Autonomic Nervous System: Introduction

Correct Answer

B — Trimethaphan

Rationale

Trimethaphan is a ganglionic blocker — it blocks nicotinic N-N receptors at autonomic ganglia, disrupting transmission in both the sympathetic and parasympathetic divisions simultaneously. Atropine is a muscarinic receptor antagonist acting at postganglionic neuroeffector junctions. Propranolol is a beta-adrenergic receptor antagonist. Neostigmine is a reversible acetylcholinesterase inhibitor.

Correct Answer

A — Muscarinic receptor antagonist

Rationale

Atropine is a muscarinic receptor antagonist — it competitively blocks muscarinic acetylcholine receptors at parasympathetic neuroeffector junctions and in the central nervous system. It is not a nicotinic antagonist, an adrenergic agonist, or an acetylcholinesterase inhibitor.

Correct Answer

C — Direct-acting muscarinic agonist

Rationale

Pilocarpine is a direct-acting muscarinic agonist — it binds and activates muscarinic receptors without requiring conversion or acetylcholinesterase inhibition. It is not indirect-acting (which would mean it works by inhibiting acetylcholinesterase), not a ganglionic blocker, and not an adrenergic agent.

Correct Answer

D — Reversible acetylcholinesterase inhibitor

Rationale

Neostigmine is a reversible acetylcholinesterase inhibitor — it binds the enzyme transiently, allowing recovery of enzyme activity over time. This distinguishes it from irreversible organophosphate inhibitors such as nerve agents and insecticides, which bind covalently. Neostigmine is not a muscarinic antagonist or an adrenergic antagonist.

Correct Answer

A — Norepinephrine

Rationale

Norepinephrine is the primary neurotransmitter released by postganglionic sympathetic neurons at their target organs. Acetylcholine is the postganglionic neurotransmitter of the parasympathetic division and is also used at the ganglionic synapse of both divisions. Dopamine is a precursor in catecholamine synthesis and a neurotransmitter in its own right; postganglionic sympathetic terminals release norepinephrine, not dopamine. Epinephrine is released primarily by the adrenal medulla into the bloodstream, not by postganglionic sympathetic nerve terminals.

Correct Answer

C — Presynaptic blocker of acetylcholine release

Rationale

Botulinum toxin is classified as a presynaptic blocker of acetylcholine release — it cleaves proteins required for vesicular fusion at cholinergic nerve terminals, preventing exocytosis of acetylcholine. It is not an acetylcholinesterase inhibitor, a muscarinic receptor antagonist, or a ganglionic blocker. Those categories describe drugs acting at different anatomical sites along the cholinergic pathway.

Correct Answer

B — Both divisions use nicotinic N-N receptors at the ganglionic synapse

Rationale

Both the sympathetic and parasympathetic divisions use acetylcholine acting on nicotinic N-N receptors at the ganglionic synapse. Ganglionic blockers such as trimethaphan and mecamylamine block these N-N receptors non-selectively, disrupting transmission in both divisions simultaneously. The ganglionic synapse does not use muscarinic receptors — those are found at parasympathetic neuroeffector junctions. Only parasympathetic postganglionic terminals release acetylcholine; sympathetic terminals release norepinephrine. Parasympathetic ganglia are terminal ganglia near target organs, but sympathetic ganglia are in the paravertebral chain — the location differs between divisions.

Correct Answer

D — Atropine blocks muscarinic M2 receptors at the sinoatrial node, removing resting parasympathetic tone

Rationale

The sinoatrial node is under dominant resting parasympathetic tone mediated by the vagus nerve, which releases acetylcholine onto muscarinic M2 receptors. This tonic input slows the heart below its intrinsic rate. Atropine blocks these M2 receptors, removes the vagal brake, and allows heart rate to rise. Atropine does not stimulate adrenergic receptors, does not increase norepinephrine release, and does not act at nicotinic receptors at the node.

Correct Answer

A — A modified sympathetic ganglion whose cells release catecholamines directly into the bloodstream

Rationale

The adrenal medulla is a modified sympathetic ganglion. Its chromaffin cells are developmentally equivalent to postganglionic sympathetic neurons but have differentiated to release catecholamines — approximately 80 percent epinephrine and 20 percent norepinephrine — directly into the systemic circulation rather than through axonal projections to a target organ. It receives preganglionic sympathetic fibers directly from the spinal cord, just as any other sympathetic ganglion would. It is not parasympathetic and does not release acetylcholine.

Correct Answer

C — Blood vessels are maintained under continuous sympathetic alpha-1 vasoconstrictor tone with no parasympathetic innervation

Rationale

Vascular smooth muscle is innervated by the sympathetic division only — there is no parasympathetic innervation of most blood vessels. Resting sympathetic tone via alpha-1 receptors maintains a degree of vasoconstriction. Blocking alpha-1 receptors removes this tonic input, allowing smooth muscle to relax and vessels to dilate, lowering peripheral resistance and blood pressure. There is no parasympathetic vasodilatory tone to be unmasked. Alpha-1 blockade does not activate beta-2 receptors and has no effect on acetylcholinesterase.

Correct Answer

B — Opioids activate mu receptors in enteric neurons, reducing propulsive motility and increasing sphincter tone

Rationale

Mu opioid receptors are densely expressed throughout the enteric nervous system. Activation of these receptors by opioid analgesics reduces propulsive peristaltic activity and increases tone in gastrointestinal sphincters, slowing transit and producing constipation. This effect occurs locally within the gut wall and is distinct from the central analgesic mechanism. Peripherally restricted opioid antagonists such as methylnaltrexone are designed to reverse this enteric effect without crossing the blood-brain barrier to antagonize central analgesia. Opioids do not block muscarinic receptors, do not activate the sympathetic nervous system, and do not primarily work by blocking serotonin release.

