Medical School · Year 1 · Anatomy Pelvis Head Neck · includes a quiz and discussion video
Lecture 8: Pelvic Nerves
Unit 1.5: Human Gross Anatomy III - Pelvis and Head/Neck
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the sacral plexus and its major branches
- Explain the autonomic innervation of pelvic viscera
- Describe the course and distribution of the pudendal nerve
- Identify the components of the inferior hypogastric plexus
- Explain the neural control of bladder, bowel, and sexual function
- Correlate anatomical features with clinical conditions
Overview of Pelvic Innervation
The pelvis receives both somatic and autonomic innervation, each serving distinct functions. Understanding pelvic neuroanatomy is essential for interpreting symptoms of dysfunction and for preserving neural structures during pelvic surgery.
Somatic innervation to the pelvis derives from the sacral plexus (L4-S4), which provides motor and sensory innervation to the pelvic floor muscles, the perineum, the gluteal region, and the lower limb. The pudendal nerve (S2-S4), the most important somatic nerve of the perineum, controls the voluntary sphincters and provides sensation to the external genitalia.
Autonomic innervation controls the pelvic viscera—bladder, rectum, and reproductive organs. The sympathetic supply reaches the pelvis via the hypogastric nerves and the sacral sympathetic trunk, generally promoting storage functions. The parasympathetic supply comes from the pelvic splanchnic nerves (S2-S4), generally promoting emptying and erectile functions. These fibers integrate at the inferior hypogastric plexus before distributing to individual organs.
<image>Panel A: Sagittal section showing sacral plexus on posterior pelvic wall with branches exiting through greater sciatic foramen. Panel B: Sympathetic fibers descending via hypogastric nerves and sacral sympathetic trunk. Panel C: Parasympathetic fibers from pelvic splanchnic nerves (S2-S4) converging at inferior hypogastric plexus on lateral pelvic wall. Panel D: Subsidiary branches extending from plexus to bladder, rectum, and reproductive organs.</image>
The Lumbosacral Trunk
The lumbosacral trunk is a crucial neural structure that links the lumbar and sacral plexuses, allowing contributions from L4 and L5 to participate in forming nerves of the sacral plexus.
This thick nerve trunk forms from descending fibers of the L4 and L5 ventral rami. It descends over the ala (wing) of the sacrum, passing behind the common iliac vessels, to join the S1 ventral ramus within the pelvis. Through this connection, L4 and L5 contribute to the formation of the sciatic nerve and other sacral plexus branches.
The lumbosacral trunk is vulnerable to compression against the sacral ala by pelvic masses or by the fetal head during prolonged labor. Such compression can result in foot drop and numbness in the leg—a form of maternal obstetric palsy.
<image>Panel A: Lumbosacral trunk with L4 and L5 ventral rami contributions descending over clearly labeled sacral ala. Panel B: Trunk joining S1 root to form sacral plexus with common iliac vessels crossing anteriorly. Panel C: Vulnerable position between sacral ala and pelvic contents emphasized. Panel D: Inset showing fetal head or pelvic mass compression causing maternal obstetric palsy with foot drop.</image>
The Sacral Plexus
The sacral plexus is a large nerve network formed from the ventral rami of L4-S4 (with contributions via the lumbosacral trunk). It lies on the anterior surface of the piriformis muscle, deep within the pelvis.
The plexus has a triangular configuration, with nerve roots emerging from the anterior sacral foramina and converging toward the greater sciatic foramen. Most branches exit the pelvis through the greater sciatic foramen to supply the gluteal region, posterior thigh, and entire lower leg and foot.
Relations of the sacral plexus include: anteriorly, the internal iliac vessels, ureter, sigmoid colon (on the left), and rectum; posteriorly, the piriformis muscle. The sacral nerves emerge through the anterior sacral foramina, with the lumbosacral trunk joining from above.
