Autonomic Nervous System Drugs Practice Questions

Q: Intravenous administration of norepinephrine in a patient already taking an effective dose of atropine will often:

Increase heart rate. **Explanation:** The correct answer is **A. Increase heart rate.** **Mechanism:** Norepinephrine (NE) is a potent agonist at $\alpha_1$, $\alpha_2$, and $\beta_1$ receptors. When administered alone, NE causes intense vasoconstriction ($\alpha_1$), leading to a significant rise in mean arterial pressure. This triggers a **compensatory baroreceptor reflex**, which increases vagal (parasympathetic) tone to the heart, resulting in **reflex bradycardia**. However, **Atropine** is a muscarinic antagonist that blocks the effects of the vagus nerve on the heart. By "pre-treating" with atropine, the baroreceptor-mediated reflex bradycardia is abolished. Consequently, the direct stimulatory effect of NE on cardiac **$\beta_1$ receptors** is unmasked, leading to an increase in heart rate (tachycardia). **Analysis of Incorrect Options:** * **B. Decrease total peripheral resistance:** Incorrect. NE causes profound vasoconstriction via $\alpha_1$ receptors, which significantly *increases* total peripheral resistance. * **C. Decrease pupil size:** Incorrect. NE acts on $\alpha_1$ receptors in the radial muscle of the iris to cause mydriasis (dilation), not miosis (contraction). * **D. Have no effect:** Incorrect. NE is a potent sympathomimetic with significant hemodynamic effects. **NEET-PG High-Yield Pearls:** * **Vagal Escape:** Atropine "unmasks" the direct $\beta_1$ effect of drugs that otherwise cause reflex bradycardia (like NE or Phenylephrine). * **Norepinephrine vs. Epinephrine:** NE lacks significant $\beta_2$ activity; therefore, it does not cause vasodilation or a drop in diastolic blood pressure, unlike Epinephrine. * **Drug of Choice:** NE is the first-line vasopressor for **septic shock**.

Q: What is the effect of a Beta 2 agonist?

Bronchial muscle relaxation. **Explanation:** The correct answer is **D. Bronchial muscle relaxation.** **Mechanism of Action:** Beta-2 ($\beta_2$) receptors are G-protein coupled receptors (Gs) primarily located in smooth muscles. When a $\beta_2$ agonist binds to these receptors, it stimulates adenylyl cyclase, increasing intracellular cAMP levels. This leads to the activation of Protein Kinase A, which causes sequestration of calcium and inhibits myosin light chain kinase, resulting in **smooth muscle relaxation**. This effect is most prominent in the bronchioles, making $\beta_2$ agonists (like Salbutamol) the mainstay for treating asthma and COPD. **Analysis of Incorrect Options:** * **A. Mydriasis:** This is caused by the contraction of the radial muscle of the iris, mediated by **Alpha-1 ($\alpha_1$)** receptors. * **B. Contraction of urinary sphincters:** This is an excitatory response mediated by **Alpha-1 ($\alpha_1$)** receptors. $\beta_2$ stimulation actually causes relaxation of the detrusor muscle (though $\beta_3$ is the primary receptor for detrusor relaxation). * **C. Vasoconstriction:** This is mediated by **Alpha-1 ($\alpha_1$)** receptors. In contrast, $\beta_2$ stimulation causes **vasodilation** in skeletal muscle blood vessels, which can lead to a decrease in peripheral resistance. **High-Yield Clinical Pearls for NEET-PG:** * **Tocolysis:** $\beta_2$ agonists (e.g., Ritodrine, Isoxsuprine) are used to relax the uterus and delay premature labor. * **Metabolic effects:** $\beta_2$ stimulation increases glycogenolysis in the liver and promotes the uptake of potassium into skeletal muscles (hence, they are used to treat **Hyperkalemia**). * **Side Effects:** Muscle tremors (most common), tachycardia (due to $\beta_1$ cross-reactivity and reflex response), and hypokalemia.

Q: Carvedilol is an effective antihypertensive agent that, like propranolol, is capable of blocking beta receptors. An important difference between the two drugs is that carvedilol

