A pharmacology demonstration shows that a muscarinic antagonist abolishes sweating induced by electrical stimulation of the sympathetic chain, while an alpha blocker has little effect on this response. This reflects which special feature of sympathetic innervation to eccrine sweat glands?

• A Sweat glands receive their entire supply from parasympathetic postganglionic fibres travelling with the facial nerve
• B Eccrine sweat glands are supplied by dopaminergic postganglionic fibres unique among sympathetics
• C Postganglionic sympathetic fibres to eccrine sweat glands release acetylcholine acting on muscarinic receptors ✓
• D Sweat glands lack postganglionic neurons entirely, receiving preganglionic fibres directly

Explanation

Although eccrine sweat glands are sympathetically innervated, their postganglionic fibres are cholinergic, releasing acetylcholine onto muscarinic receptors. Together with the adrenal medulla and renal vasculature, they form the classic exceptions to the noradrenergic rule for sympathetic postganglionic neurons. This explains why anticholinergic drugs cause anhidrosis. Option A confuses the pathway with facial secretomotor supply to salivary and lacrimal glands.

Reference: Ganong Review of Medical Physiology, 26th ed.

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