Autonomic Nervous System Drugs Practice Questions

Q: Pirenzepine acts on which receptor?

Muscarinic. **Explanation:** **Pirenzepine** is a selective **Muscarinic (M1) receptor antagonist**. It belongs to the class of anticholinergic drugs. 1. **Why Muscarinic is correct:** Muscarinic receptors (M1–M5) are G-protein coupled receptors that mediate the parasympathetic effects of acetylcholine. Pirenzepine specifically targets **M1 receptors** located on gastric parietal cells and autonomic ganglia. By blocking these receptors, it reduces gastric acid secretion, which is why it was historically used in the treatment of peptic ulcers. 2. **Why other options are incorrect:** * **Nicotinic:** These are ligand-gated ion channels found at the neuromuscular junction (Nm) and autonomic ganglia (Nn). Drugs acting here include Succinylcholine or Hexamethonium, not Pirenzepine. * **Alpha & Beta:** These are adrenergic receptors that respond to adrenaline and noradrenaline. Drugs acting here (e.g., Prazosin or Propranolol) affect the sympathetic nervous system, not the parasympathetic system where Pirenzepine acts. **High-Yield Clinical Pearls for NEET-PG:** * **Selectivity:** While Pirenzepine is M1-selective, **Telenzepine** is another more potent M1 blocker. * **Clinical Use:** Its use has largely been superseded by Proton Pump Inhibitors (PPIs) and H2 blockers due to better efficacy and fewer side effects. * **M3 Antagonists:** For exams, remember **Darifenacin and Solifenacin** are selective M3 blockers used for overactive bladder (OAB). * **M2 Antagonists:** **Methoctramine** is a selective M2 blocker (primarily experimental). * **Rule of Odds:** M1, M3, and M5 are excitatory (Gq), while M2 and M4 are inhibitory (Gi).

Q: Which of the following antiglaucoma drugs can cause heterochromia iridis?

Latanoprost. **Latanoprost** is the correct answer. It is a **Prostaglandin F2α (PGF2α) analog**, which is currently the first-line treatment for Open-Angle Glaucoma. **Mechanism of Side Effect:**Latanoprost causes **heterochromia iridis** (specifically, increased brown pigmentation of the iris) by stimulating the proliferation and activity of melanocytes in the iris stroma [1]. This leads to increased melanin content. This change is often permanent. Other characteristic side effects of prostaglandin analogs include **hypertrichosis** (increased eyelash growth/thickness) and **periorbital fat atrophy).**Analysis of Incorrect Options:** * **A. Timolol:** A non-selective beta-blocker. It reduces aqueous humor production. Its primary side effects are local (stinging) and systemic (bradycardia, bronchospasm in asthmatics), but it does not affect iris color. * **C. Apraclonidine:** An alpha-2 agonist. It reduces aqueous production and increases uveoscleral outflow. Common side effects include lid retraction and local allergic reactions (blepharoconjunctivitis). * **D. Acetazolamide:** A carbonic anhydrase inhibitor. It reduces aqueous humor formation. Systemic side effects include paresthesia, metabolic acidosis, and hypokalemia. **High-Yield NEET-PG Pearls:** * **Mechanism of Action:** Prostaglandin analogs (Latanoprost, Bimatoprost, Travoprost) lower IOP by increasing **uveoscleral outflow**. * **Bimatoprost** is specifically FDA-approved for the treatment of eyelash hypotrichosis. * **Contraindication:** Prostaglandin analogs should be avoided in **uveitic glaucoma** as they may exacerbate intraocular inflammation (cystoid macular edema).

Q: Which of the following is NOT a cholinergic action?

Mydriasis. **Explanation:** Cholinergic actions are mediated by the parasympathetic nervous system (PNS) through the neurotransmitter Acetylcholine (ACh) acting on Muscarinic (M) receptors. To remember these actions, use the mnemonic **DUMBBELLS** (Diarrhea, Urination, Miosis, Bradycardia, Bronchoconstriction, Emesis, Lacrimation, Lethargy, Salivation/Sweating). **Why Mydriasis is the Correct Answer:** **Mydriasis** refers to the dilation of the pupil, which is a **Sympathetic (Adrenergic)** action mediated by $\alpha_1$ receptors on the radial dilator pupillae muscle. In contrast, cholinergic stimulation causes **Miosis** (pupillary constriction) by acting on the $M_3$ receptors of the circular sphincter pupillae muscle. **Analysis of Incorrect Options:** * **Salivation:** Cholinergic stimulation of $M_3$ receptors in the salivary glands increases secretions. * **Sweating:** Although mediated by the sympathetic nervous system anatomically, the postganglionic fibers to eccrine sweat glands are **cholinergic** (releasing ACh onto $M_3$ receptors). Thus, sweating is a cholinergic effect. * **Bradycardia:** ACh acts on $M_2$ receptors in the SA node of the heart to decrease the heart rate (negative chronotropy). **High-Yield Clinical Pearls for NEET-PG:** 1. **Accommodation:** Cholinergic drugs cause contraction of the ciliary muscle ($M_3$), leading to "spasm of accommodation" and a decrease in intraocular pressure (useful in Glaucoma). 2. **Sweat Glands Exception:** Remember that sweat glands are the "exception to the rule"—they are sympathetic in origin but **cholinergic** in transmission. 3. **Atropine:** As a muscarinic antagonist, Atropine will cause the opposite effects: Mydriasis, dry mouth, and tachycardia.

