Autonomic Nervous System Drugs Practice Questions

Q: Which muscarinic receptor causes vasodilation?

M3. **Explanation:** The correct answer is **M3**. **Mechanism of Vasodilation:** Muscarinic receptors are not typically found on the smooth muscle of blood vessels in a way that receives direct parasympathetic innervation. However, **M3 receptors** are present on the **vascular endothelium**. When these receptors are stimulated by a muscarinic agonist (like Acetylcholine), they activate the Gq-phospholipase C pathway, leading to an increase in intracellular calcium. This triggers the enzyme **Nitric Oxide Synthase (NOS)** to produce **Nitric Oxide (NO)**, also known as Endothelium-Derived Relaxing Factor (EDRF). NO diffuses into the adjacent vascular smooth muscle, increases cGMP, and causes relaxation (vasodilation). *Note: If the endothelium is damaged, M3 stimulation directly on smooth muscle causes vasoconstriction.* **Why other options are incorrect:** * **M1:** These are primarily "Neural" receptors located in the CNS and gastric parietal cells (increasing HCl secretion). They are not involved in systemic vasodilation. * **M2:** These are "Cardiac" receptors located in the SA and AV nodes. Their stimulation leads to a decrease in heart rate (negative chronotropy) and conduction velocity via Gi protein (decreasing cAMP). * **M4:** These are primarily located in the CNS and act via Gi proteins to inhibit adenylate cyclase; they do not play a role in peripheral vascular tone. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Receptors:** M1, M3, M5 are **Gq** coupled (Stimulatory); M2, M4 are **Gi** coupled (Inhibitory). * **The "Rule of 1, 2, 3":** M1 = Brain/Glands; M2 = Heart; M3 = Smooth Muscle/Exocrine Glands/Endothelium. * **Atropine** is a competitive antagonist for all these receptors; it can block the vasodilatory effect of injected choline esters.

Q: Which one of the following is FALSE regarding atropine?

Long acting miotic. ### Explanation **Why Option D is the Correct Answer (The False Statement):** Atropine is a potent **competitive antagonist at muscarinic receptors**. In the eye, it blocks the $M_3$ receptors on the circular muscles (sphincter pupillae) of the iris. This leads to relaxation of the sphincter muscle, resulting in **Mydriasis** (dilation of the pupil), not miosis. Therefore, Atropine is a **long-acting Mydriatic** (and cycloplegic), with effects lasting up to 7–10 days. **Analysis of Incorrect Options (True Statements):** * **Option A:** In inflammatory conditions like iridocyclitis, atropine is used to prevent **synechiae** (adhesions between the iris and lens). By keeping the pupil dilated and the ciliary muscle paralyzed, it reduces pain and prevents the iris from sticking to the lens. * **Option B:** The standard clinical concentration of Atropine sulfate used in ophthalmic practice for adults is **1%** (drops or ointment). * **Option C:** Atropine is the **drug of choice for symptomatic sinus bradycardia**. It blocks the vagal (parasympathetic) input to the SA node, thereby increasing the heart rate. **High-Yield Clinical Pearls for NEET-PG:** 1. **Cycloplegia:** Atropine causes paralysis of the ciliary muscle, leading to loss of accommodation. It is the most potent cycloplegic available. 2. **Contraindication:** Atropine is strictly contraindicated in patients with **Angle-Closure Glaucoma**, as mydriasis can further obstruct the aqueous outflow. 3. **Antidote:** Physostigmine (a tertiary amine) is the preferred antidote for atropine poisoning because it can cross the blood-brain barrier. 4. **Duration:** Atropine has the longest duration of action among mydriatics; for shorter procedures, Tropicamide (6 hours) or Cyclopentolate (24 hours) are preferred.

Q: Stimulation of which receptor will lead to the release of renin?