<image>Panel A: Sacral plexus on piriformis muscle with contributing roots (lumbosacral trunk L4-L5, S1-S4) emerging from sacral foramina. Panel B: Roots joining to form triangular plexus configuration. Panel C: Major branches (sciatic, superior gluteal, inferior gluteal, pudendal, posterior femoral cutaneous) traced to greater sciatic foramen exit. Panel D: Relationship to piriformis muscle and internal iliac vessels illustrated.</image>
Branches of the Sacral Plexus: Nerves to Pelvic Muscles
The sacral plexus gives rise to several nerves that supply muscles of the pelvic wall and floor.
The nerve to piriformis arises from S1-S2 and passes directly into the piriformis muscle without leaving the pelvis.
The nerve to obturator internus arises from L5-S2, exits through the greater sciatic foramen below the piriformis, crosses the ischial spine, and enters the perineum through the lesser sciatic foramen to supply the obturator internus and superior gemellus muscles from their pelvic surface.
The nerve to quadratus femoris arises from L4-S1, exits below the piriformis, and descends deep to the sciatic nerve to supply the quadratus femoris and inferior gemellus muscles.
The nerves to the levator ani arise directly from S3-S4, passing to the pelvic (superior) surface of the levator ani muscle. These are distinct from the pudendal nerve and provide the primary motor innervation to the pelvic floor muscles. The coccygeus muscle receives similar direct innervation from S4-S5.
<image>Panel A: Nerve to piriformis (S1-S2) entering muscle directly without leaving pelvis. Panel B: Nerve to obturator internus (L5-S2) coursing through greater and lesser sciatic foramina to reach muscle. Panel C: Nerve to quadratus femoris (L4-S1) descending deep to sciatic nerve. Panel D: Direct S3-S4 branches to levator ani from pelvic surface and S4-S5 branches to coccygeus with root values labeled.</image>
Major Branches Exiting the Pelvis
The principal branches of the sacral plexus exit through the greater sciatic foramen, with the piriformis muscle serving as a key anatomical landmark.
The superior gluteal nerve (L4-S1) is the only nerve to exit above the piriformis muscle, accompanying the superior gluteal vessels. It supplies the gluteus medius, gluteus minimus, and tensor fasciae latae—the hip abductors. Injury causes a Trendelenburg gait (the pelvis drops on the unsupported side during walking).
The sciatic nerve (L4-S3), the largest nerve in the body, exits below the piriformis. Though appearing as a single nerve, it actually comprises two components bound in a common sheath: the tibial nerve (L4-S3, medial) and the common fibular (peroneal) nerve (L4-S2, lateral). The sciatic nerve supplies the posterior thigh muscles via the tibial component and continues to innervate the entire leg below the knee via both components.
The inferior gluteal nerve (L5-S2) exits below the piriformis to supply the gluteus maximus, the primary hip extensor. Injury causes difficulty rising from a seated position and climbing stairs.
The posterior femoral cutaneous nerve (S1-S3) exits below the piriformis to provide sensory innervation to the skin of the posterior thigh, inferior buttock (inferior cluneal nerves), and the posterior perineum (perineal branch).
The pudendal nerve (S2-S4) exits below the piriformis but follows a unique course, crossing the ischial spine and re-entering the perineum through the lesser sciatic foramen to become the principal nerve of the perineum.
<image>Panel A: Piriformis muscle as key landmark with suprapiriform compartment above showing superior gluteal nerve and vessels exit. Panel B: Infrapiriform compartment below piriformis with sciatic nerve (largest), inferior gluteal, posterior femoral cutaneous, and pudendal nerves. Panel C: Each nerve's course and distribution indicated with arrows showing muscle targets. Panel D: Inset showing Trendelenburg sign from superior gluteal nerve injury with pelvis drop on unsupported side.</image>
The Pudendal Nerve
The pudendal nerve (S2-S4) is the principal somatic nerve of the perineum, providing motor innervation to the striated sphincters and perineal muscles, and sensory innervation to the external genitalia and perianal skin.
The pudendal nerve follows a distinctive course that can be remembered as a double transit through the greater and lesser sciatic foramina. After forming on the piriformis, it exits the pelvis through the greater sciatic foramen below the piriformis, curves around the posterior aspect of the ischial spine (where it can be blocked for obstetric anesthesia), and re-enters the perineum through the lesser sciatic foramen. It then travels in the pudendal canal (Alcock's canal), a fascial tunnel formed by the obturator fascia on the lateral wall of the ischioanal fossa.
