Pelvic Nerves

Formed by the union of part of the ventral ramus of L4 (the portion not contributing to the femoral/obturator nerves of the lumbar plexus) with the entire ventral ramus of L5.

• Descends over the pelvic brim (ala of the sacrum) to join the sacral ventral rami, forming the primary structural link between the lumbar and sacral plexuses.

• Formed by the lumbosacral trunk (L4-L5) together with the ventral rami of S1-S4, lying on the anterior (pelvic) surface of piriformis.
• Gives rise to the majority of the nerves supplying the lower limb (sciatic, superior/inferior gluteal, posterior cutaneous nerve of thigh, and others) as well as the pudendal nerve, which is unique among the sacral plexus's branches in supplying the perineum rather than the lower limb.

Pudendal nerve (S2, S3, S4):

• The principal somatic (both motor and sensory) nerve of the perineum.
• Course: Exits the pelvis via the greater sciatic foramen (below piriformis), crosses the ischial spine (a key surface/palpable landmark, alongside the sacrospinous ligament here), re-enters the perineum via the lesser sciatic foramen, then runs within the pudendal (Alcock's) canal on the lateral wall of the ischioanal fossa.
• Branches: Inferior rectal nerve (motor to external anal sphincter, sensory to the lower anal canal/perianal skin), perineal nerve (motor to the perineal muscles, sensory to the posterior scrotum/labia), dorsal nerve of the penis/clitoris (sensory, terminal branch).
• Full detail (including the "figure-of-eight" course around the ischial spine, shared with the internal pudendal vessels).

Sciatic nerve (L4-S3):

• Arises from the sacral plexus, exits via the greater sciatic foramen (below piriformis) to enter the gluteal region and posterior thigh; full detail in Lower Limb content; included here for completeness given its sacral plexus origin.

Superior gluteal nerve (L4, L5, S1):

• Arises from the sacral plexus, exits above piriformis: full detail in Lower Limb content.

Inferior gluteal nerve (L5, S1, S2):

• Arises from the sacral plexus, exits below piriformis: full detail in Lower Limb content.

Nerve to obturator internus (L5, S1, S2):

• Arises from the sacral plexus, exits via the greater sciatic foramen (below piriformis), re-enters via the lesser sciatic foramen (alongside the pudendal nerve and internal pudendal vessels) to supply obturator internus from its pelvic aspect.

Nerve to quadratus femoris (L4, L5, S1):

• Arises from the sacral plexus, exits via the greater sciatic foramen (below piriformis, running deep to the sciatic nerve) to supply quadratus femoris (and gives an articular branch to the hip joint); full detail in Lower Limb content.

Superior hypogastric plexus:

• Lies anterior to the sacral promontory/L5 vertebral body, formed by the continuation of the aortic (intermesenteric) plexus, receiving sympathetic contributions from the lumbar splanchnic nerves.
• Divides into the right and left hypogastric nerves, descending into the pelvis.

Hypogastric nerves:

• Paired nerves (right and left) continuing from the superior hypogastric plexus, descending on either side of the rectum to join the inferior hypogastric plexuses: carry sympathetic (and some visceral afferent) fibres into the pelvis.

Inferior hypogastric plexus (pelvic plexus):

• Paired plexuses lying on the lateral pelvic wall (close to the pelvic viscera, within the endopelvic fascia near the cervix/lateral rectum/bladder base), formed by the convergence of the hypogastric nerves (sympathetic) and the pelvic splanchnic nerves (parasympathetic).
• Distributes named sub-plexuses to each pelvic organ (vesical plexus, rectal plexus, and in females the uterovaginal plexus), the final common autonomic pathway supplying bladder, rectum, and reproductive organs.
• Clinically critical surgical landmark: its close proximity to the lateral aspects of the rectum, cervix, and prostate/seminal vesicles means that pelvic surgery (particularly rectal cancer surgery, total mesorectal excision, and radical hysterectomy/prostatectomy) carries a significant risk of inadvertent plexus injury if dissection strays outside the correct anatomical planes, producing bladder, bowel, and/or sexual dysfunction (see Clinical Anatomy below).

