates $\alpha$ and $\beta$ receptors. Stimulation of $\beta_1$ receptors in the heart leads to significant **tachycardia** and increased contractility. * **D. Dopamine:** At moderate to high doses, dopamine stimulates $\beta_1$ receptors and triggers the release of norepinephrine, resulting in **tachycardia**. **High-Yield Clinical Pearls for NEET-PG:** * **Succinylcholine & Bradycardia:** Pre-treatment with **Atropine** is often recommended, especially in pediatric anesthesia, to prevent succinylcholine-induced bradycardia. * **Hyperkalemia:** Succinylcholine causes a transient rise in serum potassium (approx. 0.5 mEq/L). It is strictly contraindicated in patients with burns, crush injuries, or upper motor neuron lesions due to the risk of fatal hyperkalemia. * **Malignant Hyperthermia:** Succinylcholine is a known trigger for Malignant Hyperthermia; the antidote is **Dantrolene**. * **Phase II Block:** Prolonged exposure to succinylcholine can lead to a Phase II block, which resembles a non-depolarizing block.

Question 532: Metoprolol is preferred over Propranolol as it:

A. Is more potent in blocking beta-1 receptors (Correct Answer)
B. Is more potent in blocking beta-2 receptors
C. Is more effective in suppressing essential tremors
D. Impairs exercise capacity

Explanation: **Explanation:** The core pharmacological difference between Metoprolol and Propranolol lies in their **receptor selectivity**. **1. Why Option A is Correct:** Metoprolol is a **cardioselective (second-generation) beta-blocker**. It has a significantly higher affinity for **$\beta_1$ receptors** (located primarily in the heart and juxtaglomerular cells) than for $\beta_2$ receptors. This selectivity makes it "preferable" because it provides the desired therapeutic effects (reduction in heart rate, contractility, and blood pressure) while minimizing side effects associated with $\beta_2$ blockade, such as bronchospasm or peripheral vasoconstriction. **2. Why the Other Options are Incorrect:** * **Option B:** Propranolol is a non-selective beta-blocker; it blocks both $\beta_1$ and $\beta_2$ receptors equally. Metoprolol is specifically designed to *avoid* potent $\beta_2$ blockade. * **Option C:** Essential tremors are mediated by peripheral **$\beta_2$ receptors**. Therefore, **Propranolol** (non-selective) is the drug of choice for tremors, not Metoprolol. * **Option D:** All beta-blockers can impair exercise capacity to some extent, but non-selective blockers (Propranolol) do so more severely by blocking $\beta_2$-mediated skeletal muscle vasodilation and glycogenolysis. This is a disadvantage, not a reason for preference. **High-Yield Clinical Pearls for NEET-PG:** * **Cardioselective Beta-blockers (Mnemonic: New Beta Blockers Are Exclusive):** **N**ebivolol, **B**isoprolol, **B**etaxolol, **A**tenolol, **E**smolol, **M**etoprolol. * **Asthma/COPD:** Cardioselective blockers (like Metoprolol) are safer in patients with reactive airway disease, though they should still be used with caution at high doses where selectivity is lost. * **Diabetes:** Metoprolol is preferred in diabetics because it is less likely to mask hypoglycemic tachycardia or delay recovery from hypoglycemia (both $\beta_2$ mediated). * **Esmolol:** The shortest-acting beta-blocker (T½ ≈ 9 mins), administered IV for hypertensive emergencies or arrhythmias.

Question 533: What is the preferred preoperative drug for managing blood pressure in a patient with pheochromocytoma?

A. Prazosin
B. Phenoxybenzamine (Correct Answer)
C. Propranolol
D. Labetalol

Explanation: ***Phenoxybenzamine***- It is the standard first-line agent, administered 10–14 days preoperatively, because it is a **non-selective**, **irreversible alpha-adrenergic blocker**.- This irreversible blockade prevents the high concentrations of circulating **catecholamines** (epinephrine and norepinephrine) resulting in severe **hypertensive crisis** during surgical tumor manipulation.*Propranolol*- Propranolol and other pure **beta-blockers** should only be initiated *after* adequate **alpha-blockade** is established, typically to control tachycardia.- Giving a beta-blocker alone leaves the **alpha-1 receptors** unopposed, leading to unchecked systemic **vasoconstriction** and severe, potentially fatal, hypertension.*Prazosin*- Although Prazosin is an **alpha-1 selective blocker**, it is generally less preferred because it is a *reversible* antagonist.- **Phenoxybenzamine** is favored because its irreversible action provides superior, more stable, and prolonged protection against massive **catecholamine surges** during surgery.*Labetalol*- Labetalol is generally avoided for initial control because its **beta-blocking action** significantly outweighs its alpha-blocking action (high beta:alpha ratio).- This dominant beta blockade can induce **unopposed alpha vasoconstriction**, similar to pure beta-blockers, resulting in paradoxical and severe **hypertension**.