Q: Which of the following receptors does not require the action of adrenaline for the treatment of anaphylactic shock?

α2. **Explanation:** In the management of anaphylactic shock, **Adrenaline (Epinephrine)** is the drug of choice because it acts as a physiological antagonist to the mediators of anaphylaxis (like histamine). Its therapeutic efficacy is derived from its action on **$\alpha_1$, $\beta_1$, and $\beta_2$ receptors**, while $\alpha_2$ stimulation plays no significant role in treating the acute crisis. * **Why $\alpha_2$ is the correct answer:** $\alpha_2$ receptors are primarily located presynaptically and function to inhibit the release of norepinephrine (negative feedback). Stimulation of $\alpha_2$ receptors does not contribute to the reversal of shock; in fact, it could theoretically decrease sympathetic outflow, which is counterproductive in a circulatory collapse. **Role of other receptors (Incorrect Options):** * **$\alpha_1$ (Option A):** Stimulation causes **vasoconstriction**, which increases peripheral vascular resistance and blood pressure, effectively reversing the hypotension and reducing mucosal edema (e.g., laryngeal edema). * **$\beta_1$ (Option C):** Stimulation increases **myocardial contractility (inotropy) and heart rate (chronotropy)**, improving cardiac output to combat shock. * **$\beta_2$ (Option D):** This is a life-saving action that causes **bronchodilation** to relieve bronchospasm and **stabilizes mast cells**, preventing further release of inflammatory mediators. **NEET-PG High-Yield Pearls:** * **Drug of Choice:** Adrenaline is the DOC for Anaphylactic Shock. * **Route of Administration:** **Intramuscular (IM)** in the anterolateral thigh (vastus lateralis) is preferred over SC or IV in the initial setting due to faster absorption and safety. * **Concentration:** 1:1000 (1 mg/ml) for IM; 1:10,000 (0.1 mg/ml) for IV. * **Standard Dose:** 0.5 mg IM for adults; 0.01 mg/kg for children.

Q: Which of the following sympathomimetic amines has agonistic action on a1, a2, b1, and b3 adrenoceptors, but not on b2 receptors?

Noradrenaline. **Explanation:** The correct answer is **Noradrenaline (Norepinephrine)**. **1. Why Noradrenaline is Correct:** Noradrenaline is a potent agonist at **$\alpha_1$, $\alpha_2$, and $\beta_1$** receptors. Crucially, it has **negligible or no action on $\beta_2$ receptors**. Recent pharmacological studies also confirm its agonistic activity on **$\beta_3$** receptors (involved in thermogenesis and lipolysis). Because it lacks $\beta_2$ activity (which mediates vasodilation), Noradrenaline causes intense peripheral vasoconstriction, leading to a significant rise in both systolic and diastolic blood pressure. **2. Why the Other Options are Incorrect:** * **Phenylephrine (Option A):** This is a selective **$\alpha_1$ agonist**. It lacks significant action on $\beta$ receptors. * **Isoprenaline (Option B):** This is a non-selective **$\beta$ agonist** ($\beta_1$, $\beta_2$, and $\beta_3$). It has virtually no action on $\alpha$ receptors. * **Adrenaline (Option C):** Adrenaline is a potent agonist at **all** adrenoceptors: $\alpha_1$, $\alpha_2$, $\beta_1$, **and $\beta_2$**. The presence of $\beta_2$ activity distinguishes it from Noradrenaline and allows it to cause bronchodilation and vasodilation in skeletal muscle beds. **3. NEET-PG High-Yield Pearls:** * **Drug of Choice:** Noradrenaline is the first-line vasopressor for **Septic Shock**. * **Reflex Bradycardia:** Unlike Adrenaline, Noradrenaline often causes a "reflex bradycardia" because the intense $\alpha_1$-mediated vasoconstriction triggers the baroreceptor reflex, which overrides its direct $\beta_1$ stimulatory effect on the heart rate. * **Metabolism:** Both Adrenaline and Noradrenaline are metabolized by **COMT** and **MAO**; the end product measured in urine is **Vanillylmandellic acid (VMA)**. * **Rule of Thumb:** If a question mentions "no $\beta_2$ action" among catecholamines, always think of Noradrenaline.

