Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs Indian Medical PG Practice Questions and MCQs

Question 591: What is the mechanism of action of neuromuscular blocking agents such as curare?

A. Inhibits ACh synthesis
B. Causes persistent depolarization
C. Acts as an agonist at ACh receptors
D. Antagonizes ACh receptors (Correct Answer)

Explanation: ***Antagonizes ACh receptors*** - Curare and related non-depolarizing neuromuscular blocking agents act as **competitive antagonists** at the **nicotinic acetylcholine (ACh) receptors** on the motor end plate of skeletal muscle. - By binding to these receptors without activating them, they prevent ACh from binding and initiating muscle contraction, leading to **flaccid paralysis**. *Inhibits ACh synthesis* - Drugs that inhibit **ACh synthesis**, such as hemicholinium, interfere with the uptake of choline into the presynaptic neuron. - This mechanism would eventually reduce ACh release but is not the direct action of curare-like agents, which block postsynaptic receptors. *Causes persistent depolarization* - This is the mechanism of action of **depolarizing neuromuscular blockers** such as **succinylcholine**, which activate the ACh receptor and cause prolonged depolarization, rendering the muscle unresponsive. - Curare-like drugs are **non-depolarizing** and do not cause persistent depolarization. *Acts as an agonist at ACh receptors* - A drug that acts as an **agonist at ACh receptors** would **activate muscle contraction**, initially causing fasciculations followed by paralysis due to persistent depolarization (as seen with succinylcholine). - Curare, however, **blocks** these receptors, preventing activation.

Question 592: Which of the following substances is commonly known as an arrow poison used by indigenous South American tribes?

A. Opium
B. Curare (Correct Answer)
C. Cannabis
D. Cyanide

Explanation: ***Curare*** - **Curare** is the traditional name for South American arrow poisons derived from plants, primarily *Chondrodendron tomentosum* and *Strychnos* species - It acts as a **competitive non-depolarizing neuromuscular blocking agent**, blocking nicotinic receptors at the neuromuscular junction - Causes **skeletal muscle paralysis** by competing with acetylcholine, leading to respiratory failure in prey - **Clinical relevance:** Tubocurarine (d-tubocurarine), derived from curare, was historically used as a muscle relaxant in surgery; modern derivatives include atracurium, vecuronium, and rocuronium *Opium* - **Opium** is derived from *Papaver somniferum* (opium poppy) and contains alkaloids like morphine and codeine - Acts on **opioid receptors** in the CNS to produce analgesia and sedation - Not used as an arrow poison by South American tribes; its effects are analgesic rather than paralytic *Cannabis* - **Cannabis** (*Cannabis sativa*) contains psychoactive compounds like THC (tetrahydrocannabinol) - Acts on **cannabinoid receptors** producing psychoactive and analgesic effects - Not used as an arrow poison; lacks the rapid paralytic action needed for hunting *Cyanide* - **Cyanide** inhibits cytochrome c oxidase, blocking cellular respiration and causing rapid cell death - While highly toxic, it is **not the traditional arrow poison** of South American indigenous tribes - Traditional arrow poisons like curare cause neuromuscular paralysis rather than cellular asphyxiation

Question 593: Which selective alpha-2 agonist is used in the treatment of glaucoma?

A. Epinephrine
B. Dipivefrine
C. Brimonidine (Correct Answer)
D. Timolol

Explanation: ***Brimonidine*** - **Brimonidine** is an **alpha-2 adrenergic agonist** specifically designed for use in glaucoma treatment [1]. - It works by reducing **aqueous humor production** and increasing **uveoscleral outflow**, thereby lowering intraocular pressure [1]. *Timolol* - **Timolol** is a **non-selective beta-blocker** used in glaucoma to reduce **aqueous humor production**. - It does not primarily act as an alpha-2 agonist and has a different mechanism of action. *Epinephrine* - **Epinephrine** is a **non-selective adrenergic agonist** with both alpha- and beta-receptor activity. - While it can lower intraocular pressure, its broad effects and potential side effects make it less suitable as a first-line agent, and it is not a selective alpha-2 agonist. *Dipivefrine* - **Dipivefrine** is a **prodrug of epinephrine** that is converted to epinephrine within the eye. - Like epinephrine, it is a non-selective adrenergic agonist and not a selective alpha-2 agonist.

Question 594: In primary open-angle glaucoma, pilocarpine eye drops lower intraocular pressure primarily by acting on which of the following?