Question 534: Mechanism of action of botulinum toxin is:

A. Increases cAMP
B. Inhibits the release of acetylcholine (Correct Answer)
C. Inhibits the release of GABA
D. Inhibits Elongation Factor 2

Explanation: ***Inhibits the release of Acetylcholine***- Botulinum toxin (BoNT) is a **zinc metalloprotease** that cleaves **SNARE proteins** (e.g., SNAP-25) essential for vesicular fusion and neurotransmitter release.- By preventing the exocytosis of **acetylcholine (ACh)** vesicles at the **neuromuscular junction**, BoNT causes irreversible chemical denervation and resulting **flaccid paralysis**.*Inhibits Elongation factor 2*- Inhibition of **Elongation factor 2 (EF-2)** targets the host cell's **protein synthesis** machinery, leading to cell death.- This mechanism is characteristic of toxins like **Diphtheria toxin** and *Pseudomonas aeruginosa* **Exotoxin A**, not botulinum toxin.*Inhibits the release of GABA*- While BoNT can inhibit the release of various neurotransmitters, its defining clinical effect of **flaccid paralysis** is primarily due to inhibition of **acetylcholine** at the peripheral neuromuscular junction.- Inhibition of **GABA** (an inhibitory neurotransmitter) in the CNS would typically lead to increased CNS excitability or spasms, which is contrary to the effects of botulism.*Increases cAMP*- Increasing intracellular **cAMP** levels, usually via activation of **adenylate cyclase**, is the mechanism used by toxins such as **cholera toxin** and **pertussis toxin**.- This mechanism primarily causes large fluid shifts (diarrhea) or cellular dysfunction (like impaired phagocytosis) and is unrelated to botulinum toxin's paralytic action.

Question 535: What is the mechanism of action of botulinum toxin?

A. Blocks the postsynaptic acetylcholine receptor
B. Inhibits acetylcholine reuptake
C. Inhibits acetylcholine release from the presynaptic terminal (Correct Answer)
D. Inhibits acetylcholinesterase

Explanation: ***Inhibits acetylcholine release from the presynaptic terminal***- Botulinum toxin (BoNT) is a potent **neurotoxin** that acts by cleaving specific proteins known as **SNARE proteins** (e.g., **SNAP-25, VAMP, Syntaxin**).- The cleavage of these proteins prevents the fusion of **acetylcholine (ACh)-containing vesicles** with the presynaptic membrane, effectively blocking ACh release and causing **flaccid paralysis**.*Inhibits acetylcholine reuptake*- Acetylcholine is primarily inactivated by enzymatic degradation via **acetylcholinesterase (AChE)**, not by reuptake into the nerve terminal.- Inhibiting reuptake would typically lead to increased synaptic ACh levels, which is the opposite of the action of **botulinum toxin**.*Blocks the postsynaptic acetylcholine receptor*- This is the mechanism of action for neuromuscular blocking agents like **curare** and **non-depolarizing paralytics**.- **Botulinum toxin** targets the presynaptic terminal machinery responsible for **vesicle fusion**, not the postsynaptic receptors.*Inhibits acetylcholinesterase*- Drugs that inhibit this enzyme (like **neostigmine** or **organophosphates**) lead to high levels of ACh in the synapse, causing excessive stimulation and potentially **cholinergic crisis**.- This mechanism increases muscle tone and activity, whereas **botulinum toxin** causes profound muscle relaxation and **paralysis**.

Question 536: A patient came to the hospital with muscle weakness, diarrhea, and bradycardia. He was diagnosed with organophosphate poisoning for which atropine was administered. After atropine administration, which of the following changes are not seen?

