Splanchnic nerves

Nervi splanchnici

Also known as: greater splanchnic nerve, lesser splanchnic nerve, least splanchnic nerve, lumbar splanchnic nerves, pelvic splanchnic nerves, nervi erigentes

Visceral nerves that carry autonomic fibres to the organs of the body cavities. The thoracic, lumbar and sacral splanchnic nerves carry sympathetic fibres to prevertebral ganglia. The pelvic splanchnic nerves (S2–S4) are the sacral parasympathetic outflow.

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Overview

Greater splanchnicT5–T9 → coeliac ganglion

Lesser splanchnicT10–T11 → aorticorenal/SMG

Least splanchnicT12 → renal plexus

Pelvic splanchnicsS2–S4, parasympathetic

Except for the cardiopulmonary splanchnics, the abdominopelvic splanchnic nerves are made of preganglionic sympathetic fibres. These pass through the sympathetic trunk without synapsing and synapse in the prevertebral (pre-aortic) ganglia around the major branches of the abdominal aorta:
• the greater splanchnic nerve (T5–T9) pierces the crus of the diaphragm and ends in the coeliac ganglion, supplying foregut derivatives;
• the lesser (T10–T11) ends in the aorticorenal and superior mesenteric ganglia, supplying the midgut and kidneys;
• the least (T12) ends in the renal plexus;
• the lumbar splanchnics (L1–L2) go to the inferior mesenteric ganglion and the superior hypogastric plexus, supplying the hindgut and pelvic viscera;
• the sacral splanchnics run from the sacral trunk to the inferior hypogastric plexus.

Some greater splanchnic fibres go directly to the adrenal medulla, whose chromaffin cells act as modified postganglionic neurons and secrete adrenaline into the blood.

The pelvic splanchnic nerves (S2–S4) are entirely different. They carry preganglionic parasympathetic fibres from the sacral spinal cord to the inferior hypogastric (pelvic) plexus. They supply:
• the bladder (detrusor);
• the hindgut beyond the left colic flexure;
• the reproductive organs, including the erectile tissue (hence the old name nervi erigentes).

Visceral afferent (pain) fibres run back to the spinal cord alongside the sympathetic splanchnics. This explains why visceral pain is referred to the dermatomes of the segments that give rise to them.

Location: Posterior mediastinum (thoracic splanchnics on the vertebral bodies) → through the diaphragmatic crura → coeliac, superior mesenteric and aorticorenal plexuses on the aorta; lumbar and sacral splanchnics in the retroperitoneum and pelvis.

Parts & subdivisions

Prevertebral ganglia
The coeliac, superior mesenteric, aorticorenal and inferior mesenteric ganglia lie on the front of the aorta at the origins of their arteries. Postganglionic fibres travel along those arteries to the corresponding gut regions: foregut, midgut and hindgut.
Pelvic splanchnic nerves
The only splanchnic nerves that are parasympathetic (S2–S4 ventral rami). They are responsible for micturition (detrusor contraction), defecation (hindgut motility, relaxation of the internal anal sphincter) and erection (vasodilatation of erectile tissue).

Anatomy

Innervation: Smooth muscle, glands and vessels of the abdominal and pelvic viscera, and the adrenal medulla. Visceral afferents return pain from the organs.
Nerve roots: - Greater: T5–T9.
• Lesser: T10–T11.
• Least: T12.
• Lumbar: L1–L2.
• Sacral (sympathetic): from the sacral trunk.
• Pelvic (parasympathetic): S2–S4.
Branches: - Cardiopulmonary splanchnics (postganglionic sympathetic, from the cervical and T1–T5 ganglia): cardiac and pulmonary plexuses.
• Greater: coeliac ganglion and adrenal medulla.
• Lesser: aorticorenal and superior mesenteric ganglia.
• Least: renal plexus.
• Lumbar: inferior mesenteric ganglion and superior hypogastric plexus.
• Sacral: inferior hypogastric plexus.
• Pelvic: inferior hypogastric plexus, then intramural ganglia of the descending and sigmoid colon, rectum, bladder and genital organs.

Function

Sympathetic control of the gut (reduced motility and secretion, sphincter contraction, vasoconstriction of splanchnic vessels)
Stimulation of adrenaline release from the adrenal medulla
Sympathetic control of the kidney (renin release, vasoconstriction)
Parasympathetic control of the bladder, hindgut and erection (pelvic splanchnics)
Carrying visceral pain afferents to T5–L2 and S2–S4 spinal segments

Clinical significance

Referred visceral pain

Visceral afferents enter the cord at the segments of their splanchnic nerves, so pain is felt in the matching dermatomes:
• foregut (T5–T9): epigastrium;
• midgut (T10–T11): periumbilical region;
• hindgut (T12–L2): suprapubic region.

Early appendicitis is therefore felt around the umbilicus (T10) before parietal peritoneal irritation localises it to the right iliac fossa.

Coeliac plexus block

Neurolytic blockade of the coeliac plexus or the splanchnic nerves (by alcohol or phenol, CT- or EUS-guided) relieves intractable upper abdominal pain, especially from pancreatic cancer and chronic pancreatitis. Side-effects include orthostatic hypotension and diarrhoea, from unopposed parasympathetic activity.

Pelvic surgery and autonomic injury

Damage to the pelvic splanchnic nerves or the inferior hypogastric plexus during rectal or prostate cancer surgery, such as total mesorectal excision or radical prostatectomy, causes urinary retention and erectile dysfunction. Nerve-sparing techniques aim to preserve these nerves.

Mnemonics

Thoracic splanchnic levels

"5–9, 10–11, 12": the greater (5 to 9), lesser (10 and 11) and least (12) splanchnic nerves get progressively smaller and lower.

Point and shoot

"Point (erection, Parasympathetic pelvic splanchnics), Shoot (emission, Sympathetic lumbar splanchnics)." Ejaculation itself also needs the somatic pudendal nerve (bulbospongiosus).

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