Autonomic nervous system
The autonomic nervous system controls viscera, glands and vessels through two-neuron chains, two transmitters and a handful of receptor types. CPSP asks about the outflows, which receptor mediates each effect, where the ganglia and plexuses lie on CT for blocks, and the autonomic consequences of spinal anaesthesia, cord injury, diabetes and drugs used in radiology.
One of the most tested topics in past papers
Organisation
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Outflow | Thoracolumbar (T1–L2), lateral horn | Cranial nerves III, VII, IX, X and S2–S4 |
| Ganglia | Paravertebral chain and prevertebral (coeliac, mesenteric) | Close to or in the organ |
| Preganglionic transmitter | Acetylcholine (nicotinic) | Acetylcholine (nicotinic) |
| Postganglionic transmitter | Noradrenaline (acetylcholine at eccrine sweat glands) | Acetylcholine (muscarinic) |
- Adrenal medulla: chromaffin cells are modified postganglionic neurons, supplied directly by preganglionic fibres acting on nicotinic receptors; they release adrenaline into the blood within seconds.
- Postganglionic axons have no end-plates: transmitter leaks from varicosities along the axon into a wide gap and diffuses to the smooth muscle.
- The hypothalamus integrates autonomic reflexes, temperature and stress responses. The enteric nervous system (myenteric for motility, submucosal for secretion) works alone: a denervated small-bowel transplant still shows coordinated peristalsis.
Receptors
| Receptor | Second messenger | Key effects |
|---|---|---|
| α1 | Gq, calcium | Vasoconstriction of skin and gut vessels, pupil dilator, bladder neck and prostate, internal anal sphincter |
| α2 | Gi, lowers cAMP | Presynaptic brake; clonidine acts centrally to cut sympathetic outflow (basis of the clonidine suppression test) |
| β1 | Gs, raises cAMP | SA node rate (steeper phase 4 depolarisation), contractility, renin |
| β2 | Gs | Bronchodilation, uterine relaxation, muscle vasodilatation |
| M2 | Gi | Slows the heart |
| M3 | Gq | Glands, gut and bladder smooth muscle, sphincter pupillae, ciliary muscle |
Effects on organs
| Organ | Sympathetic | Parasympathetic |
|---|---|---|
| Pupil | Dilates | Constricts; ciliary muscle contracts for near vision |
| Heart | Faster, stronger | Slower; right vagus mainly SA node, left vagus mainly AV conduction |
| Bronchi | Dilate | Constrict, secrete |
| Gut | Reduces motility, closes sphincters | Increases motility and secretion |
| Bladder | Relaxes detrusor, closes neck | Detrusor contraction (S2–S4) |
| Male sex organs | Emission, bladder neck closure | Erection |
| Salivary glands | Scanty viscous saliva | Copious watery saliva (both stimulate: same direction) |
| Skin | Sweating, arrector pili (goosebumps), vasoconstriction | No supply |
Ganglia and plexuses on imaging
- Cervical sympathetic trunk lies directly behind the carotid sheath on the prevertebral fascia. The stellate ganglion lies in front of the neck of the first rib; it is blocked for upper limb sympathetic pain.
- Horner syndrome (apical lung tumour invading the chain): ptosis, miosis, anhidrosis. The pupil difference is greatest in dim light, when the normal pupil dilates and the affected one cannot. Congenital Horner leaves a lighter iris, because sympathetic input is needed for iris pigment.
- Ciliary ganglion lies between the optic nerve and lateral rectus; only the parasympathetic fibres from the oculomotor nerve synapse there. Pupillary fibres run on the surface of the oculomotor nerve, so a posterior communicating aneurysm dilates the pupil early.
- Greater splanchnic nerve (T5–T9) runs beside the thoracic vertebrae, pierces the crus and synapses in the coeliac ganglia around the coeliac trunk. A coeliac plexus block relieves pancreatic cancer pain but removes splanchnic sympathetic tone, causing diarrhoea and postural hypotension.
- Lumbar sympathetic chain lies on the anterolateral vertebral bodies, medial to psoas: the needle for chemical sympathectomy goes there. Afterwards the foot is warm, pink and dry (loss of α1 skin vasoconstriction and sweating).
- Submandibular ganglion relays chorda tympani fibres to the submandibular gland. A pontine haemorrhage cuts descending sympathetic fibres: pinpoint pupils.
Drugs met in radiology
- Hyoscine butylbromide (bowel relaxation) blocks ciliary muscle muscarinic receptors: near vision blurs. Phenylephrine dilates the pupil through α1 without affecting focusing.