A. All of the options
B. Trabecular meshwork
C. Ciliary epithelium
D. Longitudinal fibres of the ciliary muscle (Correct Answer)

Explanation: ***Longitudinal fibres of the ciliary muscle***- Pilocarpine is a **muscarinic agonist** that contracts the **longitudinal fibers of the ciliary muscle** [1, 3].- This contraction pulls on the **scleral spur**, separating the **trabecular meshwork** sheets, which increases conventional **aqueous humor outflow** [2, 3].*Trabecular meshwork*- While the **trabecular meshwork** is the site where aqueous humor exits the eye, pilocarpine primarily acts on the ciliary muscle to **indirectly affect** the meshwork's outflow facility [2, 3].- Pilocarpine does not directly alter the structure or function of the trabecular meshwork cells.*Ciliary epithelium*- The **ciliary epithelium** is responsible for **aqueous humor production** [1, 2].- Pilocarpine primarily affects **outflow**, not production, through its action on the ciliary muscle [1, 2].*All of the options*- Pilocarpine does not act on **all** these structures; its primary mechanism is through the ciliary muscle to enhance outflow.- It has no direct significant effect on **ciliary epithelium** or direct action on the **trabecular meshwork** itself.

Question 595: Which antimuscarinic drug is used in overactive bladder?

A. Trospium (Correct Answer)
B. Atropine
C. Tropicamide
D. Pirenzepine

Explanation: ***Trospium*** - **Trospium** is a quaternary ammonium compound that acts as an **antimuscarinic agent** primarily used to treat **overactive bladder (OAB)**. - Its **polar nature** limits its ability to cross the blood-brain barrier, reducing central nervous system side effects common with other antimuscarinics. *Tropicamide* - **Tropicamide** is an **antimuscarinic** agent primarily used as a **mydriatic** (to dilate pupils) and **cycloplegic** (to paralyze the ciliary muscle) for ophthalmic examinations. - It has a short duration of action, making it unsuitable for chronic conditions like overactive bladder. *Atropine* - **Atropine** is a non-selective **muscarinic antagonist** with a wide range of uses, including bradycardia, organophosphate poisoning, and ophthalmic procedures. - While it has antimuscarinic effects on the bladder, its systemic side effects (e.g., dry mouth, blurred vision, tachycardia) limit its use for overactive bladder. *Pirenzepine* - **Pirenzepine** is a selective **M1 muscarinic antagonist** primarily used to treat **peptic ulcers** by reducing gastric acid secretion. - Its selectivity for M1 receptors means it has limited efficacy for relieving bladder symptoms, which are primarily mediated by M2 and M3 receptors.

Question 596: Which sympathomimetic drug is primarily known to increase heart rate?

A. Isoprenaline (Correct Answer)
B. Phenylephrine
C. Noradrenaline
D. Adrenaline

Explanation: ***Isoprenaline*** - **Isoprenaline** (isoproterenol) is a non-selective beta-adrenergic agonist, with a strong affinity for **β1 and β2 receptors** [1]. - Its activation of **β1 receptors** in the heart leads to a significant increase in **heart rate (positive chronotropy)** and contractility (positive inotropy) [1]. - It is the **most potent chronotropic agent** among sympathomimetics and is primarily known for increasing heart rate [2]. *Phenylephrine* - **Phenylephrine** is a selective **α1 adrenergic agonist** that causes vasoconstriction [4]. - It increases blood pressure but typically causes **reflex bradycardia** (decreased heart rate) due to baroreceptor activation. - Does NOT directly increase heart rate. *Noradrenaline* - **Noradrenaline** (norepinephrine) primarily acts on **α1 receptors** causing vasoconstriction, and to a lesser extent on **β1 receptors** [3]. - While it can stimulate β1 receptors, its predominant effect is to increase **mean arterial pressure** through vasoconstriction, often causing **reflex bradycardia** [3]. *Adrenaline* - **Adrenaline** (epinephrine) acts on **α1, β1, and β2 receptors** [4]. While it does increase heart rate via **β1 receptor** stimulation, it also causes significant **vasoconstriction** (via α1) and **vasodilation** (via β2). - Its cardiovascular effects are more complex and dose-dependent compared to isoprenaline's specific chronotropic action.

Question 597: Which of the following is NOT a side effect of atropine?

A. Diarrhoea (Correct Answer)
B. Urinary retention
C. Confusion of elderly
D. Blurring of vision

Explanation: ***Diarrhoea*** - Atropine is a **muscarinic antagonist** that blocks the action of **acetylcholine** on muscarinic receptors in the gastrointestinal tract. - This leads to **decreased GI motility** and **decreased secretions**, typically causing **constipation**, NOT diarrhoea. - Diarrhoea would be associated with **cholinergic agonists** or anticholinesterases, which increase GI motility. *Blurring of vision* - Atropine causes **mydriasis** (pupil dilation) and **cycloplegia** (paralysis of the ciliary muscle). - **Cycloplegia** impairs accommodation for near vision, leading to **blurring of vision**. - This is a common anticholinergic side effect. *Urinary retention* - Atropine blocks **M3 receptors** on the **detrusor muscle**, causing bladder relaxation and reduced contractility. - This leads to **urinary retention**, especially in patients with pre-existing conditions like **prostatic hypertrophy**. *Confusion in elderly* - Atropine can cross the **blood-brain barrier** and cause **CNS effects** including confusion, agitation, and delirium. - Elderly patients are particularly susceptible to these **central anticholinergic effects**.