Beta-1. **Explanation:** The release of renin from the **Juxtaglomerular (JG) cells** of the kidney is primarily mediated by the sympathetic nervous system through **Beta-1 ($\beta_1$) receptors**. When these receptors are stimulated, they activate the Gs-protein-adenylyl cyclase pathway, increasing intracellular cAMP, which triggers the exocytosis of renin into the bloodstream. This is the rate-limiting step of the Renin-Angiotensin-Aldosterone System (RAAS). **Analysis of Options:** * **Beta-1 (Correct):** Predominantly located in the heart (increasing heart rate and contractility) and the JG cells of the kidney (stimulating renin release). * **Alpha-1 ($\alpha_1$):** Found mainly in vascular smooth muscle; stimulation causes vasoconstriction and pupillary dilation (mydriasis), but does not directly trigger renin release. * **Alpha-2 ($\alpha_2$):** These are primarily presynaptic inhibitory receptors. Stimulation in the CNS decreases sympathetic outflow, which would actually lead to a *decrease* in renin release. * **Beta-2 ($\beta_2$):** Primarily located in the bronchioles (bronchodilation) and skeletal muscle blood vessels (vasodilation). While they have metabolic effects, they are not the primary mediators of renin secretion. **High-Yield Clinical Pearls for NEET-PG:** 1. **Beta-Blockers and RAAS:** Non-selective and $\beta_1$-selective blockers (e.g., Propranolol, Atenolol) reduce blood pressure partly by inhibiting $\beta_1$-mediated renin release. 2. **Other Stimuli for Renin:** Apart from $\beta_1$ stimulation, renin is released in response to **decreased renal perfusion pressure** (baroreceptors in afferent arterioles) and **decreased sodium delivery** to the macula densa. 3. **Rule of 1:** Remember $\beta_1$ for **1** Heart and **1** Kidney (JG cells).

Q: What is the effect of sympathetic stimulation?

Vasoconstriction of skin and mucus membranes. The sympathetic nervous system (SNS) is the "fight or flight" system, designed to redirect blood flow to vital organs and prepare the body for physical exertion. ### **Explanation of the Correct Answer** **D. Vasoconstriction of skin and mucus membranes:** This occurs via the stimulation of **$\alpha_1$-adrenergic receptors**. During sympathetic activation, the body shunts blood away from non-essential peripheral areas (like the skin and mucosa) toward the skeletal muscles, heart, and brain. This is why a person may appear pale during a "fight or flight" response. ### **Analysis of Incorrect Options** * **A. Pupillary constriction:** This is a **parasympathetic** effect (miosis) mediated by $M_3$ receptors. Sympathetic stimulation causes **pupillary dilation (mydriasis)** via $\alpha_1$ receptors on the radial dilator pupillae muscle. * **B. Contraction of bladder detrusor:** This is a **parasympathetic** effect ($M_3$) to facilitate voiding. Sympathetic stimulation causes **relaxation** of the detrusor ($\beta_2/\beta_3$) and **contraction** of the internal sphincter ($\alpha_1$) to promote urinary retention. * **C. Bronchial smooth muscle contraction:** This is a **parasympathetic** effect ($M_3$). Sympathetic stimulation causes **bronchodilation** via **$\beta_2$ receptors** to increase airflow for oxygenation. ### **High-Yield NEET-PG Pearls** * **Dual Supply Exception:** Most blood vessels only have sympathetic innervation. Vasoconstriction is $\alpha_1$ mediated, while vasodilation in skeletal muscle is $\beta_2$ mediated. * **Sweat Glands:** Anatomically sympathetic but **cholinergic** (mediated by Acetylcholine acting on $M$ receptors). * **Mnemonic for SNS:** "Dilation" (Pupils, Bronchioles) and "Constriction" (Sphincters, Peripheral Vessels).

