Which postganglionic sympathetic fibre population is unusual because it releases acetylcholine rather than norepinephrine at its terminal?

• A Vasoconstrictor fibres to skin arterioles
• B Fibres to radial muscle of the iris
• C Fibres to the bronchial smooth muscle
• D Sudomotor fibres to eccrine sweat glands ✓

Explanation

Most postganglionic sympathetic neurons are adrenergic and release norepinephrine. Eccrine sweat glands are the classic exception: their sudomotor fibres are sympathetic but cholinergic, acting on muscarinic receptors. Bronchial smooth muscle receives no sympathetic postganglionic innervation directly, only circulating epinephrine acting on beta-2 receptors. Iris dilator fibres and cutaneous vasoconstrictor fibres are adrenergic as expected.

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

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