Question 598: Hyoscine is an antagonist at which cholinergic receptor?

A. Muscarinic (Correct Answer)
B. Nicotinic
C. Both
D. None of the above

Explanation: ***Muscarinic*** - **Hyoscine** (scopolamine) is a well-known **antagonist** at **muscarinic cholinergic receptors**. - It blocks the action of **acetylcholine** at these receptors, leading to its anticholinergic effects like treating motion sickness and reducing secretions. *Nicotinic* - **Hyoscine** does not primarily act on **nicotinic cholinergic receptors**. - Drugs acting on nicotinic receptors include **neuromuscular blockers** (e.g., succinylcholine, rocuronium) and **ganglionic blockers**, which have different clinical applications. *Both* - While some drugs may have activity at both receptor types, **hyoscine's primary and clinically significant antagonism is at muscarinic receptors**. - Its therapeutic effects are attributed almost exclusively to its **muscarinic blockade**. *None of the above* - This option is incorrect because **hyoscine is a clear antagonist at muscarinic cholinergic receptors**. - Its widespread use in medicine is based on this specific pharmacological action.

Question 599: What is a contraindication of antimuscarinic drugs?

A. Peptic ulcer
B. Glaucoma (Correct Answer)
C. Asthma
D. Urinary incontinence

Explanation: ***Correct: Glaucoma*** - Antimuscarinic drugs cause **mydriasis (pupil dilation)** and **cycloplegia**, which increases intraocular pressure, especially in individuals with **narrow-angle glaucoma**. - In narrow-angle glaucoma, pupil dilation causes the peripheral iris to bunch up and block the trabecular meshwork, obstructing aqueous humor outflow. - This can precipitate an **acute angle-closure glaucoma attack**, a medical emergency, making glaucoma an **absolute contraindication** to antimuscarinic drugs. - This is one of the most important contraindications to remember for all anticholinergic medications. *Incorrect: Peptic ulcer* - Antimuscarinic drugs were **historically used to treat** peptic ulcer disease by reducing gastric acid secretion and gastrointestinal motility. - While they are no longer first-line therapy (replaced by proton pump inhibitors and H2 blockers), peptic ulcer is **not a contraindication**. - The main reason they fell out of favor was due to side effects and less efficacy compared to modern alternatives, not because they worsen the condition. *Incorrect: Asthma* - Some antimuscarinics (e.g., **ipratropium, tiotropium**) are actually used as **bronchodilators** in asthma and COPD management. - They work by blocking muscarinic receptors in airway smooth muscle, causing bronchodilation. - Therefore, asthma is a **treatment indication**, not a contraindication. *Incorrect: Urinary incontinence* - Antimuscarinic drugs are the **primary pharmacological treatment** for overactive bladder and urge incontinence. - They work by blocking M3 muscarinic receptors in the detrusor muscle, reducing bladder contractions. - Common drugs include oxybutynin, tolterodine, and solifenacin. - Urinary incontinence is a **treatment indication**, not a contraindication.

Question 600: Which class of drugs does botulinum toxin mimic in its action?

A. Adrenergics
B. Antiadrenergic
C. Cholinergics
D. Anticholinergics (Correct Answer)

Explanation: ***Anticholinergics*** - **Botulinum toxin** inhibits the release of **acetylcholine** from **presynaptic nerve terminals** [2] by preventing vesicle fusion, leading to muscle paralysis [1]. - While the **mechanism differs** (botulinum acts presynaptically, anticholinergics act postsynaptically at receptors), the **functional outcome** is similar: reduced cholinergic neurotransmission. - In terms of **clinical effect** at the neuromuscular junction, both reduce acetylcholine's action, making anticholinergics the closest functional parallel among the given options. *Cholinergics* - **Cholinergics** enhance acetylcholine activity, either by increasing its release, mimicking its effects at receptors, or inhibiting its breakdown. - This is the **opposite** of botulinum toxin's action, which reduces acetylcholine's impact. *Adrenergics* - **Adrenergics** stimulate the **sympathetic nervous system** via **adrenergic receptors** (α and β receptors). - They act on **norepinephrine/epinephrine pathways**, not the cholinergic system where botulinum toxin acts. *Antiadrenergic* - **Antiadrenergic drugs** block **adrenergic receptors** or inhibit sympathetic activity. - These are unrelated to botulinum toxin's effect on **cholinergic neuromuscular transmission**.

Practice by Chapter

Cholinergic Agonists

Practice Questions

Cholinergic Antagonists

Practice Questions

Adrenergic Agonists

Practice Questions

Adrenergic Antagonists

Practice Questions

Ganglionic Agents

Practice Questions

Neuromuscular Blocking Agents

Practice Questions

Autonomic Drugs in Ophthalmology

Practice Questions

Autonomic Drugs in Cardiovascular Disease

Practice Questions

Autonomic Drugs in Respiratory Disease

Practice Questions

Autonomic Drugs in Urological Disorders

Practice Questions

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