has a1 receptor blocking actions. ### Explanation **Core Concept:** Carvedilol belongs to a unique class of **third-generation non-selective beta-blockers**. Unlike Propranolol (a first-generation non-selective beta-blocker), Carvedilol possesses additional **$\alpha_1$-receptor blocking activity**. This dual action results in peripheral vasodilation (via $\alpha_1$ blockade) alongside decreased heart rate and contractility (via $\beta$ blockade), making it highly effective in treating hypertension and chronic heart failure. **Analysis of Options:** * **Option D (Correct):** Carvedilol provides a "balanced" hemodynamic effect. While $\beta$-blockade prevents reflex tachycardia, the $\alpha_1$-blockade reduces peripheral vascular resistance. * **Option A (Incorrect):** Carvedilol is primarily administered **orally**. It is not used as eye drops; Timolol and Betaxolol are the standard beta-blockers used topically for glaucoma. * **Option B (Incorrect):** Carvedilol does **not** have Intrinsic Sympathomimetic Activity (ISA). Drugs with ISA (like Pindolol or Acebutolol) act as partial agonists. * **Option C (Incorrect):** Carvedilol is **non-selective**, blocking $\beta_1$, $\beta_2$, and $\alpha_1$ receptors. Selective $\beta_1$ blockers (cardioselective) include Metoprolol, Atenolol, and Bisoprolol. **High-Yield NEET-PG Pearls:** 1. **Antioxidant Properties:** Carvedilol has unique antioxidant and anti-proliferative properties, which contribute to its mortality benefit in **Congestive Heart Failure (CHF)**. 2. **Labetalol vs. Carvedilol:** Both block $\alpha$ and $\beta$ receptors. However, Labetalol is the drug of choice for **hypertensive emergencies in pregnancy**, while Carvedilol is a cornerstone for **chronic heart failure** management. 3. **Membrane Stabilizing Activity:** Like Propranolol, Carvedilol possesses local anesthetic (membrane stabilizing) activity. 4. **Metabolism:** It is highly lipophilic and undergoes extensive hepatic metabolism.

Q: Which muscles are affected first by tubocurare?

Head and neck muscles. **Explanation:** Tubocurarine is a prototype **non-depolarizing neuromuscular blocker** that acts as a competitive antagonist at the nicotinic acetylcholine receptors ($N_m$) of the neuromuscular junction. **Why Head and Neck Muscles are correct:** Neuromuscular blockers do not affect all muscles simultaneously. There is a characteristic sequence of paralysis based on the muscle's size, vascularity, and metabolic activity. Small, rapidly moving, and highly perfused muscles are affected first. Therefore, the sequence begins with the **extrinsic eye muscles** (causing diplopia), followed by the **small muscles of the face, head, and neck**. **Analysis of Incorrect Options:** * **Limbs (B) and Abdominal muscles (D):** These are larger muscle groups. After the head and neck, paralysis spreads to the muscles of the fingers, then the limbs, and finally the trunk/abdominal muscles. * **Respiratory muscles (C):** These are the **last** to be paralyzed. The diaphragm is the most resistant muscle to non-depolarizing blockers. This is clinically significant as it provides a safety margin during anesthesia; however, it also means the diaphragm is the **first to recover** when the drug wears off. **High-Yield NEET-PG Pearls:** * **Sequence of Paralysis:** Small muscles (Eyes/Face) → Fingers → Limbs → Trunk → Diaphragm. * **Sequence of Recovery:** Exactly the reverse (Diaphragm recovers first). * **Antidote:** Neostigmine (acetylcholinesterase inhibitor) is used to reverse the blockade by increasing ACh levels at the synapse. * **Clinical Sign:** "Train-of-Four" (TOF) monitoring is used in practice to assess the depth of the blockade.

Q: Which of the following is a selective alpha-2 blocker?

Yohimbine. **Explanation:** **Correct Answer: C. Yohimbine** **Mechanism and Rationale:** Yohimbine is a classic, selective **alpha-2 adrenergic receptor antagonist (blocker)** [1]. Alpha-2 receptors are primarily located presynaptically; their stimulation normally inhibits the release of norepinephrine (negative feedback) [3]. By blocking these receptors, Yohimbine increases sympathetic outflow and norepinephrine release [1]. Clinically, it was historically used for erectile dysfunction, though it is now largely obsolete due to more effective agents like PDE-5 inhibitors. **Analysis of Incorrect Options:** * **A. Tizanidine:** This is a centrally acting **alpha-2 agonist**. It is used as a skeletal muscle relaxant (spasticity) by inhibiting motor neurons in the spinal cord. * **B. Terazosin:** This is a selective **alpha-1 blocker** [2]. It is primarily used in the management of Benign Prostatic Hyperplasia (BPH) and hypertension by causing vasodilation and relaxing prostatic smooth muscle [2]. * **D. Dexmedetomidine:** This is a highly selective **alpha-2 agonist**. It is used in clinical settings (ICU/Operation Theatre) for sedation and analgesia without causing significant respiratory depression. **High-Yield NEET-PG Pearls:** * **Alpha-2 Blockers:** Other examples include **Idazoxan** and **Rauwolscine**. * **Mirtazapine:** An important antidepressant that acts as an alpha-2 blocker, increasing serotonin and norepinephrine levels. * **Non-selective Alpha Blockers:** Remember **Phenoxybenzamine** (Irreversible) and **Phentolamine** (Reversible) [4]. * **Clinical Caution:** Alpha-2 blockers can cause tachycardia and hypertension due to the unchecked release of norepinephrine.

