Adrenal Medulla Innervation — FRCA Primary MCQ
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Correct answer: A — Thoracic sympathetic preganglionic fibres traverse sympathetic ganglia without synapsing and release acetylcholine onto predominantly nicotinic receptors
Explanation lettering: C = shown as A · A = shown as C
The correct answer is C. Preganglionic sympathetic axons arise from thoracic spinal sympathetic neurons, travel mainly in the splanchnic nerves and pass through peripheral sympathetic ganglia without synapsing. They terminate directly on adrenal medullary chromaffin cells, which function as modified postganglionic sympathetic neurons. Acetylcholine activates predominantly neuronal nicotinic receptors, causing depolarisation, calcium entry and exocytosis of catecholamine-containing granules. PACAP can also be released as a cotransmitter, particularly during sustained high-frequency stress, so the innervation should not be described as chemically exclusively cholinergic. There is no equivalent parasympathetic secretomotor supply. Options A and D incorrectly insert a postganglionic neuron, while B and E incorrectly attribute vagal or equivalent parasympathetic innervation.
Reference: Carbone E, Borges R, Eiden LE, García AG, Hernández-Cruz A. Chromaffin Cells of the Adrenal Medulla: Physiology, Pharmacology, and Disease. Comprehensive Physiology. 2019;9:1443–1502. https://pubmed.ncbi.nlm.nih.gov/31688964/