Pudendal nerve, course and branches in a male.
|Sacral nerves S2, S3, S4|
|Inferior rectal nerves
dorsal nerve of the penis
dorsal nerve of the clitoris
|Anatomical terms of neuroanatomy|
The pudendal nerve is the main nerve of the perineum.:274 It carries sensation from the external genitalia of both sexes and the skin around the anus and perineum, as well the motor supply to various pelvic muscles, including the male or female external urethral sphincter and the external anal sphincter. If damaged, most commonly by childbirth, lesions may cause sensory loss or fecal incontinence. The nerve may also be temporarily blocked as part of an anaesthetic procedure.
The pudendal canal is also known by the eponymous term "Alcock's canal", after Benjamin Alcock, an Irish anatomist who documented the canal in 1836.
The pudendal nerve is paired, meaning there are two nerves, one on the left and one on the right side of the body. Each is formed as three roots immediately above the upper border of the sacrotuberous ligament and the ischiococcygeus muscle. The three roots become two cords when the middle and lower root join to form the lower cord, and these in turn unite to form the pudendal nerve proper just proximal to the sacrospinous ligament. The three roots are derived from the ventral rami of the second, third, and fourth sacral spinal nerves, with the primary contribution coming from the fourth.:215:157
The pudendal nerve passes between the piriformis muscle and coccygeus (ischiococcygeus) muscles and leaves the pelvis through the lower part of the greater sciatic foramen. It crosses over the lateral part of the sacrospinous ligament and reenters the pelvis through the lesser sciatic foramen. After reentering the pelvis, it accompanies the internal pudendal artery and internal pudendal vein upwards and forwards along the lateral wall of the ischiorectal fossa, being contained in a sheath of the obturator fascia termed the pudendal canal, along with the internal pudendal blood vessels.:8
Inside the pudendal canal, the nerve divides into branches, first giving off the inferior anal nerve, then the perineal nerve, before continuing as the dorsal nerve of the penis (in males) or the dorsal nerve of the clitoris (in females).:34
The pudendal nerve may vary in its origins. For example, the pudendal nerve may actually originate off of the sciatic nerve. Consequently, damage to the sciatic nerve can affect the pudendal nerve as well. Sometimes dorsal rami of the first sacral nerve contribute fibers to the pudendal nerve, and even more rarely S5.
The pudendal nerve supplies sensation to the penis in males and the clitoris in females, through the branches dorsal nerve of penis and dorsal nerve of clitoris.:422 The posterior scrotum in males and the labia in females are also supplied, via the posterior scrotal nerves (males) or posterior labial nerves (females). The pudendal nerve is one of several nerves supplying sensation to these areas. Branches also supply sensation to the anal canal.:8 By providing sensation to the penis and the clitoris, the pudendal nerve is responsible for the afferent component of penile erection and clitoral erection. :147
Branches also innervate muscles of the perineum and pelvic floor; namely the bulbospongiosus and ischiocavernosus muscles, the levator ani muscle (including the Iliococcygeus, pubococcygeus, puborectalis and either pubovaginalis in females or pubourethralis in males),:422 the external anal sphincter (via the inferior anal branch),:7 and male or female external urethral sphincter.:424–425
As it functions to innervate the External Urethral Sphincter it is responsible for the tone of the sphincter mediated via acetylcholine release. This means that during periods of increased acetylcholine release the smooth muscle in the external urethral sphincter contracts, causing urinary retention. Whereas in periods of decreased acetylcholine release the smooth muscle in the external urethral sphincter relaxes, allowing voiding of the bladder to occur. 
A pudendal nerve block, also known as a saddle nerve block, is a local anesthesia technique used in a obstetric procedure to anesthetize the perineum during labor. In this procedure, an anesthetic agent such as lidocaine is injected through the inner wall of the vagina about the pudendal nerve.
The pudendal nerve can be compressed or stretched, resulting in temporary or permanent neuropathy. Irreversible nerve injury may occur when nerves are stretched by 12% or more of their normal length.:655 If the pelvic floor is over-stretched, acutely (e.g. prolonged or difficult childbirth) or chronically (e.g. chronic straining during defecation caused by constipation), the pudendal nerve is vulnerable to stretch-induced neuropathy.:655 Pudendal nerve entrapment, also known as Alcock canal syndrome, is very rare and is associated with professional cycling. Systemic diseases such as diabetes and multiple sclerosis can damage the pudendal nerve via demyelination or other mechanisms.:37 A pelvic tumor (most notably a large sacrococcygeal teratoma), or surgery to remove the tumor, can also cause permanent damage.
Unilateral pudendal nerve neuropathy inconsistently causes fecal incontinence in some, but not others. This is because crossover innervation of the external anal sphincter occurs in some individuals.:34
The pudendal nerve is difficult to visualize on routine CT or MR imaging, however under CT guidance, a needle may be placed adjacent to the pudendal neurovascular bundle. The ischial spine, an easily identifiable structure on CT, is used as the level of injection. A spinal needle is advanced via the gluteal muscles and advanced within several millimeters of the ischial spine. Contrast (X-ray dye) is then injected, highlighting the nerve in the canal and allowing for confirmation of correct needle placement. The nerve may then be injected with cortisone and local anesthetic to confirm and also treat chronic pain of the external genitalia (known as vulvodynia in females), pelvic and anorectal pain.
Nerve latency testing
The time taken for a muscle supplied by the pudendal nerve to contract in response to an electrical stimulus applied to the sensory and motor fibers can be quantified. Increased conduction time (terminal motor latency) signifies damage to the nerve.:46 2 stimulating electrodes and 2 measuring electrodes are mounted on the examiner's gloved finger ("St Mark's electrode").:46
The term pudendal comes from Latin pudenda, meaning external genitals, derived from pudendum, meaning "parts to be ashamed of". The pudendal canal is also known by the eponymous term "Alcock's canal", after Benjamin Alcock, an Irish anatomist who documented the canal in 1836. Alcock documented the existence of the canal and pudendal nerve in a contribution about iliac arteries in Robert Bentley Todd's "The Cyclopaedia of Anatomy and Physiology".
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|Wikimedia Commons has media related to Pudendal nerve.|
- Anatomy figure: 41:04-11 at Human Anatomy Online, SUNY Downstate Medical Center - "Inferior view of female perineum, branches of the internal pudendal artery."
- figures/chapter_32/32-2.HTM — Basic Human Anatomy at Dartmouth Medical School
- figures/chapter_32/32-3.HTM — Basic Human Anatomy at Dartmouth Medical School
- Cross section image: pelvis/pelvis-female-17 - Plastination Laboratory at the Medical University of Vienna
- Diagnosis and treatment at www.nervemed.com
- Pudendal nerve entrapment at chronicprostatitis.com
- CT sequence showing a pudendal nerve block.