Anatomy chapters
Autonomic nervous system
How the sympathetic and parasympathetic systems are built, where their neurons and ganglia lie, which transmitters they use, and the clinical signs of damage such as Horner's syndrome.
- The sympathetic system arises from T1 to L2 (thoracolumbar); the parasympathetic from cranial nerves III, VII, IX, X and S2 to S4 (craniosacral).
- Sympathetic ganglia lie near the spinal cord; parasympathetic ganglia lie near or in the organ.
- All preganglionic fibres use acetylcholine; most sympathetic postganglionic fibres use noradrenaline.
On this page
Overview
The autonomic nervous system controls the organs automatically: heart, blood vessels, glands, gut, bladder and the pupil.
It has two main parts:
- Sympathetic: "fight or flight". It speeds the heart, widens the pupils and airways, and diverts blood to muscles.
- Parasympathetic: "rest and digest". It slows the heart, narrows the pupils, and increases gut activity and secretions.
Both use a two-neuron chain: a preganglionic neuron in the brainstem or cord, and a postganglionic neuron in a ganglion.
Sympathetic and parasympathetic compared
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Origin | Thoracolumbar (T1–L2), lateral horn | Craniosacral: cranial nerves III, VII, IX, X; S2–S4 |
| Ganglia | Near the cord: sympathetic trunk and prevertebral ganglia | Near or in the organ |
| Preganglionic fibre | Short | Long |
| Postganglionic fibre | Long | Short |
| Preganglionic transmitter | Acetylcholine (nicotinic) | Acetylcholine (nicotinic) |
| Postganglionic transmitter | Noradrenaline (mostly) | Acetylcholine (muscarinic) |
Sweat glands are supplied by sympathetic fibres that release acetylcholine, not noradrenaline.
The adrenal medulla is supplied directly by preganglionic sympathetic fibres; its cells act like postganglionic neurons and release adrenaline into the blood.
The sympathetic system
The sympathetic trunk
A chain of ganglia on each side of the vertebral column, from the base of the skull to the coccyx.
- Cervical: superior, middle and inferior cervical ganglia. The inferior often joins the first thoracic to form the stellate ganglion.
- Thoracic, lumbar and sacral ganglia, roughly one per segment.
- The two trunks meet in front of the coccyx at the ganglion impar.
Rami communicantes
- White rami: carry myelinated preganglionic fibres from the spinal nerve to the trunk. They exist only from T1 to L2, because only these segments have a lateral horn.
- Grey rami: carry unmyelinated postganglionic fibres back to every spinal nerve, to supply sweat glands, skin vessels and hair muscles all over the body.
White rami only at T1–L2; grey rami at all spinal nerves.
What preganglionic fibres do
Once in the trunk, a preganglionic fibre can:
- Synapse in the ganglion at the same level.
- Travel up or down the trunk to synapse at another level (this is how the head and lower limbs are reached).
- Pass through without synapsing as a splanchnic nerve to a prevertebral ganglion (coeliac, superior mesenteric, aorticorenal, inferior mesenteric) near the abdominal organs.
| Splanchnic nerve | Segments | Goes to |
|---|---|---|
| Greater | T5–T9 | Coeliac ganglion |
| Lesser | T10–T11 | Aorticorenal / superior mesenteric ganglion |
| Least | T12 | Renal plexus |
| Lumbar | L1–L2 | Inferior mesenteric and pelvic plexuses |
Sympathetic supply of the head
Preganglionic fibres from T1 (and T2) rise in the trunk to the superior cervical ganglion. Postganglionic fibres travel on the internal and external carotid arteries to reach the eye (dilator pupillae and the smooth muscle of the upper lid), sweat glands of the face and blood vessels.
The parasympathetic system
Cranial part
| Cranial nerve | Nucleus | Ganglion | Supplies |
|---|---|---|---|
| III | Edinger-Westphal | Ciliary | Sphincter pupillae (constricts the pupil) and ciliary muscle (accommodation) |
| VII | Superior salivatory | Pterygopalatine | Lacrimal gland, glands of the nose and palate |
| VII | Superior salivatory | Submandibular | Submandibular and sublingual glands |
| IX | Inferior salivatory | Otic | Parotid gland |
| X | Dorsal motor nucleus of vagus | Ganglia in the organ walls | Heart, lungs, gut up to the left colic flexure |
The parotid gland gets its parasympathetic supply from the glossopharyngeal nerve (IX) via the otic ganglion, carried by the auriculotemporal nerve.
Sacral part
The pelvic splanchnic nerves (S2–S4) supply the hindgut (from the left colic flexure), the bladder (contracting the detrusor) and the erectile tissue.
"S2, 3, 4 keeps the penis off the floor": erection is parasympathetic, through the pelvic splanchnic nerves.
Effects on organs
| Organ | Sympathetic | Parasympathetic |
|---|---|---|
| Pupil | Dilates | Constricts |
| Heart | Faster and stronger | Slower |
| Bronchi | Widen | Narrow; more secretions |
| Gut | Less movement; sphincters close | More movement and secretion; sphincters relax |
| Bladder | Detrusor relaxes; internal sphincter closes | Detrusor contracts (emptying) |
| Male sexual function | Ejaculation | Erection |
| Sweat glands | Sweating | - |
Clinical relevance
Horner's syndrome
How it happens: interruption of the sympathetic supply to the head anywhere along its path: the brainstem (lateral medullary syndrome), the cord at T1, the lung apex (Pancoast tumour), the cervical sympathetic trunk (neck surgery) or along the carotid artery.
What you see on the same side:
- Ptosis: mild drooping of the upper lid (paralysis of the smooth muscle in the lid).
- Miosis: small pupil (dilator pupillae unopposed).
- Anhidrosis: no sweating on that side of the face (if the lesion is before the carotid branches).
- Apparent sinking of the eye (enophthalmos).
Hirschsprung disease
How it happens: neural crest cells fail to reach part of the colon, so it has no ganglion cells in its wall.
What you see: a newborn with delayed passage of meconium and a swollen abdomen; the bowel above the aganglionic segment is dilated.
Sympathectomy
Cutting the sympathetic trunk (for example, the upper thoracic ganglia) stops excessive sweating of the hands and improves blood flow to the skin.
Expect: the origin of each system, white vs grey rami, transmitters (and the sweat gland exception), the cranial parasympathetic ganglia and what they supply (especially the parotid via the otic ganglion), the pelvic splanchnic nerves, and the features of Horner's syndrome.
Quick revision
- Sympathetic: T1–L2. Parasympathetic: III, VII, IX, X and S2–S4.
- White rami T1–L2 only; grey rami to every spinal nerve.
- All preganglionic: acetylcholine. Sympathetic postganglionic: noradrenaline (sweat glands: acetylcholine).
- Ciliary (III), pterygopalatine and submandibular (VII), otic (IX).
- Parotid: IX via the otic ganglion.
- Horner's: ptosis, miosis, anhidrosis.
Written by The House Job from standard neuroanatomy references (Snell's Clinical Neuroanatomy).
FAQ
Where does the sympathetic outflow come from?
From the lateral horn of the spinal cord between T1 and L2.
Which ganglion supplies the parotid gland?
The otic ganglion, with preganglionic fibres from the glossopharyngeal nerve.
Which sympathetic target uses acetylcholine?
Most sweat glands.
What causes Horner's syndrome?
Damage to the sympathetic supply of the head, for example by a tumour at the apex of the lung.