Q: Reflex tachycardia is most likely to occur after the systemic administration of which of the following drugs?

albuterol. **Explanation:** The correct answer is **albuterol**. This question tests your understanding of the baroreceptor reflex and the vascular effects of adrenergic agonists. **1. Why Albuterol is Correct:** Albuterol is a selective **$\beta_2$-adrenergic agonist**. While primarily used for bronchodilation, systemic administration causes stimulation of $\beta_2$ receptors in the peripheral vasculature, leading to **vasodilation** and a decrease in peripheral vascular resistance (PVR). This drop in blood pressure triggers the **baroreceptor reflex**, resulting in a compensatory increase in sympathetic outflow to the heart (reflex tachycardia). Additionally, at higher doses, albuterol may lose selectivity and directly stimulate $\beta_1$ receptors in the heart, further increasing the heart rate. **2. Why the Other Options are Incorrect:** * **Methoxamine & Phenylephrine:** Both are selective **$\alpha_1$-adrenergic agonists**. They cause potent vasoconstriction, which increases peripheral vascular resistance and blood pressure. This rise in pressure triggers the baroreceptor reflex to increase vagal (parasympathetic) tone, leading to **reflex bradycardia**, not tachycardia. * **Propranolol:** This is a non-selective **$\beta$-blocker**. It blocks $\beta_1$ receptors in the heart, directly causing a decrease in heart rate (**bradycardia**). It would actually inhibit a reflex tachycardic response. **Clinical Pearls for NEET-PG:** * **Rule of Thumb:** Drugs that cause significant vasodilation (e.g., Hydralazine, Nitrates, $\alpha$-blockers, $\beta_2$-agonists) typically cause reflex tachycardia. * **Vasoconstrictors** that increase mean arterial pressure (e.g., Phenylephrine, Norepinephrine) typically cause reflex bradycardia. * **Isoproterenol** ($\beta_1 + \beta_2$) causes the most profound tachycardia because it combines direct cardiac stimulation ($\beta_1$) with reflex tachycardia due to vasodilation ($\beta_2$).

Q: Which of the following is an alpha-1 blocker?

Tamsulosin. **Explanation:** The question asks for an alpha-1 blocker, and while options A, B, and C are all technically alpha-1 antagonists, **Tamsulosin** is the most specific answer in many clinical contexts due to its subtype selectivity. **1. Why Tamsulosin is the Correct Answer:** Tamsulosin is a **selective alpha-1A blocker**. Alpha-1A receptors are primarily located in the smooth muscle of the bladder neck and prostate. By blocking these, Tamsulosin causes muscle relaxation, improving urine flow in patients with **Benign Prostatic Hyperplasia (BPH)**. Unlike non-selective alpha-1 blockers, it has minimal effect on alpha-1B receptors (found in blood vessels), resulting in less orthostatic hypotension. **2. Analysis of Other Options:** * **Prazosin & Terazosin:** These are **non-selective alpha-1 blockers** (blocking 1A, 1B, and 1D subtypes). While they are used for BPH, they are also potent antihypertensives. In many MCQ formats, if a "uroselective" drug like Tamsulosin is present, it is highlighted for its specific clinical utility in BPH. * **Clonidine:** This is an **alpha-2 agonist** (central acting). It stimulates alpha-2 receptors in the medulla, decreasing sympathetic outflow, and is used as an antihypertensive. **3. NEET-PG High-Yield Pearls:** * **First-Dose Phenomenon:** Prazosin is notorious for causing marked orthostatic hypotension and syncope after the first dose. * **Uroselectivity:** Tamsulosin and Silodosin are the preferred "uroselective" agents for BPH because they do not require dose titration and have minimal cardiovascular side effects. * **IFIS:** Tamsulosin is associated with **Intraoperative Floppy Iris Syndrome (IFIS)** during cataract surgery; patients should inform their ophthalmologist if they are on this medication.

Q: Which of the following statements regarding the use of adrenaline in anaphylactic shock is true?

