Pelvic nerves and plexuses
Lumbosacral plexus, pelvic splanchnic nerves, sacral sympathetic trunk and hypogastric plexuses
Pelvic nerves are either somatic plexuses on the walls (lower limb and perineum) or autonomic plexuses around the viscera (bladder, bowel, uterus, sexual function). CPSP asks which nerve is injured at surgery or delivery, which segments carry labour pain, and why an operation relieves one pain but not another.
Lumbar plexus (L1–L4)
- Formed within psoas major from ventral rami L1–L4, so a psoas haematoma (on anticoagulants) compresses femoral and obturator roots together.
| Nerve | Roots | Emerges | Clinical point |
|---|---|---|---|
| Iliohypogastric | L1 | Lateral border | Suprapubic skin; cut by lateral extension of a Pfannenstiel or a gridiron incision |
| Ilioinguinal | L1 | Lateral border | Pierces internal oblique near the ASIS, exits the superficial ring; mons, labium majus, upper medial thigh; trapped at lateral port sites |
| Genitofemoral | L1–L2 | Front of psoas | Genital branch through the deep ring; femoral branch to skin below the inguinal ligament |
| Femoral | L2–L4, posterior divisions | Lateral border, between psoas and iliacus | Pressed by deep self-retaining retractor blades, or kinked under the inguinal ligament in exaggerated lithotomy |
| Obturator | L2–L4, anterior divisions | Medial border | Runs on the lateral pelvic wall in the floor of the ovarian fossa to the obturator canal |
- Obturator nerve injury at lymphadenectomy or ovarian surgery: weak adduction, numb medial thigh. Ovarian disease irritating it refers pain to the inner thigh and knee. The obturator nodes lie between the external iliac vein above and the nerve below.
- Lumbosacral trunk (L4–L5) crosses the ala of the sacrum to join the sacral plexus. The fetal head or forceps can compress it in a long second stage: foot drop with weak inversion and weak hip abduction (L4–L5 pattern), which separates it from a common fibular palsy at the fibular neck (inversion spared).
Sacral plexus (L4–S4)
- Lies on piriformis, behind the internal iliac vessels and ureter: sidewall tumour gives sciatica, leg oedema and hydronephrosis. Endometriosis at the sciatic notch gives sciatica only with periods.
| Anterior divisions | Posterior divisions |
|---|---|
| Tibial part of sciatic | Common fibular part of sciatic |
| Pudendal (S2–S4) | Superior gluteal (L4–S1), inferior gluteal (L5–S2) |
| Nerve to obturator internus, nerve to quadratus femoris | Nerve to piriformis |
- The pudendal nerve is fixed as it passes round the ischial spine, so a long second stage or big baby stretches it, causing stress incontinence.
- Sacral nerve stimulation uses the S3 foramen: correct placement gives a perineal bellows contraction and great toe flexion.
Sympathetic trunk in the abdomen and pelvis
- The lumbar trunk lies on the vertebral bodies medial to psoas (left beside the aorta, right behind the IVC). Injury at para-aortic surgery gives a warm, pink, dry leg with normal pulses.
- The sacral trunk lies on the sacrum medial to the anterior sacral foramina; the two trunks meet in front of the coccyx at the ganglion impar.
- Preganglionic sympathetic cells exist only in the lateral horn of T1–L2, so white rami exist only there. Sacral ganglia send grey rami only to sacral nerves.
Hypogastric plexuses
- Superior hypogastric plexus (presacral nerve): in front of L5 and the sacral promontory, below the aortic bifurcation, between the common iliac arteries; the left common iliac vein lies just beneath it. Formed by the aortic plexus plus lumbar splanchnic nerves (L1–L2); divides into right and left hypogastric nerves.
- Presacral neurectomy relieves midline uterine (central) dysmenorrhoea but not ovarian or adnexal pain, because ovarian afferents run with the ovarian vessels to T10–T11.
- Inferior hypogastric (pelvic) plexus: on each side of the rectum, cervix, vaginal fornices and bladder base, lateral to the uterosacral ligament. It combines sympathetic fibres (hypogastric and sacral splanchnic nerves) with parasympathetic pelvic splanchnic nerves. Its uterovaginal part beside the cervix is Frankenhäuser's plexus, the target of a paracervical block at 3 and 9 o'clock.
- Damage at radical hysterectomy causes urinary retention with reduced bladder sensation, plus bowel and sexual dysfunction.
Pelvic splanchnic nerves (nervi erigentes)
- Preganglionic parasympathetic fibres from the ventral rami of S2–S4; they do not pass through the sympathetic trunk.
- Long preganglionic fibres synapse in the pelvic plexus and organ-wall ganglia; short postganglionic fibres release acetylcholine.
