Pelvic nerves and vessels

• Sacral plexus

  Between them, the lumbar plexus and the sacral plexus supply innervation to the lower limb and pelvis. The lumbar plexus has been described during the posterior abdominal wall presentation and will not be included here. L4 and L5 nerve roots form a lumbosacral trunk that carries nerve fibres to join S1 nerve root at the start of the sacral plexus. The sacral component of the plexus is formed from S1 to S4 roots.

  
• Sciatic nerve (SN)

  •L4, L5, S1-3

  Posterior compartment of thigh and branches supply all muscles leg/foot

  
• Sciatic nerve (SN)

  The largest nerve from the sacral plexus is the sciatic nerve. It carries nerve fibres from the lumbosacral trunk and roots within the pelvis to form a nerve root value of L4-S3. It forms on the anterior surface of piriformis muscle on the lateral internal wall of the pelvis and then passes out of the pelvis via the greater sciatic notch to reach the gluteal region.

  
• Sciatic nerve (SN)

  It is easy to identify in the gluteal region as a very large nerve passing from beneath piriformis to then pass down the posterior surface of the thigh. It supplies the posterior compartment of the thigh (the hamstring muscles) and then divides into branches that supply all of the muscles of the leg and foot. It also carries sensory fibres

  
• Gluteal nerves (superior SG and inferior IG)

  •Superior (L4, L5, S1) gluteal to gluteus medius and minimus

  •Inferior (L5, S1, S2) gluteal to gluteus maximus

  
• Gluteal nerves (superior SG and inferior IG)

  The gluteal nerves supply the gluteal muscles. The nerves and accompanying vessels are easy to identify in the gluteal region passing through the greater sciatic foramen immediately above and below piriformis.

  
• Nerve to obturator internus  (OI)

  •Direct L5-S1

  
• Nerve to piriformis

  •Direct from S1-S2

  
• Nerves to levator ani and coccygeus

  •Levator ani direct S4 and pudendal

  •Coccygeus direct S4, S5

  
• Pudendal nerve

  •S2-S4

  •Levator ani, ext. anal and ext. urethral sphincters, bulbospongiosus, ischiocavernosus

  •Motor and sensory

  The pudendal nerve is a very important nerve for muscles of the pelvic floor and the perineum. It supplies both the external (voluntary) sphincters of the urethra and anal canal so is important in the maintenance of continence. It also carries sensory fibres to the perineum in males and females.

  
• Pudendal nerve

  It forms from nerve roots S2 to S4 in the pelvis, close to the pelvic floor then passes through the greater sciatic foramen, around the ischial spine and then through the lesser sciatic foramen to reach the anal triangle and then the perineum. The ischial spine is used as a landmark during a pudendal nerve block.

  
• Autonomic

  The internal organs of the pelvis and the erectile tissues of the perineum are supplied by autonomic nerves. The superior hypogastric plexus is a sympathetic nerve plexus that sits at the bifurcation of the aorta into the common iliac arteries. The inferior hypogastric plexus is a mix of sympathetic and parasympathetic fibres that sits on the internal lateral wall of the pelvis (one each side).

  
• The vagus nerve supplied the abdominal viscera with parasympathetic innervation up until two-thirds of the way along the transverse colon. The hindgut was innervated by the pelvic splanchnic nerves.

  The sympathetic supply to the pelvic organs is from lumbar and sacral splanchnic nerves derived from the sympathetic chains. The pelvic splanchnic nerves from roots S2 to S4 supply parasympathetic innervation to the pelvis. The diagram shows the interaction with the superior and inferior hypogastric plexus, in life there are splanchnic nerves on the left and right sides.

  
• Sympathetic

  •Rectum = decrease movement

  •Contract internal anal and urethral sphincters

  •Relax bladder detrusor

  •Ejaculation = contract ductus deferens and seminal vesicles

  •Uterus = relax/contract (hormonal)

  •Vasoconstriction of arteries

  
• Parasympathetic

  •Pelvic splanchnic nerves S2-4

  •Increase motility of rectum and anal canal

  •Relax internal anal sphincter

  •Bladder = contracts detrusor and inhibits contraction of internal sphincter

  •No effect on seminal vesicles, ductus deferens, uterus

  •Vasodilation of arteries = erection

  
• Nerve supply of perineum (male)

  
• Nerves of perineum (female)

  
• Dermatomes

  Sensory  to anterior skin = ilioinguinal, genitofemoral and pudendal

  Pudendal sensory to genitalia

  
• Vessels

  •At L4 the aorta divides to form 2 common iliac arteries

  •Common iliac arteries divide into internal and external iliac branches

  •The blood supply of the pelvis is from the internal iliac arteries

  •Each internal iliac artery divides into an anterior and posterior division

  
• Vessels

  The main arterial supply of the pelvic structures is the internal iliac artery. The aorta divides to form the common iliac arteries at L4 and then each common iliac artery divides to form an external iliac artery that supplies the lower limb and an internal iliac artery to the pelvis. Each internal iliac artery then further divides into an anterior and a posterior division to supply the pelvic structures.

  
• Vessels

  Variation of the vessels is common, even large structures may vary – for example the internal iliac may not divide to form a posterior and an anterior division but all vessels may branch from one single common vessel.

  
• Branches of the anterior division

  The umbilical artery ends as a piece of connective tissue but before it does this is forms at least two arteries to supply the bladder – the superior vesical arteries. The obturator artery passes through the obturator foramen and into the medial compartment of the thigh – this vessel is often absent and instead a small vessel crosses the superior ramus of the pubic bone from the femoral or external iliac arteries to then enter the pelvis and pass through the obturator foramen.

  
• Branches of the anterior division

  The inferior vesical artery is usually replaced by the vaginal or uterine artery in a female. The middle rectal artery supplies the rectum in both sexes and also the prostate gland in males. The inferior gluteal artery passes through the greater sciatic notch to supply the gluteal region with blood. The final branch is the internal pudendal artery that supplies an inferior rectal branch in the anal triangle and several branches to the perineum.

  
• Branches of the anterior division - male

  •Inferior vesical and middle rectal supply prostate, seminal vesicles and ductus deferens

  •Testicular arteries from abdominal aorta at L2

  The inferior vesical artery also supplies the bladder. Remember that although blood supply to the internal male reproductive organs and tissues of the perineum are from a local artery (the internal iliac) the arterial supply for the testis is direct from the abdominal aorta.

  
• Branches of the anterior division - female

  Ovarian arteries from the abdominal aorta at L2

  The inferior vesical artery does not exist in a female but is instead replaced by vaginal or uterine branches that also supply the bladder. The distal parts of the vagina are supplied by perineal branches of the internal pudendal artery. Again, note that the arterial supply to the ovaries is via a direct branch of the abdominal aorta.

  
• Branches of the posterior division

  The iliolumbar artery supplies part of the posterior abdominal wall; the lateral sacral artery contributes blood to the vertebral canal and the superior gluteal artery supplies part of the gluteal region. Variation of the branches of the posterior division is also possible.

  
• The majority of the pelvic organs are supplied by branches of the internal iliac artery.

  
• Internal iliac veins

  There are also small venous plexus associated with some of the pelvic structures – for example the prostate gland, the rectum and the sacrum.