Pelvic splanchnic nerves (nervi erigentes):

• Arise directly from the ventral rami of S2, S3, S4, carrying parasympathetic preganglionic fibres to the inferior hypogastric plexus and onward to the pelvic viscera (bladder detrusor, distal colon/rectum, and (the origin of the traditional name "nervi erigentes") the erectile tissue of the penis/clitoris, mediating erection via parasympathetically-induced vasodilation).
• Represent the sole parasympathetic supply to the hindgut/pelvic viscera, distinct from and additional to the vagus nerve's foregut/midgut territory (see Abdomen content), a key organising principle: the vagus does NOT supply the hindgut or pelvic organs; this territory is exclusively S2-S4 parasympathetic.

Sympathetic fibres:

• Reach the pelvis via two routes: (1) the hypogastric nerves, descending directly from the superior hypogastric plexus (carrying fibres that originated from the lumbar sympathetic chain/lumbar splanchnic nerves), and (2) the sacral sympathetic chain itself, continuing directly from the lumbar sympathetic trunk over the sacral ala, giving grey rami communicantes to the sacral spinal nerves and additional direct branches to the inferior hypogastric plexus.
• Sympathetic activity generally promotes bladder relaxation/sphincter contraction (urinary continence), reduces gut motility, and (in males) mediates emission and ejaculation: a functional division of labour with the parasympathetic system's role in erection ("point and shoot": Parasympathetic = Point (erection); Sympathetic = Shoot (ejaculation)).

• The pelvic splanchnic nerves (S2-S4) provide the parasympathetic supply to the descending colon (via retrograde travel up through the inferior hypogastric and inferior mesenteric plexuses, a notable exception where parasympathetic fibres travel "against" the expected sympathetic-plexus-only direction to reach hindgut structures otherwise supplied by the IMA), sigmoid colon, rectum, bladder, and erectile tissue.
• Promotes bladder detrusor contraction (micturition), increases hindgut motility, and mediates erection.

• Originate from lower thoracic/upper lumbar spinal cord levels, travel via the lumbar splanchnic nerves to the superior hypogastric plexus/hypogastric nerves (the principal route), or via the sacral sympathetic chain directly (a secondary route), both converging on the inferior hypogastric plexus for final distribution to pelvic organs.

• Pain fibres from pelvic organs above the pelvic pain line (an anatomical/functional boundary, roughly corresponding to structures still in contact with peritoneum, upper rectum, uterine fundus/body, upper bladder) travel with the sympathetic fibres retrograde to the superior hypogastric plexus and thence to the lower thoracic/upper lumbar spinal cord, producing pain referred to the lower abdomen/suprapubic region.
• Pain fibres from structures below the pelvic pain line (lower rectum/anal canal above the pectinate line, cervix, upper vagina, bladder base/trigone) travel with the parasympathetic pelvic splanchnic nerves to the S2-S4 spinal cord segments, producing pain that may be less well localised or referred differently, and explains why pathology in these lower structures can sometimes present with perineal or sacral discomfort rather than typical suprapubic pain.

Pudendal nerve block:

• Performed by injecting local anaesthetic around the pudendal nerve as it crosses the ischial spine (transvaginal approach in obstetric practice, or transperineal/transgluteal approaches in other contexts), providing anaesthesia to the perineum, lower vagina, and vulva/scrotum for procedures such as instrumental vaginal delivery or perineal repair.
• The ischial spine's palpability (per vaginam in obstetric practice) makes this a reliable, reproducible landmark-based block.

Pelvic autonomic nerve injury:

• Occurs most commonly during radical pelvic surgery (total mesorectal excision for rectal cancer, radical hysterectomy for cervical cancer, radical prostatectomy) where the inferior hypogastric plexus and its branches lie close to the surgical dissection planes around the rectum, cervix, and prostate/seminal vesicles.
• Consequences depend on which fibres are injured: sympathetic injury can impair bladder neck closure (retrograde ejaculation in males, or urinary symptoms) and reduce ejaculatory function; parasympathetic (pelvic splanchnic nerve) injury impairs erectile function (males) and bladder detrusor contraction (both sexes), potentially causing urinary retention/atonic bladder.
• Nerve-sparing surgical techniques (developed particularly for radical prostatectomy and, increasingly, rectal and gynaecological cancer surgery) are specifically designed around detailed knowledge of the inferior hypogastric plexus's course to minimise this risk while still achieving adequate oncological resection margins.