Correct Answer

A — Nicotinic N-N receptors on the postganglionic neuron

Rationale

At the ganglionic synapse, acetylcholine released from the preganglionic neuron acts on nicotinic N-N receptors on the postganglionic neuron. This is the universal arrangement in both the sympathetic and parasympathetic divisions. Muscarinic receptors are found at parasympathetic neuroeffector junctions — on smooth muscle, glands, and cardiac muscle — not at ganglia. Nicotinic N-M receptors are found at the skeletal neuromuscular junction, not in autonomic ganglia. Alpha-1 adrenergic receptors respond to norepinephrine and epinephrine, not acetylcholine.

Correct Answer

D — Phenylethanolamine-N-methyltransferase converts norepinephrine to epinephrine in chromaffin cells, induced by high local glucocorticoid concentrations from the adrenal cortex

Rationale

Chromaffin cells of the adrenal medulla contain phenylethanolamine-N-methyltransferase, the enzyme that methylates norepinephrine to form epinephrine. This enzyme is induced by high concentrations of glucocorticoids that drain from the adrenal cortex into the medulla via the portal sinusoids, producing the local glucocorticoid concentration needed to sustain enzyme expression. Without an intact adrenal cortex, epinephrine synthesis falls. The adrenal medulla does possess dopamine beta-hydroxylase and therefore does synthesize norepinephrine as an intermediate. The adrenal medulla receives sympathetic, not parasympathetic, innervation.

Correct Answer

C — Blood vessels — dominant sympathetic vasoconstrictor tone with no parasympathetic innervation

Rationale

Blood vessels are innervated by the sympathetic division only — there is no parasympathetic innervation of most systemic blood vessels. Resting sympathetic tone via alpha-1 receptors maintains vasoconstriction, so alpha-1 blockade produces vasodilation. The sinoatrial node is under dominant resting parasympathetic tone — not sympathetic — which keeps heart rate below the intrinsic pacemaker rate. Bronchi are under dominant resting parasympathetic bronchoconstrictor tone, which is why muscarinic antagonists such as ipratropium produce bronchodilation. Sympathetic activation dilates bronchi via beta-2 receptors, but this is not the resting dominant tone.

Correct Answer

B — Accumulation of acetylcholine at muscarinic receptors due to acetylcholinesterase inhibition

Rationale

Organophosphate pesticides are irreversible inhibitors of acetylcholinesterase. When this enzyme is blocked, acetylcholine accumulates at all cholinergic synapses. At muscarinic receptors, excess acetylcholine produces the SLUDGE syndrome — salivation, lacrimation, urination, defecation, gastrointestinal cramps, and emesis — along with bradycardia and bronchospasm, as seen in this patient. Treatment requires atropine in large doses to block the muscarinic effects, and pralidoxime to reactivate the enzyme if given before covalent aging occurs. Muscarinic blockade would produce the opposite picture — dry mouth, tachycardia, and urinary retention. Reuptake blockade would produce sympathetic, not parasympathetic, excess. Ganglionic blockade would suppress rather than amplify autonomic activity.

Correct Answer

D — Activation of muscarinic M3 receptors, contracting the ciliary muscle and opening the trabecular meshwork to increase aqueous humor outflow

Rationale

Pilocarpine is a direct-acting muscarinic agonist that activates M3 receptors in the ciliary body and iris sphincter. Contraction of the ciliary muscle pulls on the trabecular meshwork, widening the spaces through which aqueous humor drains into the canal of Schlemm. This increases outflow and lowers intraocular pressure. The accompanying miosis from iris sphincter contraction is a side effect, not the therapeutic mechanism. Pilocarpine does not block muscarinic receptors, does not activate adrenergic receptors, and does not inhibit carbonic anhydrase — those mechanisms belong to other glaucoma drug classes.

Correct Answer

A — Muscarinic receptor antagonist

Rationale

This patient presents with the classic anticholinergic toxidrome — tachycardia, mydriasis, dry flushed skin, urinary retention, and confusion — produced by blockade of muscarinic receptors throughout the body. Removal of parasympathetic tone at the sinoatrial node causes tachycardia; removal of glandular secretion causes dry mouth and dry skin; removal of bladder detrusor tone causes urinary retention; and central muscarinic blockade causes confusion. Many commonly prescribed drugs in older adults carry muscarinic antagonist activity, including antihistamines, tricyclic antidepressants, antipsychotics, and bladder medications. A beta-1 antagonist would cause bradycardia, not tachycardia. An acetylcholinesterase inhibitor would produce the opposite picture — excess cholinergic activity. An alpha-1 agonist would cause hypertension and reflex bradycardia.

Correct Answer

C — Propranolol blocks beta-2 adrenergic receptors in bronchial smooth muscle, removing bronchodilatory tone and causing bronchoconstriction

Rationale

Propranolol is a non-selective beta-adrenergic antagonist that blocks both beta-1 and beta-2 receptors. Beta-2 receptors in bronchial smooth muscle mediate bronchodilation when activated by epinephrine or sympathomimetic drugs. Blocking these receptors removes endogenous bronchodilatory tone and opposes the action of the patient's albuterol inhaler, which works precisely by activating beta-2 receptors. This makes propranolol and other non-selective beta-blockers contraindicated in patients with asthma or reactive airway disease. Cardioselective beta-1 antagonists such as metoprolol carry lower risk at therapeutic doses but are still used with caution. Propranolol does not block alpha-1 receptors, does not increase parasympathetic acetylcholine release, and its cardiac effects do not produce airway edema.