Within the pudendal canal and beyond, the nerve gives rise to three major branches. The inferior rectal nerve branches early (often before or at the entrance to the canal), crossing the ischioanal fossa to supply the external anal sphincter and the perianal skin. The perineal nerve continues forward to divide into deep branches (motor to the muscles of the urogenital triangle and the external urethral sphincter) and superficial branches (sensory to the posterior scrotum or labia). The dorsal nerve of the penis/clitoris continues forward along the inferior pubic ramus to provide sensory innervation to the glans and body of the penis or clitoris.
<image>Panel A: Pudendal nerve on piriformis exiting through greater sciatic foramen below piriformis and curving around ischial spine with block site marked. Panel B: Nerve entering through lesser sciatic foramen and traveling in pudendal canal on ischioanal fossa lateral wall. Panel C: Sequential branches - inferior rectal nerve to anal canal and perineal nerve with deep muscular and superficial scrotal/labial branches. Panel D: Dorsal nerve of penis/clitoris continuing to external genitalia with color-coded distribution territories.</image>
The Coccygeal Plexus
The coccygeal plexus is a small network formed by the ventral rami of S4, S5, and the coccygeal nerve (Co). It lies on the pelvic surface of the coccygeus muscle and sacrotuberous ligament.
The principal branches are the anococcygeal nerves, which pierce the anococcygeal ligament to provide sensory innervation to the skin over the coccyx and in the intergluteal cleft.
Though small, the coccygeal plexus may be relevant in patients with coccydynia (tailbone pain) and in understanding the complete innervation pattern of the perineal region.
<image>Panel A: Coccygeal plexus on pelvic surface of coccygeus muscle with S4, S5, and coccygeal nerve contributions. Panel B: Small network formation from contributing roots. Panel C: Anococcygeal nerves piercing anococcygeal ligament to reach skin overlying coccyx. Panel D: Relationship to sacrotuberous ligament and transition from sacral plexus above.</image>
Autonomic Innervation: Sympathetic Pathways
The sympathetic innervation of the pelvis derives from the thoracolumbar outflow (T10-L2) and reaches pelvic organs via two pathways: the hypogastric nerves descending from the superior hypogastric plexus, and the sacral sympathetic trunk with its gray rami.
The sacral sympathetic trunk is a direct continuation of the lumbar sympathetic trunk, descending on the anterior surface of the sacrum. It typically contains four to five ganglia, decreasing in size inferiorly, and terminates at the ganglion impar—a single midline ganglion anterior to the coccyx where the two trunks fuse. Because there are no white rami communicantes below L2, all sympathetic fibers in the sacral trunk have descended from higher levels. Gray rami from each ganglion rejoin the sacral and coccygeal nerves, distributing sympathetic fibers to the lower limb and perineum.
The superior hypogastric plexus (also called the presacral nerve) lies anterior to the L5 vertebral body and sacral promontory. It forms as a continuation of the intermesenteric plexus and receives contributions from the lumbar splanchnic nerves (L1-L2). This plexus is predominantly sympathetic. It divides into the right and left hypogastric nerves, which descend into the pelvis on either side of the rectum to reach the inferior hypogastric plexus.
<image>Panel A: Lumbar sympathetic trunk continuing as sacral sympathetic trunk on anterior sacrum with ganglia decreasing in size. Panel B: Ganglion impar at coccyx where two trunks fuse as single midline ganglion. Panel C: Superior hypogastric plexus at L5/sacral promontory receiving lumbar splanchnic input. Panel D: Right and left hypogastric nerves descending to inferior hypogastric plexuses with arrows showing sympathetic flow direction.</image>
Autonomic Innervation: Parasympathetic Pathways
The parasympathetic innervation of the pelvis derives from the sacral outflow (S2-S4) via the pelvic splanchnic nerves, which are unique as the only parasympathetic outflow below the head.
The pelvic splanchnic nerves (also called nervi erigentes, Latin for "nerves of erection") arise from the S2, S3, and S4 ventral rami within the pelvis. These nerves leave the sacral roots and pass directly to the inferior hypogastric plexus, where they merge with sympathetic fibers from the hypogastric nerves.