A. Nasal discharge
B. Bradycardia
C. Muscle weakness (Correct Answer)
D. Diaphoresis

Explanation: ***Muscle weakness*** - Muscle weakness in organophosphate poisoning is a **nicotinic effect** caused by excessive acetylcholine at the **neuromuscular junction (NMJ)** - This leads to initial fasciculations followed by depolarization blockade and paralysis - **Atropine is a muscarinic antagonist** and has **no effect on nicotinic receptors** - Therefore, atropine does **not reverse muscle weakness** - this requires **pralidoxime** (2-PAM) to reactivate acetylcholinesterase - Muscle weakness is the change that is **NOT SEEN** after atropine administration *Bradycardia* - Bradycardia is a **muscarinic effect** on the heart (M2 receptors on SA node) - Atropine effectively blocks cardiac muscarinic receptors, leading to **increased heart rate** - This change (reversal of bradycardia) **IS SEEN** with atropine administration *Diaphoresis* - Excessive sweating is mediated by **muscarinic receptors** on eccrine sweat glands - Atropine blocks these receptors, causing **dry skin** (anticholinergic effect) - This change (cessation of sweating) **IS SEEN** with atropine *Nasal discharge* - Rhinorrhea (nasal secretions) is a **muscarinic effect** - part of SLUDGE syndrome - Atropine blocks glandular muscarinic receptors, causing **drying of secretions** - This change (reduction in nasal discharge) **IS SEEN** with atropine **Key Concept**: Atropine reverses only **muscarinic** effects (SLUDGE, bradycardia, bronchospasm) but NOT **nicotinic** effects (muscle weakness, fasciculations). Pralidoxime is needed for nicotinic symptoms.

Question 537: Which of the following conditions is not reversed with the administration of atropine?

A. Bradycardia
B. Bronchoconstriction
C. Excessive salivation
D. Muscle weakness due to neuromuscular blockade (Correct Answer)

Explanation: ***Muscle weakness due to neuromuscular blockade*** - Atropine is a **muscarinic receptor antagonist** and therefore has no direct effect on the **nicotinic receptors** responsible for neuromuscular transmission and muscle strength. - The reversal of **non-depolarizing neuromuscular blockade** requires increasing acetylcholine via a **cholinesterase inhibitor** (e.g., neostigmine), which acts at the **neuromuscular junction**. *Bradycardia* - Atropine blocks **M2 receptors** on the heart's sinoatrial (SA) node, inhibiting **vagal tone** and increasing the heart rate, thereby reversing bradycardia. - It is the primary drug used to treat hemodynamically significant **bradyarrhythmias**. *Excessive salivation* - Atropine is an anticholinergic agent that blocks **M3 receptors** on exocrine glands, effectively reducing all secretions, including **saliva**. - This is why it is often used as a pre-anesthetic agent to dry secretions and reduce the risk of **aspiration**. *Bronchoconstriction* - Atropine blocks **M3 receptors** in the bronchial smooth muscle, leading to relaxation and **bronchodilation**. - While effective, related synthetic compounds like **ipratropium** are often preferred for treating bronchoconstriction due to fewer systemic anticholinergic side effects.

Question 538: The most appropriate description of the vasomotor reversal of Dale is that

A. Repeated administration of ephedrine decreases its effect on blood pressure
B. An increase in pulse pressure is produced by the intravenous administration of isoprenaline
C. A patient pretreated with phentolamine develops severe hypotension on the administration of adrenaline (Correct Answer)
D. High dose of acetylcholine after alpha blockade produces vasomotor reversal

Explanation: ***A patient pretreated with phentolamine develops severe hypotension on the administration of adrenaline***%@%@%@%@In the **vasomotor reversal of Dale**, the typical pressor (vasoconstrictor) response of **adrenaline** is reversed to a depressor (vasodilator) response after the **alpha-receptors** have been non-selectively blocked.%@%@%@%@**Phentolamine** (an alpha-blocker) abolishes the alpha-1 mediated vasoconstriction, thus unmasking the predominant **beta-2 mediated vasodilation** effect of adrenaline, leading to **hypotension**.%@%@*Repeated administration of ephedrine decreases its effect on blood pressure*%@%@This describes **tachyphylaxis**, a rapid decrease in response to a drug following repeated administration over a short period.%@%@**Ephedrine** is an indirect sympathomimetic whose reduced effect is due to the rapid depletion of stored **norepinephrine** from the nerve terminals it acts upon.%@%@*High dose of acetylcholine after*%@%@The reversal of Dale is fundamentally an interaction within the **adrenergic system** (alpha and beta receptors) and requires an alpha-receptor blocker.%@%@**Acetylcholine** is the primary neurotransmitter of the **cholinergic system**, and its effects or paradoxical reversal effects do not define the vasomotor reversal of adrenaline.%@%@*An increase in pulse pressure is produced by the intravenous administration of isoprenaline*%@%@This is the expected, direct pharmacological effect of **isoprenaline** (isoproterenol), a potent non-selective **beta-agonist** (B1 and B2).%@%@Isoprenaline increases pulse pressure by significantly increasing cardiac output (B1) and causing marked peripheral vasodilation (B2), but this does not involve the required alpha-blockade and subsequent drug reversal.

