Learn / Autonomic receptors

α, β, muscarinic & nicotinic

Which receptor sits where, what it couples to, and what happens when it fires — with the memory hooks (1 heart, 2 lungs; odd squeezes, even inhibits, β stimulates) and the one question everyone gets stuck on: why the same Gs → cAMP that speeds the heart relaxes the airway.

Gq

sQueeze — PLC → IP₃/DAG → ↑ Ca²⁺ → contraction / secretion

α1 TARGET ORGAN Vascular smooth muscle · Pupil dilator · GI and urinary internal sphincters

M1 TARGET ORGAN CNS neurons · Enteric nervous system · Gastric secretion — indirectly

M3 TARGET ORGAN Bronchiolar smooth muscle · Pupil sphincter · Ciliary muscle of the lens

Gi

Inhibit — ↓ cAMP. M2 adds K⁺-channel opening (slows the heart); α2 adds ↓ Ca²⁺ entry (less NE release)

α2 TARGET ORGAN Presynaptic sympathetic nerve terminal · Pancreatic β-cells · GI tract wall

M2 TARGET ORGAN SA node · AV node · Atrial myocardium

Gs

Stimulate — ↑ cAMP → PKA (excites heart, relaxes smooth muscle)

β1 TARGET ORGAN SA node · Ventricular & atrial myocardium · AV node

β2 TARGET ORGAN Bronchiolar smooth muscle · Ciliary muscle of the lens · Skeletal-muscle arterioles

β3 TARGET ORGAN Adipocytes · Bladder detrusor

Ligand-gated Na⁺ channel

Fast depolarisation — no second messenger

Nicotinic GANGLION ALL autonomic ganglia — sympathetic and parasympathetic · Adrenal medulla chromaffin cells

Nicotinic NMJ Skeletal muscle end plate

Memory hooks

1 heart, 2 lungs. β1 = heart (↑ rate, ↑ contractility, ↑ conduction) — then tack on renin, the one β1 job outside the heart (kidney JG cells). β2 = lungs, plus every smooth muscle that relaxes under adrenaline: bronchioles, uterus, ciliary muscle, skeletal-muscle arterioles.
Odd squeezes, even inhibits, β stimulates. α1, M1, M3 → Gq (sQueeze). α2, M2 → Gi (Inhibit). β1, β2, β3 → Gs (Stimulate cAMP). The wider version: qiss qiq siq sqs for α1 α2 β1 β2 M1 M2 M3 D1 D2 H1 H2 V1 V2; or HAVe 1 M&M (H1, α1, V1, M1, M3 = Gq) and MAD 2s (M2, α2, D2 = Gi), everything else Gs.
The heart has a gas and a brake on the same pedal box. β1 (Gs) speeds it, M2 (Gi) slows it — opposite G proteins on the same SA node.
Sympathetic to the skin sweats with ACh. Eccrine sweat glands are the one sympathetic target with muscarinic (M3) receptors — that's why anticholinergics give dry, hot skin.
Ganglion receptors vs target-organ receptors. Only Nₙ lives in the ganglion. α1, α2, β1–3 and M1–3 are all on the target organ — the classic mistake is to put a β1 or M3 in the ganglion.
Every ganglion is nicotinic. Both divisions, plus the adrenal medulla, use ACh on Nₙ. The adrenal medulla is a modified sympathetic ganglion: its chromaffin cells are modified postganglionic neurons that release mostly epinephrine, plus some norepinephrine, into the blood.
Who talks to β2. NE released by most sympathetic postganglionic neurons acts strongly at α1, α2 and β1 but only weakly at β2. Circulating epinephrine (adrenal medulla) strongly activates β2 — so airway and muscle-arteriole dilation are largely a hormonal, adrenal effect.

Source: FMK-03.3 Neural communications I (autonomic receptors); FMK-04.3 Synaptic transmission; FMK-05.5 GPCR signalling.