Autonomic Nervous System Pharmacology: Transmitters & Receptors

Autonomic pharmacology looks huge — dozens of drugs, dozens of effects. But it all collapses into one idea: know the transmitter and the receptor at each site, and every drug action follows. The map below is the framework the entire topic hangs on.

The two divisions

• Sympathetic (“fight or flight”) — thoracolumbar outflow; short pre-ganglionic, long post-ganglionic fibres.
• Parasympathetic (“rest and digest”) — craniosacral outflow; long pre-ganglionic, short post-ganglionic fibres.

The transmitter–receptor rules (memorise these)

• All pre-ganglionic fibres release ACh acting on nicotinic (N_N) receptors in the ganglion.
• Parasympathetic post-ganglionic releases ACh on muscarinic receptors.
• Sympathetic post-ganglionic releases noradrenaline on adrenergic (α/β) receptors.
• Three exceptions worth marks: sympathetic fibres to sweat glands are cholinergic (muscarinic); the adrenal medulla is innervated by a pre-ganglionic fibre (ACh→N) and secretes adrenaline; the renal vascular bed uses dopamine.

Cholinergic receptors

• Nicotinic (N): ligand-gated ion channels — N_M (neuromuscular junction) and N_N (ganglia, CNS).
• Muscarinic (M1–M5): GPCRs — M1 (CNS, gastric glands), M2 (heart, slows it), M3 (smooth muscle, glands, eye).

Adrenergic receptors & their effects

• α1 — vasoconstriction, mydriasis, bladder sphincter contraction.
• α2 — pre-synaptic; inhibits noradrenaline release (clonidine lowers BP centrally).
• β1 — heart: ↑ rate, force, conduction; renin release.
• β2 — bronchodilation, vasodilation, uterine relaxation, glycogenolysis.
• β3 — lipolysis, bladder relaxation.

The drugs, by what they do

• Cholinergic agonists: pilocarpine (glaucoma), bethanechol (urinary retention).
• Anticholinesterases: neostigmine (myasthenia), physostigmine (crosses BBB), organophosphates (poisoning → treat with atropine + pralidoxime).
• Antimuscarinics: atropine, ipratropium, oxybutynin.
• Adrenergic agonists: adrenaline (anaphylaxis), noradrenaline (shock), phenylephrine (α1), salbutamol (β2, asthma), dobutamine (β1).
• Adrenergic antagonists: prazosin (α1), propranolol (non-selective β), atenolol/metoprolol (β1-selective).

Exam tip: don’t memorise effects — derive them. “Atropine blocks M” → no muscarinic action → tachycardia, dry mouth, mydriasis, urinary retention. Get the receptor right and the whole answer writes itself.

Master this single framework — transmitter, receptor, effector — and autonomic pharmacology stops being a list to cram and becomes a system you can reason through in the exam hall.