Q: Which of the following adverse drug reactions is caused by Apraclonidine?

Apraclonidine. Apraclonidine is a selective alpha-2 (α2) adrenergic agonist primarily used to control or prevent post-surgical elevations in intraocular pressure (IOP) after laser procedures [1]. The question asks for a drug that causes a specific adverse reaction (implied by the context of the options provided). Apraclonidine is notorious for causing local allergic reactions, specifically blepharoconjunctivitis (itching, hyperemia, and lid edema). Because of this high incidence of follicular conjunctivitis and tachyphylaxis (loss of efficacy over time), it is restricted to short-term use rather than chronic glaucoma management. Analysis of Options: * A. Pilocarpine: A miotic (cholinergic agonist). Its classic side effects include miosis, brow ache (ciliary muscle contraction), and retinal detachment [2]. It does not typically cause the allergic blepharoconjunctivitis associated with alpha-agonists. * C. Brimonidine: Also an α2 agonist, but it is more selective and lipid-soluble than Apraclonidine. While it can cause similar reactions, it has a lower incidence of ocular allergy, making it suitable for long-term therapy [1]. * D. Latanoprost: A Prostaglandin analog. Its signature side effects are increased iris pigmentation (permanent), eyelash thickening, and hypertrichosis. NEET-PG High-Yield Pearls: * Mechanism: Apraclonidine reduces IOP by decreasing aqueous humor production [3]. * Clinical Use: Prophylaxis against IOP spikes after Argon Laser Trabeculoplasty (ALT) or Iridotomy [1]. * Lid Retraction: Unlike other glaucoma drugs, Apraclonidine can cause eyelid retraction due to its weak α1 activity on Müller’s muscle. * Horner’s Syndrome: Apraclonidine is used as a diagnostic test; it reverses ptosis in a Horner’s eye due to denervation supersensitivity of α1 receptors [4].

Question 1

A 32-year-old woman had abdominal surgery, after which she complains about difficulty in urination. Cholinomimetic stimulation with bethanechol or neostigmine is beneficial in relieving post-operative urinary retention. Neostigmine and bethanechol in moderate doses are likely to have different effects on what?

Accepted Answer

Vascular endothelium

Answer

Salivary glands

Answer

Ureteral tone

Answer

Gastric secretory cells

Question 2

Intravenous administration of norepinephrine in a patient already taking an effective dose of atropine will often:

Accepted Answer

Increase heart rate

Answer

Decrease total peripheral resistance

Answer

Decrease pupil size

Answer

Have no effect on the cardiovascular system

Question 3

What is the effect of a Beta 2 agonist?

Accepted Answer

Bronchial muscle relaxation

Answer

Mydriasis

Answer

Contraction of urinary sphincters

Answer

Vasoconstriction

Question 4

Carvedilol is an effective antihypertensive agent that, like propranolol, is capable of blocking beta receptors. An important difference between the two drugs is that carvedilol

Accepted Answer

has a1 receptor blocking actions

Answer

is available only as eye drops

Answer

has intrinsic sympathomimetic activity

Answer

is a selective blocker of cardiac b1 receptors

Question 5

Which muscles are affected first by tubocurare?

Accepted Answer

Head and neck muscles

Answer

Limbs

Answer

Respiratory muscles

Answer

Abdominal muscles

Question 6

A child has swallowed the contents of two bottles of nasal decongestant, whose primary ingredient is an alpha-adrenergic receptor agonist. What signs of alpha activation may occur in this patient?

Accepted Answer

Mydriasis (pupil dilation)

Answer

Tachycardia

Answer

Vasoconstriction

Answer

All the above

Question 7

Which of the following is a selective alpha-2 blocker?

Accepted Answer

Yohimbine

Answer

Tizanidine

Answer

Terazosin

Answer

Dexmedetomidine

Question 8

Which of the following statements regarding the parasympathetic nervous system is true?

Accepted Answer

Acetylcholine is the neurotransmitter at both the pre- and post-synaptic junctions.

Answer

It causes dilation of skeletal muscles.

Answer

Post-ganglionic fibers are longer than pre-ganglionic fibers.

Answer

Noradrenaline is the neurotransmitter at the post-ganglionic junction.

Question 9

Which of the following drugs will decrease heart rate in a patient with a normal heart rate but will have little effect on heart rate in a cardiac transplant recipient?

Accepted Answer

Phenylephrine

Answer

Adrenaline

Answer

Noradrenaline

Answer

Isoproterenol

Question 10

Which of the following adverse drug reactions is caused by Apraclonidine?

Accepted Answer

Apraclonidine

Answer

Pilocarpine

Answer

Brimonidine

Answer

Latanoprost

Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs — MCQs

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