Sweat glands are innervated by sympathetic nerves, yet the postganglionic neurotransmitter released at thermoregulatory (eccrine) sweat glands is acetylcholine rather than noradrenaline. This is an exception to the general rule of sympathetic neurotransmission. Which receptor subtype on eccrine sweat glands mediates their response?

• A Nicotinic Nm receptors at the neuroeffector junction because acetylcholine always acts on nicotinic receptors at all autonomic effector organs
• B Adrenergic beta-2 receptors because noradrenaline co-released with acetylcholine mediates the actual secretory response
• C Muscarinic M3 receptors, since the glands respond to cholinergic stimulation; hence atropine (muscarinic blocker) produces anhidrosis and hyperthermia ✓
• D Muscarinic M2 receptors that activate Gi-coupled K⁺ channels in sweat gland secretory coils to drive Cl⁻ secretion

Explanation

Eccrine sweat glands are innervated by sympathetic cholinergic fibres (the neurons are sympathetic but switched to cholinergic transmission developmentally). The secretory coils respond to acetylcholine via M3 muscarinic receptors (Gq-coupled), activating PLC→IP3→Ca2+→Cl⁻ secretion with secondary Na⁺ and water secretion. Blocking M3 receptors with atropine causes anhidrosis and can precipitate life-threatening hyperthermia. Nicotinic Nm receptors are found at the neuromuscular junction of skeletal muscle, not sweat glands. Adrenergic receptors mediate apocrine sweat gland secretion (axillary), not thermoregulatory eccrine sweating.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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