Sympathetic is thoracolumbar with short pre and long postganglionic fibres; parasympathetic is craniosacral and the reverse
Mnemonic
“Sympathetic is thoracolumbar and fight or flight; parasympathetic is craniosacral and rest and digest”:
| Sympathetic | Parasympathetic | |
|---|---|---|
| Outflow | T1 to L2 | CN III, VII, IX, X and S2 to S4 |
| Ganglia | Near the cord (paravertebral chain) | Near the organ |
| Preganglionic fibre | Short | Long |
| Postganglionic transmitter | Noradrenaline | Acetylcholine |
“S2, 3, 4 keeps the pelvis off the floor” for the sacral parasympathetic outflow, and “1, 9, 7, 3” counts the four cranial parasympathetic nerves: III, VII, IX, X.
Both are cholinergic at the ganglion (nicotinic), which is why ganglion blockers affect both systems.
Two sympathetic exceptions: sweat glands use acetylcholine on muscarinic receptors despite being sympathetic, and the adrenal medulla is directly innervated by preganglionic fibres, acting as a modified ganglion that releases adrenaline into the blood.
Expansion
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Outflow | Thoracolumbar (T1 to L2) | Craniosacral (III, VII, IX, X and S2 to S4) |
| Preganglionic fibre | Short | Long |
| Postganglionic fibre | Long | Short |
| Ganglia | Paravertebral chain and prevertebral | In or near the target organ |
| Postganglionic transmitter | Noradrenaline (except sweat glands, which are cholinergic) | Acetylcholine |
| Divergence | High: widespread effect | Low: discrete effect |
All preganglionic fibres release acetylcholine acting on nicotinic receptors in both systems, which is why ganglion blockers affect both.
Two special cases: the adrenal medulla is innervated by preganglionic fibres directly, and sweat glands receive sympathetic fibres that are cholinergic and muscarinic, which is why antimuscarinic drugs cause dry skin.
Vagal outflow supplies everything down to the distal third of the transverse colon, with the pelvic splanchnics (S2 to S4) supplying the rest, mirroring the hindgut boundary.