From the inferior hypogastric plexus, parasympathetic fibers distribute to all pelvic viscera, generally promoting functions opposite to sympathetic effects: bladder detrusor contraction (micturition), rectal smooth muscle contraction and internal sphincter relaxation (defecation), and vasodilation in erectile tissues (erection).
A distinctive feature of the pelvic splanchnic nerves is their ascending distribution. Some fibers ascend retroperitoneally to join the inferior mesenteric plexus and follow the inferior mesenteric artery branches to supply the hindgut—the descending colon, sigmoid colon, and rectum (from the splenic flexure distally). This pathway explains why the parasympathetic innervation to these structures differs from that of the rest of the colon (which receives vagal parasympathetic fibers).
<image>Panel A: Pelvic splanchnic nerves arising from S2, S3, S4 leaving sacral roots and passing to inferior hypogastric plexus. Panel B: Branches to bladder causing detrusor contraction for micturition. Panel C: Branches to rectum promoting defecation and to erectile tissues causing erection. Panel D: Distinctive colored ascending pathway along inferior mesenteric vessels supplying descending colon, sigmoid, and rectum.</image>
The Inferior Hypogastric Plexus
The inferior hypogastric plexus (also called the pelvic plexus) is the principal integration center for autonomic control of pelvic viscera. It is a bilateral structure, located on either side of the rectum—between the rectum and bladder in males, and between the rectum and vagina in females.
The plexus receives input from three sources: sympathetic fibers via the hypogastric nerves descending from the superior hypogastric plexus, parasympathetic fibers via the pelvic splanchnic nerves from S2-S4, and additional sympathetic fibers from the sacral splanchnic nerves (from the sacral sympathetic trunk ganglia).
From the inferior hypogastric plexus, subsidiary plexuses extend to each pelvic organ along the visceral blood vessels. The vesical plexus supplies the bladder, controlling storage (sympathetic) and voiding (parasympathetic). The prostatic plexus (in males) supplies the prostate, seminal vesicles, and erectile tissue of the penis; it contains the cavernous nerves (nervi erigentes), which are the parasympathetic fibers responsible for erection and are vulnerable during radical prostatectomy. The uterovaginal plexus (in females) supplies the uterus, vagina, and clitoral erectile tissue. The middle rectal plexus supplies the rectum.
The nerves of the inferior hypogastric plexus lie in the endopelvic fascia lateral to the rectum and viscera. Their position makes them vulnerable during pelvic surgery, particularly radical hysterectomy and radical prostatectomy, where nerve-sparing techniques aim to preserve sexual and urinary function.
<image>Panel A: Inferior hypogastric plexus positioned lateral to rectum with hypogastric nerve input from above (sympathetic). Panel B: Pelvic splanchnic nerves from S2-S4 (parasympathetic) and sacral splanchnic nerves from sacral sympathetic trunk as inputs. Panel C: Subsidiary branches to bladder (vesical plexus), prostate/uterus-vagina, and rectum (rectal plexus). Panel D: Cavernous nerves running posterolateral to prostate highlighted as site at risk during radical prostatectomy.</image>
Neural Control of Pelvic Organ Function
Bladder
The bladder operates in two phases: storage and voiding. During storage, sympathetic tone (T11-L2) relaxes the detrusor muscle and contracts the internal sphincter (in males), while somatic tone (pudendal nerve) keeps the external sphincter contracted. During voiding, parasympathetic activation (S2-S4) contracts the detrusor and relaxes the internal sphincter, while voluntary relaxation of the external sphincter permits urine flow.
Rectum and Continence
Similar principles govern the rectum. Sympathetic activity decreases motility and maintains internal anal sphincter contraction (storage). Parasympathetic activity increases motility and relaxes the internal sphincter (defecation). The external anal sphincter, under pudendal nerve control, provides voluntary continence.