- Metoprolol before coronary CT slows the heart through β1 receptors on the SA node. Atropine does not help complete heart block below the AV node, which is beyond vagal control.
- Contrast anaphylaxis: adrenaline restores pressure mainly through α1 arteriolar constriction; in a patient on a β-blocker, glucagon raises cAMP without β receptors.
- Neostigmine treats acute colonic pseudo-obstruction by increasing acetylcholine at colonic muscarinic receptors. Postoperative ileus is driven by reflex sympathetic inhibition of the gut; thoracic epidural block shortens it.
Clinical physiology
- Spinal anaesthesia: hypotension from sympathetic block (arteriolar and venous dilatation); urinary retention from S2–S4 block.
- Spinal cord injury: after spinal shock, the bladder empties reflexly and reflex erections occur because the sacral S2–S4 arc below the lesion is intact. In lesions at or above T6, a distended bladder triggers autonomic dysreflexia: massive sympathetic vasoconstriction below the lesion that descending inhibition cannot cross; hypertension, baroreflex bradycardia and flushing above the lesion. Sit the patient up and relieve the bladder.
- Cervical cord injury: no sympathetic sweating or skin vasomotor control below the lesion, so heat causes hyperthermia with dry skin; the pineal loses its sympathetic drive and night-time melatonin.
- Diabetic autonomic neuropathy: postural fall in pressure without a rise in heart rate (no reflex vasoconstriction), loss of heart-rate variation with breathing (cardiac vagal fibres), no overshoot after Valsalva release.
- Haemorrhage: cold pale skin from α1 constriction of skin vessels; uterine arteries constrict early, so the fetus may become distressed while the mother's pressure is still normal.
- Exercise: sympathetic vasoconstriction diverts blood from the gut and inactive muscle. Heart failure and obstructive sleep apnoea keep sympathetic activity high, harmful over time.
Remember Sweat glands are the sympathetic exception: acetylcholine on muscarinic receptors.
Exam trap The adrenal medulla receives preganglionic fibres, not postganglionic ones.
Exam trap Horner anisocoria is most obvious in dim light; third nerve anisocoria is most obvious in bright light.
Test yourself on this topic
5 of the 52 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 3-year-old boy had a difficult delivery with a weak left hand afterwards. He now has mild left ptosis, a small left pupil and a left iris that is noticeably lighter than the brown right iris. Why is the left iris lighter?
Answer: Sympathetic input is needed for iris pigment
A birth injury to the lower brachial plexus, including the T1 root, can damage the preganglionic sympathetic fibres to the eye, causing Horner syndrome. In the first years of life, sympathetic innervation is needed for melanocytes in the iris stroma to develop their pigment. When it is lost early, the affected iris stays lighter, giving heterochromia. Horner syndrome acquired in adulthood does not change iris colour.
Heterochromia in Horner
- Birth injury to T1 root
- Loss of sympathetic supply early
- Iris melanocytes fail to pigment
- Affected iris is lighter
Remember A lighter iris on the Horner side means it began in early childhood.
Exam trap Acquired Horner syndrome in a child without birth trauma needs imaging to exclude neuroblastoma.
Why the other options are wrong
- Parasympathetic loss bleaches the iris stroma Parasympathetic fibres do not control iris pigment, and they are intact here.
- The iris dilator muscle has atrophied Dilator atrophy does not change iris colour.
- Lens opacity makes the iris look pale A cataract affects the pupil reflex, not the colour of the iris.
- Raised intraocular pressure thins the iris There is no glaucoma, and raised pressure does not lighten the iris.
During surgery near the left side of the aortic arch, the surgeon briefly stimulates the left vagus nerve. Compared with stimulation of the right vagus, which effect is the left vagus more likely to produce?
Answer: Slowing of atrioventricular conduction
Both vagi supply the heart, but not equally. The right vagus mainly supplies the sinoatrial node, so right-sided stimulation chiefly slows the sinus rate. The left vagus mainly supplies the atrioventricular node, so left-sided stimulation chiefly slows AV conduction, lengthening the PR interval and sometimes causing AV block. Vagal effects on ventricular contractility are small because the ventricles have little parasympathetic supply.
Right and left vagus
- Right vagus: SA node
- Right effect: sinus slowing
- Left vagus: AV node
- Left effect: AV delay, block
Remember Right for rate, left for conduction.
Exam trap Sympathetic supply shows a similar side bias, with the right side favouring rate and the left favouring contractility.
Why the other options are wrong
- Marked sinus bradycardia Mainly a right vagal effect, through the SA node.
