Autonomic Nervous System MCQs: 15 Questions with Answers

Correct answer: B. Norepinephrine

Sympathetic post‑ganglionic fibers release norepinephrine to stimulate target organs.

Correct answer: A. Alpha-1 adrenergic receptor

Activation of alpha-1 adrenergic receptors in the dilator pupillae (radial muscle) causes contraction, resulting in pupillary dilation (mydriasis). Alpha-1 receptors are coupled to the Gq protein signaling pathway.

Correct answer: B. Beta-1 adrenergic receptor

Beta-1 adrenergic receptors are predominantly located in the heart (sinoatrial node, atrioventricular node, and myocardium). Their stimulation increases heart rate (positive chronotropy) and contractility (positive inotropy).

Correct answer: C. Nicotinic Nm receptor

The skeletal neuromuscular junction contains nicotinic muscle-type receptors (Nm), which are ligand‑gated ion channels. In contrast, autonomic ganglia contain neuronal-type nicotinic receptors (Nn).

Correct answer: D. Stimulation of salivary and digestive secretions

The parasympathetic nervous system promotes 'rest and digest' activities, which include stimulating salivary and digestive secretions, enhanced GI motility, bronchoconstriction, and miosis.

Correct answer: A. Conversion of tyrosine to DOPA by tyrosine hydroxylase

Tyrosine hydroxylase catalyzes the hydroxylation of tyrosine to L‑3,4‑dihydroxyphenylalanine (DOPA), which is the slow, rate‑limiting step in catecholamine synthesis.

Correct answer: B. Inhibition of norepinephrine release

Presynaptic alpha‑2 adrenergic receptors act as inhibitory autoreceptors. Their activation by norepinephrine decreases further neurotransmitter release via negative feedback mechanisms involving Gi protein coupling.

Correct answer: C. Beta-2 receptor

Beta‑2 adrenergic receptors are located on blood vessels supplying skeletal muscle. When stimulated by low concentrations of circulating epinephrine, they mediate vasodilation, lowering peripheral resistance.

Correct answer: C. Acetylcholinesterase (AChE)

Acetylcholinesterase (AChE) is located in cholinergic synapses and rapidly hydrolyzes acetylcholine into acetate and choline, ensuring precise and brief activation of postsynaptic receptors.

Correct answer: B. Increased myocardial contractility (positive inotropy)

Activation of beta-1 adrenergic receptors in the heart increases intracellular cyclic AMP via Gs proteins, leading to positive inotropy, positive chronotropy, and positive dromotropy.

Correct answer: D. Muscarinic (M3) receptors

M3 muscarinic receptors are located on the circular smooth muscle of the iris (sphincter pupillae) and ciliary muscle, mediating contraction that results in miosis and accommodation for near vision.

Correct answer: C. Cleavage of SNARE proteins, preventing acetylcholine exocytosis

Botulinum toxin acts by specifically cleaving SNARE proteins required for the docking and fusion of acetylcholine-containing vesicles with the presynaptic membrane, thereby blocking neurotransmitter release.

Correct answer: B. Vasodilation in skeletal muscle blood vessels

Beta-2 adrenergic receptors are predominantly coupled to Gs pathways that relax vascular smooth muscle. Epinephrine has a high affinity for beta-2 receptors, causing vasodilation in skeletal muscle beds at physiological concentrations.

Correct answer: C. Atropine

Atropine is a classic competitive antagonist (anticholinergic) at all muscarinic receptor subtypes, blocking parasympathetic responses and producing signs such as mydriasis, cycloplegia, xerostomia, and tachycardia.

Correct answer: B. Tyrosine

Tyrosine is the direct precursor for catecholamine synthesis. The rate-limiting enzyme tyrosine hydroxylase converts tyrosine to L-DOPA in adrenergic and dopaminergic neurons.