Male Sexual Function
Sexual function involves precise coordination of autonomic and somatic systems. Erection is primarily a parasympathetic phenomenon ("point"): pelvic splanchnic nerves (S2-S4) release nitric oxide and acetylcholine, causing cavernosal smooth muscle relaxation and arterial dilation, leading to engorgement of the corpora cavernosa. Emission is sympathetic ("shoot"): T10-L2 sympathetic fibers cause contraction of the vas deferens, seminal vesicles, and prostate, moving semen into the prostatic urethra while the internal sphincter closes to prevent retrograde ejaculation. Ejaculation is somatic: rhythmic contractions of the bulbospongiosus and ischiocavernosus muscles (pudendal nerve, S2-S4) expel semen from the urethra.
Female Sexual Function
In females, clitoral erection and vaginal lubrication are parasympathetic (S2-S4). Uterine contraction varies with hormonal state—sympathetic activity dominates during non-pregnant states, while the significance of autonomic innervation during labor is less clear due to the strong influence of oxytocin.
<image>Panel A: Bladder control showing storage phase (sympathetic - detrusor relaxed, sphincter contracted) and voiding phase (parasympathetic - detrusor contracted, sphincter relaxed). Panel B: Colorectal control with similar storage (sympathetic) and defecation (parasympathetic) patterns. Panel C: Male erection (parasympathetic "point") and emission (sympathetic "shoot") phases with nerve pathways labeled. Panel D: Ejaculation phase (somatic pudendal) with effector organs and complete sexual function sequence.</image>
Visceral Afferent Pathways
Pain and other sensations from pelvic viscera travel via visceral afferent fibers that accompany the autonomic nerves. The pathway taken determines the spinal cord level of entry and, consequently, the pattern of referred pain.
Pain from the bladder travels with sympathetic afferents (T11-L2), referring to the suprapubic region and perineum. Pain from the prostate similarly follows sympathetic pathways, referring to the perineum and lower back.
Pain from the uterine body travels with sympathetic afferents (T10-L1), referring to the lower abdomen. This pathway explains why labor pain (uterine contractions) can be blocked with epidural anesthesia at the T10-L1 levels. Pain from the cervix, however, travels with parasympathetic afferents (S2-S4), referring to the lower back and thighs—this pathway becomes important during the later stages of labor when cervical dilation dominates.
Physiologic sensations—bladder fullness, rectal distension—travel primarily with parasympathetic fibers to S2-S4. These are stretch-mediated and trigger appropriate reflexes without being perceived as painful.
<image>Panel A: Bladder and prostate afferents traveling with sympathetic pathways to T11-L2 with dermatome map showing suprapubic referred pain. Panel B: Uterine body afferents to T10-L1 with abdominal referred pain for labor contraction analgesia. Panel C: Cervix afferents with parasympathetic pathways to S2-S4 causing back/thigh referred pain in late labor. Panel D: Physiologic sensations (fullness, distension) traveling with parasympathetics and clinical annotation for epidural placement during labor.</image>
Clinical Correlations
Pudendal Nerve Block
A pudendal nerve block is performed to provide anesthesia for vaginal delivery, episiotomy repair, or perineal procedures. The ischial spine serves as the landmark—the pudendal nerve crosses its posterior aspect as it travels from the greater to the lesser sciatic foramen. The block can be performed transvaginally (palpating the ischial spine through the vaginal wall) or through a perineal approach. Successful block anesthetizes the external anal sphincter, the perineal skin, and the external genitalia.
Cauda Equina Syndrome
Cauda equina syndrome results from compression of the sacral nerve roots below the conus medullaris (typically at L2-L3 and below), most commonly by a large central disc herniation or tumor. Classic features include saddle anesthesia (S2-S5 dermatomes over the perineum), bladder and bowel dysfunction (retention, incontinence), and bilateral leg weakness. This is a surgical emergency requiring urgent decompression to prevent permanent deficits.
Pelvic Surgery and Nerve Injury
Pelvic surgery carries risks of nerve injury with significant functional consequences. Injury to the hypogastric plexus (as during low anterior resection) can cause bladder dysfunction and retrograde ejaculation. Injury to the inferior hypogastric plexus or cavernous nerves (as during radical prostatectomy or radical hysterectomy) can cause erectile dysfunction in men or decreased vaginal sensation in women. Nerve-sparing surgical techniques aim to preserve the autonomic plexuses by careful dissection along fascial planes.