- Increased ventricular contractility Vagal stimulation does not increase contractility.
- Rise in heart rate Vagal stimulation slows the heart.
- Shortening of the PR interval Vagal stimulation lengthens the PR interval.
A 70-year-old diabetic man with inoperable critical leg ischaemia is to have a chemical lumbar sympathectomy. Where does the lumbar sympathetic chain lie, at which the needle tip must be placed?
Answer: Anterolateral vertebral bodies, medial to psoas
The lumbar sympathetic chain runs down the anterolateral surface of the lumbar vertebral bodies, along the medial margin of psoas major. On the right it lies behind the inferior vena cava and on the left beside the aorta. For a chemical sympathectomy the needle tip is placed in front of the anterolateral L2 to L3 vertebral bodies, at the psoas margin, so that the agent spreads along the chain. The coeliac plexus lies higher, in front of the aorta.
Lumbar sympathetic chain
- On anterolateral vertebral bodies
- Along the medial edge of psoas
- Right: behind IVC; left: beside aorta
- Block at L2–L3
Remember Lumbar chain: front-side of the vertebral body, just medial to psoas.
Exam trap Spread backwards onto the genitofemoral nerve causes groin neuralgia after the procedure.
Why the other options are wrong
- Posterior to the transverse processes This is the site of the dorsal rami and facet joints.
- Within the psoas muscle belly Injection into psoas spreads to the lumbar plexus and causes nerve damage.
- In front of the aorta at the coeliac trunk This is the site of a coeliac plexus block for upper abdominal pain.
- Inside the spinal canal epidural space An epidural injection would block somatic nerves as well.
A 48-year-old woman has a sudden severe headache with a dilated right pupil that does not react to light; eye movements are still almost full. She has an aneurysm at the junction of the right internal carotid and posterior communicating arteries. Why is the pupil affected first?
Answer: Pupillary fibres lie superficially in the nerve
Parasympathetic pupilloconstrictor fibres from the Edinger-Westphal nucleus run on the outer (dorsomedial) surface of the oculomotor nerve. An aneurysm of the posterior communicating artery presses on the nerve from outside, so these superficial fibres are affected early and the pupil dilates and stops reacting. In diabetic third nerve palsy the ischaemia hits the core of the nerve, so the pupil is usually spared.
Pupil and third nerve
- Pupillary fibres superficial
- Compression hits them first
- PCom aneurysm: dilated pupil
- Diabetic palsy: pupil spared
Remember Painful third nerve palsy with a blown pupil: aneurysm until proven otherwise.
Exam trap A spared pupil does not exclude an aneurysm if the palsy is partial; imaging is still needed.
Why the other options are wrong
- The aneurysm compresses the ciliary ganglion The ciliary ganglion lies in the orbit, far from the aneurysm.
- Sympathetic fibres to the pupil are overactive The pupil dilates because constrictor fibres fail, not because sympathetic fibres are overactive.
- The Edinger-Westphal nucleus has been infarcted The nucleus is in the midbrain and is not affected by this aneurysm.
- Blood in the CSF paralyses the dilator muscle Blood in the CSF does not paralyse the iris directly.
Chromaffin cells of the adrenal medulla release adrenaline into the blood within seconds of stress. Which receptor on these cells receives the nerve signal that triggers this release?
Answer: Nicotinic acetylcholine receptor
The adrenal medulla is supplied directly by preganglionic sympathetic fibres from the splanchnic nerves, without a ganglion in between. Chromaffin cells are modified postganglionic neurones derived from neural crest. Like neurones in autonomic ganglia, they respond to acetylcholine through neuronal-type nicotinic receptors, which open cation channels, depolarise the cell and trigger calcium-dependent release of adrenaline and some noradrenaline.
Adrenal medulla
- Supplied by preganglionic fibres
- Chromaffin cells = modified postganglionic neurones
- ACh on nicotinic receptors
- Release mainly adrenaline
Remember The adrenal medulla is a sympathetic ganglion that secretes into blood.
Exam trap Nicotinic receptors here differ from muscle-type receptors, so neuromuscular blockers do not block them.
Why the other options are wrong
- Muscarinic M3 acetylcholine receptor Muscarinic receptors are found on target organs of postganglionic parasympathetic fibres.
- α1 adrenoceptor on the cell Adrenoceptors respond to catecholamines, not to the preganglionic transmitter.
- β1 adrenoceptor on the cell Found mainly on the heart and juxtaglomerular cells.
- Dopamine D2 receptor Not the receptor for the preganglionic signal.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Nervous system and special senses, Autonomic nervous system
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