Adrenergic and cholinergic receptors !
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Adrenaline and acetylcholine act through different receptor types; knowing which type sits on an organ predicts what a drug will do.
Updated 2026-11-01
Also called: Alpha and beta receptors, Muscarinic and nicotinic receptors
See it in 3D: Sinoatrial (SA) node · Main bronchi
What it is
Adrenergic receptors (α1, α2, β1, β2, β3) respond to norepinephrine and epinephrine. Cholinergic receptors are nicotinic (ion channels in ganglia, adrenal medulla and skeletal muscle) or muscarinic (M1–M5, G protein receptors on parasympathetic targets). Each G protein receptor uses a second messenger: Gq raises IP₃ and Ca²⁺, Gs raises cAMP and Gi lowers it. The receptor on each organ predicts what a drug or a toxin will do there.
Receptors, messengers and effects
| Receptor | G protein / messenger | Main sites | Effect |
|---|---|---|---|
| α1 | Gq: IP₃, DAG, Ca²⁺ ↑ | Vascular smooth muscle, iris dilator, internal urethral sphincter | Vasoconstriction, mydriasis, urinary retention |
| α2 | Gi: cAMP ↓ | Presynaptic terminals, brainstem, pancreatic β cells | Less norepinephrine release, less insulin |
| β1 | Gs: cAMP ↑ | Heart, juxtaglomerular cells | ↑ rate, contractility and conduction; renin release |
| β2 | Gs: cAMP ↑ | Bronchi, skeletal muscle arterioles, uterus, liver | Bronchodilation, vasodilation, uterine relaxation, glycogenolysis |
| β3 | Gs: cAMP ↑ | Adipose tissue, bladder detrusor | Lipolysis, detrusor relaxation |
| M2 | Gi: cAMP ↓, K⁺ channels open | SA and AV nodes, atria | ↓ heart rate and AV conduction |
| M3 | Gq: IP₃, Ca²⁺ ↑ | Bronchi, gut, detrusor, glands, endothelium, pupil sphincter | Contraction, secretion, miosis, NO-mediated vasodilation |
| Nicotinic (NN, NM) | Ligand-gated cation channel | Autonomic ganglia, adrenal medulla; skeletal muscle endplate | Fast depolarization |
Clinical notes
- Nonselective β-blockers can trigger bronchospasm in asthma (β2 blockade) and hide the tremor and tachycardia that warn of hypoglycemia.
- Anticholinergic toxicity: hot dry skin, flushing, dilated pupils, urinary retention, ileus, delirium.
- Organophosphate insecticide poisoning, a real risk in rural Colombia, blocks acetylcholinesterase: salivation, bronchorrhea, miosis and bradycardia, then fasciculations and weakness.
- In anaphylaxis epinephrine acts on α1 (vasoconstriction) and β2 (bronchodilation, less mediator release from mast cells).
Test yourself
Think of the answer, then tap the question to see it.
Why can a nonselective β-blocker worsen asthma while metoprolol does so less?
Airway relaxation depends on β2 receptors raising cAMP. Nonselective drugs block β2; metoprolol is relatively β1-selective, although selectivity fades at high concentrations.
What explains the pupils and the heart rate in organophosphate poisoning?
ACh builds up: M3 on the pupil sphincter causes miosis and M2 on the SA node causes bradycardia. Nicotinic stimulation of ganglia can sometimes give tachycardia instead.
Why does stimulating α2 receptors in the brainstem lower blood pressure?
α2 receptors are Gi-coupled: they reduce norepinephrine release and sympathetic outflow from the vasomotor centre, so vascular tone and heart rate fall.
Why does a β2 agonist lower plasma potassium?
β2 stimulation raises cAMP and activates the Na⁺/K⁺-ATPase, mainly in skeletal muscle, shifting K⁺ from plasma into cells.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- The autonomic nervous system: organization
- Sympathetic and parasympathetic effects on each organ
- The adrenal medulla and catecholamines
- Cell signaling for physiology
- The neuromuscular junction
Related conditions
In Biochemistry
In Pharmacology
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 15.1 Divisions of the Autonomic Nervous System (OpenStax (Rice University), 2022); 15.4 Drugs that Affect the Autonomic System (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 61). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.