Erectile Dysfunction
Erectile dysfunction may result from damage to the cavernous nerves during pelvic surgery (prostatectomy, colorectal surgery), pelvic fracture with associated nerve injury, diabetes mellitus (affecting small autonomic nerves), or atherosclerosis affecting the pudendal vessels. The cavernous nerves run posterolateral to the prostate, explaining why this region is carefully preserved during nerve-sparing radical prostatectomy.
Neurogenic Bladder
Neurogenic bladder refers to bladder dysfunction from neurological causes. Upper motor neuron lesions (above the sacral cord, such as spinal cord injury) produce a spastic bladder with small capacity, high pressures, and involuntary reflex voiding. Lower motor neuron lesions (affecting the sacral cord or peripheral nerves) produce a flaccid, acontractile bladder with large capacity and overflow incontinence.
Piriformis Syndrome
Piriformis syndrome is compression of the sciatic nerve by the piriformis muscle, presenting as buttock pain radiating down the posterior thigh (similar to sciatica from disc herniation). Pain is often aggravated by sitting and internal rotation of the hip. In some individuals, the sciatic nerve (or its common fibular component) passes through or above the piriformis, predisposing to compression.
<image>Panel A: Pudendal nerve block at ischial spine with transvaginal approach showing needle placement and anesthesia area achieved. Panel B: Cauda equina syndrome with classic triad - saddle anesthesia shaded on perineum, distended bladder, and bilateral leg weakness. Panel C: Nerve-sparing prostatectomy with neurovascular bundles preserved posterolateral to prostate for potency. Panel D: Piriformis syndrome showing sciatic nerve compression and variant anatomy where nerve passes through muscle.</image>
Summary
The sacral plexus (L4-S4) lies on the piriformis muscle and gives rise to the sciatic nerve (the body's largest), gluteal nerves, and the pudendal nerve. The superior gluteal nerve is the only branch exiting above the piriformis; all others exit below. The pudendal nerve (S2-S4) is the principal somatic nerve of the perineum, following a distinctive course around the ischial spine before traveling in the pudendal canal to supply the external sphincters and external genitalia. Sympathetic innervation to the pelvis arrives via the hypogastric nerves from the superior hypogastric plexus (L5 level) and the sacral sympathetic trunk, promoting storage functions. Parasympathetic innervation (S2-S4) arrives via the pelvic splanchnic nerves, promoting voiding and erectile functions. These merge at the inferior hypogastric plexus, which distributes to the pelvic viscera. Erection is parasympathetic ("point"), emission is sympathetic ("shoot"), and ejaculation is somatic (pudendal). Pain from the uterine body travels with sympathetics (T10-L1), while cervical pain travels with parasympathetics (S2-S4), explaining different patterns of labor pain.
Key Terms
Sacral plexus: The nerve network formed from L4-S4 ventral rami, lying on the piriformis muscle and giving rise to nerves supplying the gluteal region, posterior thigh, entire leg, and perineum.
Pudendal nerve: The principal somatic nerve of the perineum (S2-S4), following a distinctive course around the ischial spine and through the pudendal canal to innervate the external sphincters and external genitalia.
Pelvic splanchnic nerves (nervi erigentes): The parasympathetic nerves arising from S2-S4, providing parasympathetic innervation to the pelvic viscera and hindgut; responsible for bladder contraction, rectal emptying, and erection.
Superior hypogastric plexus (presacral nerve): The predominantly sympathetic plexus at the L5/sacral promontory level, giving rise to the hypogastric nerves that carry sympathetic fibers to the pelvis.
Inferior hypogastric plexus (pelvic plexus): The bilateral autonomic plexus lateral to the rectum where sympathetic (hypogastric nerves) and parasympathetic (pelvic splanchnic nerves) fibers merge before distributing to pelvic organs.
Cavernous nerves: The parasympathetic fibers from the inferior hypogastric plexus that pass posterolateral to the prostate to innervate the erectile tissue; preservation during radical prostatectomy is essential for potency.
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