- Supply: detrusor contraction, erection (via nitric oxide), and the hindgut from the distal transverse colon to the upper anal canal (fibres ascend through the hypogastric plexuses).
| Function | Pathway |
|---|---|
| Erection, detrusor contraction | Parasympathetic S2–S4 |
| Emission, bladder neck closure | Sympathetic T10–L2 via hypogastric plexus |
| Expulsion (ejaculation) | Pudendal nerve, bulbospongiosus |
- Damage to the superior hypogastric plexus (aortic surgery) or diabetic neuropathy gives dry or retrograde ejaculation with normal erections; S2–S4 (cauda equina) injury gives retention, faecal incontinence, saddle anaesthesia and impotence.
Visceral pain pathways
- First stage of labour: uterine contraction and cervical dilatation travel with sympathetic plexuses to T10–L1; epidural must cover these segments.
- Second stage: adds somatic perineal pain via the pudendal nerve (S2–S4).
- Pelvic pain line (non-labour organs): structures touching peritoneum (bladder dome) send pain with sympathetics to T11–L2; subperitoneal structures (bladder neck) send pain with pelvic splanchnics to S2–S4.
- Referred pain: ureter T11–L2 (lower ureter to labium and medial thigh via L1); ovary T10–T11 (periumbilical); testis T10.
- Labour contractions are myogenic and hormonal, so labour progresses after a high cord lesion; above T10 it is painless, and at or above T6 there is a risk of autonomic dysreflexia.
Remember Erection is parasympathetic (S2–S4), emission is sympathetic (L1–L2), expulsion is pudendal.
Exam trap Presacral neurectomy does not relieve ovarian pain: ovarian afferents bypass the superior hypogastric plexus.
Exam trap Foot drop after a long labour with weak hip abduction is the lumbosacral trunk, not the common fibular nerve.
Test yourself on this topic
5 of the 46 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
An anaesthetist sets up epidural analgesia for a primigravida in the first stage of labour, whose pain comes from uterine contraction and cervical dilatation. Which spinal segments must the block cover to relieve this pain?
Answer: T10–L1
Pain in the first stage arises from the contracting uterus and the dilating cervix and lower segment. These visceral afferents travel with the sympathetic nerves through the uterovaginal and inferior hypogastric plexuses, the hypogastric nerves and the superior hypogastric plexus, entering the lumbar sympathetic chain and the spinal cord at T10 to L1. Pain is therefore referred to the lower abdomen and lower back. In the second stage, stretching of the vagina and perineum adds somatic pain carried by the pudendal nerves to S2–S4.
Labour pain pathways
- First stage: visceral, T10–L1
- Travels with sympathetic plexuses
- Second stage adds somatic, S2–S4
- Pudendal nerves carry perineal pain
Remember First stage T10–L1; second stage adds S2–S4.
Exam trap Pudendal block helps the second stage only; first-stage pain enters at T10–L1.
Why the other options are wrong
- T4–T6 Level needed for caesarean section, not first-stage labour pain.
- L2–L4 Lumbar plexus segments for the thigh, not the uterus.
- S2–S4 only Covers second-stage perineal pain only.
- C3–C5 Phrenic nerve roots.
A woman with acute urinary retention has suprapubic pain from distension of the peritoneum-covered superior surface of the bladder. Through which fibres do these pain impulses reach the spinal cord?
Answer: Sympathetic-related afferents to T11–L2
Visceral pain from pelvic organs follows the 'pelvic pain line'. Structures above the line, in contact with the peritoneum, such as the superior surface of the bladder and the uterine body, send pain afferents back along sympathetic pathways (hypogastric plexuses and lumbar splanchnic nerves) to the lower thoracic and upper lumbar cord. Structures below the line, such as the bladder neck and cervix, send pain with the pelvic splanchnic nerves to S2–S4. The pudendal nerve carries somatic sensation from the perineum.
Pelvic pain line
- Above the line: pain with sympathetics (T11–L2)
- Below the line: pain with pelvic splanchnics (S2–S4)
- Bladder dome is above the line
- Bladder neck is below it
Remember Peritoneum-covered pelvic viscera send pain up the sympathetic route.
Exam trap Not all bladder pain goes to S2–S4; the dome sends pain along sympathetic routes.
Why the other options are wrong
- Pelvic splanchnic afferents to S2–S4 Carry pain from structures below the pain line, such as the bladder neck.
- Pudendal nerve afferents to S2–S4 Somatic nerve of the perineum, not the bladder dome.
- Vagal afferents to the brainstem The vagus does not supply the bladder.
- Obturator nerve afferents to L2–L4 Somatic nerve of the medial thigh.
Three months after a laparoscopic ovarian cystectomy, a woman has burning pain at her left lower port site, about 2 cm medial to the anterior superior iliac spine, radiating into the left labium majus and upper medial thigh. Which nerve is most likely trapped in the fascial closure?