The usual dose is 0.5-1 mg by intramuscular route.. Adrenaline (Epinephrine) is the **drug of choice** for anaphylactic shock due to its rapid onset and multi-receptor action ($\alpha_1, \alpha_2, \beta_1, \beta_2$). ### **Explanation of Options** * **Option A (Correct):** The standard adult dose for anaphylaxis is **0.5 mg (range 0.3–0.5 mg)** of a **1:1000** solution administered via the **Intramuscular (IM)** route in the anterolateral thigh. IM is preferred over subcutaneous (SC) because it achieves higher and faster peak plasma concentrations. * **Option B (Incorrect):** While rare if dosed correctly, **cerebral hemorrhage** and cardiac arrhythmias are known life-threatening adverse effects of adrenaline, usually occurring due to rapid spikes in blood pressure if administered too quickly or in excessive doses. * **Option C (Incorrect):** Adrenaline has a very short half-life. In anaphylaxis, if the patient does not improve, the dose should be repeated every **5–15 minutes**, not every 2–4 hours. * **Option D (Incorrect):** The concentrations differ significantly. The **1:1000 (1 mg/ml)** solution is used for IM/SC routes. However, for **Intravenous (IV)** use, it must be diluted to **1:10,000 (0.1 mg/ml)** or even 1:100,000 to prevent fatal arrhythmias and severe hypertension. ### **High-Yield NEET-PG Pearls** * **Mechanism in Anaphylaxis:** $\alpha_1$ (vasoconstriction to treat hypotension/edema), $\beta_1$ (positive inotropy), and $\beta_2$ (bronchodilation and stabilization of mast cells). * **Route of Choice:** IM (Anterolateral thigh/Vastus lateralis) is the gold standard. * **Concentration Ratios:** * **1:1,000** = 1 mg in 1 ml (IM use) * **1:10,000** = 1 mg in 10 ml (IV use in Cardiac Arrest) * **Pediatric Dose:** 0.01 mg/kg (max 0.3 mg).

Question 1

Epinephrine is used for all the following, except?

Accepted Answer

Management of acute heart failure

Answer

To prolong the action of local anaesthetics

Answer

As a topical haemostatic agent

Answer

Management of cardiac arrest

Question 2

First dose hypotension is commonly seen with which of the following medications?

Accepted Answer

Prazosin

Answer

Yohimbine

Answer

Atenolol

Answer

Methyldopa

Question 3

Which muscarinic receptor causes vasodilation?

Accepted Answer

M3

Answer

M1

Answer

M2

Answer

M4

Question 4

d-Tubocurarine acts by:

Accepted Answer

Inhibiting nicotinic receptors at the myoneural junction

Answer

Inhibiting nicotinic receptors at the autonomic ganglion

Answer

Producing a depolarizing block

Answer

Inhibiting the reuptake of acetylcholine

Question 5

Which one of the following is FALSE regarding atropine?

Accepted Answer

Long acting miotic

Answer

Used to prevent adhesions in inflammatory conditions of the eye

Answer

Used in a concentration of 1%

Answer

Can be given in sinus bradycardia patients

Question 6

Stimulation of which receptor will lead to the release of renin?

Accepted Answer

Beta-1

Answer

Alpha-1

Answer

Alpha-2

Answer

Beta-2

Question 7

What is the effect of sympathetic stimulation?

Accepted Answer

Vasoconstriction of skin and mucus membranes

Answer

Pupillary constriction

Answer

Contraction of bladder detrusor

Answer

Bronchial smooth muscle contraction

Question 8

Reflex tachycardia is most likely to occur after the systemic administration of which of the following drugs?

Accepted Answer

albuterol

Answer

methoxamine

Answer

phenylephrine

Answer

propranolol

Question 9

Which of the following is an alpha-1 blocker?

Accepted Answer

Tamsulosin

Answer

Prazosin

Answer

Terazosin

Answer

Clonidine

Question 10

Which of the following statements regarding the use of adrenaline in anaphylactic shock is true?

Accepted Answer

The usual dose is 0.5-1 mg by intramuscular route.

Answer

Cerebral hemorrhage never occurs as an adverse effect to epinephrine when used in the treatment of anaphylactic shock.

Answer

It is repeated after every 2-4 hours.

Answer

The same solution can be given for subcutaneous as well as intravenous routes.

Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs — MCQs

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