CorpusHuman Anatomy
NervousNerve group

Parasympathetic nervous system

Also known as: craniosacral division, rest-and-digest system

The craniosacral division of the ANS (CN III, VII, IX, X and S2–S4) that promotes digestion, conservation of energy and visceral emptying, using long preganglionic fibres, terminal ganglia and acetylcholine on muscarinic receptors.

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Overview

Cranial nerves carrying parasympathetic fibresIII, VII, IX, X
Sacral outflowS2–S4 (pelvic splanchnic nerves)
Share carried by the vagus≈75% of all parasympathetic fibres

Parasympathetic preganglionic neurons lie in brainstem nuclei and in the sacral spinal cord (S2–S4). Their long axons run to terminal ganglia in or beside the target organ, where short postganglionic fibres release acetylcholine onto muscarinic receptors.

The cranial outflow travels in four nerves:
• III: Edinger–Westphal nucleus → ciliary ganglion → sphincter pupillae and ciliary muscle.
• VII: superior salivatory nucleus → greater petrosal nerve → pterygopalatine ganglion → lacrimal, nasal and palatal glands; and → chorda tympani → submandibular ganglion → submandibular and sublingual glands.
• IX: inferior salivatory nucleus → lesser petrosal nerve → otic ganglion → parotid gland.
• X: dorsal motor nucleus of the vagus and nucleus ambiguus → intramural ganglia of the heart, lungs and gut as far as the left colic flexure. The vagus carries ≈75% of all parasympathetic fibres.

The sacral outflow leaves in the pelvic splanchnic nerves (S2–S4) to the inferior hypogastric plexus. It supplies the descending and sigmoid colon, the rectum, the detrusor muscle and the erectile tissue. It is responsible for micturition, defecation and erection, the last through nitric oxide-mediated vasodilation (hence "nervi erigentes").

Activity is discrete rather than global and dominates at rest. Its effects are slowing of the heart, constriction of the pupils and bronchi, accommodation for near vision, secretion of tears, saliva and digestive juices, peristalsis, relaxation of sphincters and emptying of the bladder and rectum.

Parts & subdivisions

  1. Ciliary ganglion (III)
    In the orbit, lateral to the optic nerve. Postganglionic short ciliary nerves → sphincter pupillae (miosis) and ciliary muscle (accommodation).
  2. Pterygopalatine ganglion (VII)
    In the pterygopalatine fossa. Lacrimal gland (via zygomatic and lacrimal nerves), nasal, palatal and pharyngeal mucous glands. "Hay-fever ganglion".
  3. Submandibular ganglion (VII)
    Hangs from the lingual nerve. Submandibular and sublingual salivary glands.
  4. Otic ganglion (IX)
    Below foramen ovale, medial to V3. Parotid gland via the auriculotemporal nerve.
  5. Vagal outflow (X)
    Heart (M2: ↓ SA-node rate, ↓ AV conduction), bronchi (M3: constriction, secretion), oesophagus, stomach, intestines, liver, pancreas and gut to the left colic flexure (↑ motility and secretion).
  6. Pelvic splanchnic nerves (S2–S4)
    Hindgut (from the left colic flexure), bladder (detrusor contraction, internal sphincter relaxation), erectile tissue (erection via NO).
  7. Eye
    Miosis; contraction of the ciliary muscle → lens rounds up → near vision.
  8. Heart and lungs
    ↓ heart rate and AV conduction (little effect on ventricular contractility); bronchoconstriction and ↑ mucus.
  9. Gut and glands
    ↑ peristalsis, relaxation of sphincters, copious watery saliva, gastric acid, pancreatic secretion, gallbladder contraction.

Function

  • Rest and digest: promotes digestion, absorption and energy storage
  • Slows the heart (vagal tone sets the resting rate ≈60–80 bpm, below the intrinsic SA rate ≈100 bpm)
  • Near response: miosis and accommodation
  • Lacrimation and salivation
  • Micturition, defecation and erection
Physiology

Preganglionic ACh → nicotinic N_N; postganglionic ACh → muscarinic M2 (heart) and M3 (smooth muscle, glands). ACh is rapidly hydrolysed by acetylcholinesterase, so effects are brief and local. Many parasympathetic terminals co-release VIP and NO.

Clinical significance

Atropine and muscarinic blockade

Atropine, a muscarinic antagonist, treats symptomatic bradycardia and organophosphate poisoning. Its effects are tachycardia, dry mouth, mydriasis with cycloplegia, urinary retention and constipation. Antimuscarinic drugs are also used for overactive bladder (oxybutynin) and COPD (tiotropium).

Vagal manoeuvres

Carotid sinus massage and the Valsalva manoeuvre increase vagal output to the AV node and can terminate AV-nodal re-entrant tachycardia.

Pelvic nerve injury

Radical pelvic surgery (e.g. rectal or prostate cancer) can damage the pelvic splanchnic nerves and inferior hypogastric plexus, causing urinary retention and erectile dysfunction. Nerve-sparing techniques preserve these fibres.

Mnemonics

Parasympathetic cranial nerves

"1973" — cranial nerves X, IX, VII, III.

Male sexual response

"Point and Shoot": Parasympathetic erection, Sympathetic emission (with somatic pudendal-mediated ejaculation).

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.