Answer: Ilioinguinal nerve (L1)
The ilioinguinal nerve (L1) pierces the internal oblique just medial to the anterior superior iliac spine and runs forward between the abdominal muscles to enter the inguinal canal, emerging through the superficial ring to supply the skin of the mons, the anterior labium majus and the upper medial thigh. A lateral port near this point, or the stitch closing it, can catch the nerve, causing neuropathic pain in this distribution. The femoral branch of the genitofemoral supplies the femoral triangle, and the lateral cutaneous nerve supplies the outer thigh.
Ilioinguinal nerve
- From L1
- Pierces internal oblique near the ASIS
- Supplies mons, labium majus, upper medial thigh
- Trapped by lateral port closure
Remember Labial burning after a lateral port: ilioinguinal entrapment.
Exam trap Labial pain after lower port placement is nerve entrapment, not a gynaecological recurrence.
Why the other options are wrong
- Femoral branch of genitofemoral nerve Supplies skin over the femoral triangle, not the labium.
- Lateral cutaneous nerve of the thigh Supplies the lateral thigh.
- Obturator nerve Supplies the inner thigh but not the labium and is deep in the pelvis.
- Subcostal nerve (T12) Supplies skin above the iliac crest and lateral hip.
For primary dysmenorrhoea, a surgeon performs laparoscopic division of both uterosacral ligaments close to the cervix. Which nerve fibres is this procedure meant to interrupt?
Answer: Uterine and cervical pain afferents
The uterosacral ligaments carry autonomic fibres and visceral afferents passing between the cervix and lower uterus and the inferior hypogastric plexus. Uterosacral nerve ablation (LUNA) divides these ligaments close to the cervix in the hope of interrupting pain afferents from the uterus. Trials have not shown a lasting benefit, and the procedure does not affect ovarian afferents, which travel with the ovarian vessels, or the somatic pudendal and obturator nerves.
Uterosacral nerve ablation
- Ligaments carry autonomic and afferent fibres
- Fibres link cervix and pelvic plexus
- Aim: cut uterine pain afferents
- Trials: no lasting benefit
Remember LUNA targets uterine afferents in the uterosacral ligaments.
Exam trap Uterosacral ablation cannot relieve ovarian pain, which uses a separate route.
Why the other options are wrong
- Pudendal fibres to the perineum The pudendal nerve runs in the ischioanal fossa, not in the uterosacral ligaments.
- Ovarian afferents with the ovarian vessels Ovarian afferents follow the ovarian vessels in the infundibulopelvic ligament.
- Somatic fibres to the levator ani The levator ani is supplied by direct S3–S4 branches and the pudendal nerve.
- Obturator fibres to the pelvic wall The obturator nerve runs on the lateral pelvic wall.
During excision of deep endometriosis lateral to the uterosacral ligaments, the surgeon tries to preserve a nerve network beside the rectum and cervix that controls the bladder and rectum. Which fibres combine to form this network?
Answer: Hypogastric nerves joined by pelvic splanchnics
The inferior hypogastric (pelvic) plexus lies on each side of the rectum, cervix, vaginal fornix and bladder base, lateral to the uterosacral ligament. It is formed by sympathetic fibres from the hypogastric nerve (from the superior hypogastric plexus) and the sacral splanchnic nerves, joined by parasympathetic preganglionic fibres of the pelvic splanchnic nerves from S2–S4, together with visceral afferents. Damage in deep pelvic surgery causes bladder, bowel and sexual dysfunction. The vagus and greater splanchnic nerves serve the upper abdomen.
Inferior hypogastric plexus
- Sympathetic: hypogastric and sacral splanchnic nerves
- Parasympathetic: pelvic splanchnics S2–S4
- Lies lateral to the uterosacral ligament
- Damage: bladder, bowel, sexual dysfunction
Remember Pelvic plexus = hypogastric nerves + pelvic splanchnics.
Exam trap The pelvic plexus is mixed: sympathetic from above, parasympathetic from S2–S4.
Why the other options are wrong
- Vagal fibres joined by greater splanchnics These supply the foregut and midgut, not the pelvic plexus.
- Pudendal nerve joined by obturator fibres These are somatic nerves of the perineum and thigh.
- Lumbar plexus joined by ovarian fibres The lumbar plexus is somatic; the ovarian plexus follows the ovarian vessels.
- Femoral nerve joined by genitofemoral fibres These are somatic nerves of the thigh and groin.
Where this sits in the CPSP syllabus
- Obstetrics & Gynaecology, Paper 2: Pelvic organs, vessels and nerves, Lumbosacral plexus, pelvic splanchnics, sacral sympathetic trunk and hypogastric plexuses