Q: What is the antidote for datura poisoning?

Physostigmine. **Explanation:** Datura poisoning is characterized by **anticholinergic toxicity** (the "central anticholinergic syndrome"), caused by tropane alkaloids like scopolamine and hyoscyamine. These substances cross the blood-brain barrier (BBB), leading to both peripheral symptoms (tachycardia, dry mouth, dilated pupils) and central symptoms (delirium, hallucinations, seizures). **Why Physostigmine is the Correct Answer:** Physostigmine is a **tertiary amine** acetylcholinesterase inhibitor. Unlike other carbamates, its uncharged structure allows it to **cross the blood-brain barrier**. By inhibiting the breakdown of acetylcholine, it increases synaptic concentrations of the neurotransmitter, effectively reversing both the peripheral and, crucially, the **central neurotoxic effects** of Datura. **Analysis of Incorrect Options:** * **Neostigmine & Pyridostigmine:** These are **quaternary ammonium** compounds. They are polar/charged and **cannot cross the blood-brain barrier**. While they might reverse peripheral symptoms, they are ineffective against the life-threatening CNS manifestations of Datura poisoning. * **Atropine:** Atropine is a competitive muscarinic antagonist. Since Datura poisoning is already a state of atropine-like toxicity, administering more atropine would worsen the condition. (Note: Atropine is the antidote for organophosphate poisoning, not Datura). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Datura/Atropine Toxicity:** "Mad as a hatter (delirium), Red as a beet (flushing), Dry as a bone (anhidrosis), Blind as a bat (mydriasis), and Hot as a hare (hyperthermia)." * **Physostigmine Precaution:** It should be administered via slow IV injection. Rapid administration can cause bradycardia or seizures. It is contraindicated in patients with TCA (Tricyclic Antidepressant) overdose due to increased cardiotoxicity. * **DOC:** Physostigmine is also the drug of choice for Belladonna poisoning.

Question 1

Which antihistamine is used in motion sickness?

Accepted Answer

Diphenhydramine

Answer

Cetirizine

Answer

Meclizine

Answer

Fexofenadine

Question 2

Pirenzepine acts on which receptor?

Accepted Answer

Muscarinic

Answer

Nicotinic

Answer

Alpha

Answer

Beta

Question 3

Which of the following statements regarding adrenergic receptors is NOT true?

Accepted Answer

Alpha-1 receptors are usually postsynaptic

Answer

Beta-1 receptors are predominantly found in the heart

Answer

Noradrenaline stimulates beta-1 receptors

Answer

Alpha-2 receptor stimulation inhibits transmitter release

Question 4

Which of the following antiglaucoma drugs can cause heterochromia iridis?

Accepted Answer

Latanoprost

Answer

Timolol

Answer

Apraclonidine

Answer

Acetazolamide

Question 5

The beneficial effect of neostigmine in the treatment of Myasthenia gravis is due to which action?

Accepted Answer

It inhibits the action of cholinesterase

Answer

It produces more acetylcholine

Answer

It produces more acetylcholine receptors

Answer

It increases the action of cholinesterase

Question 6

Which of the following is NOT a cholinergic action?

Accepted Answer

Mydriasis

Answer

Salivation

Answer

Sweating

Answer

Bradycardia

Question 7

Which of the following receptors does not require the action of adrenaline for the treatment of anaphylactic shock?

Accepted Answer

α2

Answer

α1

Answer

β1

Answer

β2

Question 8

All of the following are actions of muscarinic antagonists except?

Accepted Answer

Prolonged AV conduction

Answer

Decreased gastric secretion

Answer

Decreased tracheobronchial secretion

Answer

Contraction of radial muscle of iris

Question 9

Which of the following sympathomimetic amines has agonistic action on a1, a2, b1, and b3 adrenoceptors, but not on b2 receptors?

Accepted Answer

Noradrenaline

Answer

Phenylephrine

Answer

Isoprenaline

Answer

Adrenaline

Question 10

What is the antidote for datura poisoning?

Accepted Answer

Physostigmine

Answer

Neostigmine

Answer

Pyridostigmine

Answer

Atropine

Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs — MCQs

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