Question 539: Which drug elicits the following response on blood pressure and heart rate, as shown in the image?

A. Epinephrine
B. Dopamine
C. Isoproterenol
D. Norepinephrine (Correct Answer)

Explanation: ***Norepinephrine*** - Norepinephrine is a potent agonist at **α1** and **β1** receptors with minimal **β2** activity. The strong **α1** stimulation causes intense vasoconstriction, leading to a marked increase in systolic, diastolic, and mean arterial pressure, as seen in the graph. - Although norepinephrine directly stimulates the heart via **β1** receptors, the significant rise in blood pressure activates the **baroreceptor reflex**. This reflex increases vagal tone, which overrides the direct chronotropic effect and results in a net decrease in heart rate (reflex bradycardia). *Isoproterenol* - Isoproterenol is a non-selective **β-agonist** (**β1** and **β2**) and lacks **α-agonist** effects. It would cause a significant increase in heart rate (**β1** effect). - Its potent **β2** receptor stimulation leads to vasodilation and a *decrease* in diastolic and mean arterial pressure, which is the opposite of the response shown. *Epinephrine* - Epinephrine stimulates **α1**, **β1**, and **β2** receptors. At typical doses, the direct **β1** effect increases the heart rate, and the **β2** effect partially counteracts **α1**-mediated vasoconstriction, leading to a smaller rise in diastolic pressure. - The pronounced reflex bradycardia and significant increase in both systolic and diastolic pressure are more characteristic of norepinephrine's lack of **β2** agonism. *Dopamine* - Dopamine's effects are dose-dependent. At pressor doses (high doses) that stimulate **α1** receptors to increase blood pressure, there is also significant **β1** receptor stimulation. - The concurrent **β1** stimulation typically causes tachycardia or prevents significant reflex bradycardia, which is inconsistent with the graph showing a decreased heart rate.

Question 540: Which of the following statement is correct regarding the mechanism of action of labetalol?

A. Blocks both alpha and beta-adrenergic receptors. (Correct Answer)
B. Primarily acts as a vasodilator with little effect on arterioles
C. Decreases peripheral vascular resistance by directly acting as a relaxant for vascular smooth muscles.
D. Directly acts as arterial vasodilator resulting in indirect effect tachycardia.

Explanation: ***Blocks both alpha and beta-adrenergic receptors*** - Labetalol is a unique **non-selective beta-blocker** (blocking $\beta_1$ and $\beta_2$) that also possesses **selective $\alpha_1$-adrenergic blocking activity**. - This combined mechanism makes it effective for conditions like **hypertensive emergencies**, providing both reduced heart rate (via beta block) and peripheral vasodilation (via alpha block). *Directly acts as arterial vasodilator resulting in indirect effect tachycardia* - While labetalol causes **vasodilation** (due to $\alpha_1$ blockade), it also **blocks $\beta$-receptors**, preventing the typical **reflex tachycardia** that occurs with direct vasodilators like hydralazine. - Its primary mechanism is receptor blockade, not direct smooth muscle relaxation. *Primarily acts as a vasodilator with little effect on arterioles* - Labetalol acts as a potent **arteriolar vasodilator** (via $\alpha_1$ blockade), which is crucial for lowering systemic vascular resistance and blood pressure. - Its vasodilatory effect is significant and targeted at arterioles. *Decreases peripheral vascular resistance by directly acting as a relaxant for vascular smooth muscles* - Labetalol decreases **peripheral vascular resistance (PVR)** through **antagonism of $\alpha_1$ receptors** in vascular smooth muscles, preventing vasoconstriction. - It does not act as a direct smooth muscle relaxant like nitroprusside or hydralazine.

Practice by Chapter

Cholinergic Agonists

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Cholinergic Antagonists

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Neuromuscular Blocking Agents

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Autonomic Drugs in Cardiovascular Disease

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