What is between greater and lesser sciatic notch?

37.4.1 Peripheral Innervation of Urethral and Anal Rhabdosphincters

The somatic motor fibers innervating the rhabdosphincters leave the spinal cord as the ventral radices and fuse with the dorsal radices to constitute the spinal nerve. The sacral roots travel within the spinal canal from levels T12–L1 as the cauda equina. After passing through the intravertebral foramen, the spinal nerve divides into a dorsal ramus and a ventral ramus.23 The pudendal nerve is derived from the S2–S4 ventral rami (also known as the sacral plexus), but there may be some contribution from S1.24 The pudendal nerve continues through the greater sciatic foramen and enters the ischiorectal fossa (Alcock’s canal) in a lateral direction through the lesser sciatic foramen. In the posterior part of Alcock’s canal, the pudendal nerve gives off the inferior rectal nerve (which innervates the EAS), then branches into the perineal nerve (which innervates the urethral sphincter) and the dorsal nerve of the penis/clitoris (Figure 37.2B).

The branches of the perineal nerve are more superficial than the dorsal penile/clitoral nerve and generally travel on the upper surface of the perineal muscle to innervate the urethral sphincter bilaterally.

There appears to be significant variability of normal human neuroanatomy. For example, there is still controversy as a result of anatomic studies of peripheral innervation of the pelvis which, as a rule, have been performed in a few cases. Interestingly, anatomic studies apparently dissect specimens only unilaterally, so that intersubject variability is noticed, but intrasubject variability (asymmetry) is not. Finally, much less is known about the finer details of central pathways to pelvic floor rhabdosphincters in humans, as most studies have been done in experimental animals.

Urethral and anal rhabdosphincters, as well as pudendal motor neurons, contain neuronal nitric oxide synthase (nNOS), which is also found in a substantial subpopulation of muscle and nerve fibers.25,26 This enzyme is responsible for producing the neurotransmitter, nitric oxide (NO). nNOS is also concentrated at the neuromuscular junctions in humans. The role of NO in controlling the EAS and urethral sphincter, and in neuromuscular transmission to these muscles, is not well established.27 In one experiment in an animal model, an NO donor was shown to decrease urethral pressures at the sphincter level, but it is not clear whether the effect was on smooth or striated muscle.28

The lymphatic system

The lymphatic vessels of the perineum and of the external genitalia follow the course of the external pudendal vessels, and end in the superficial inguinal and subinguinal glands. Those from the ovary ascend with the ovarian artery to the lateral and preaortic glands. The lymphatic vessels of the uterus consist of two sets, superficial in the peritoneum and deep inside the organ. The vessels of the cervix go into the external iliac glands, to the hypogastric glands, and to the common iliac glands. The vessels of the body of the uterus run in the broad ligament principally and with the ovarian vessels ascend to the lateral and preaortic glands (Standring, 2008).

Neural Anatomy

The thecal sac ends blindly at the S2 level. The lower sacral and coccygeal nerves emerge from the sac, as does the extradural portion of the filum terminale. The upper four roots exit the sacrum through the paired ventral and dorsal sacral foramina. The fifth sacral roots, the coccygeal roots, and the filum exit the sacrum caudally through the sacral hiatus. The filum terminale extends to its point of fusion with the periosteum of the first coccygeal segment.

The sacral plexus is formed by the ventral rami of six roots: L4 through S3 and the upper part of S4. The lumbosacral trunk (the conjoined L4–5 roots) crosses the anterior aspect of the ala of the sacrum, descends obliquely in front of the sacroiliac joint, and enters the pelvis deep to the parietal pelvic fascia. It crosses the superior gluteal vessels and joins the first sacral root. The sacral plexus is located on the anterior surface of the piriformis muscle, deep to the parietal (Waldeyer’s) fascia. Except for the nerves to the piriformis muscle, the perforating cutaneous nerves, and the nerves to the pelvic diaphragm, essentially all branches of the sacral plexus leave the pelvis through the greater sciatic foramen.

The most important derivatives of the sacral plexus are the sciatic and pudendal nerves. The latter is unique in exiting the greater sciatic foramen only to reenter the lesser sciatic foramen by hooking around the sacrospinous ligament. It supplies the muscles of the perineum, including the external anal sphincter, and provides sensory information to the external genitalia.

The coccygeal plexus is derived from the ventral rami of S4 and S5 as well as the coccygeal roots. It lies on the pelvic surface of the coccygeus muscle. It innervates the coccygeus muscle and provides some perianal sensation.

Both the sympathetic and parasympathetic components of the autonomic nervous system have an intimate relationship with the sacrum. The sacral sympathetic trunk, continuous with the lumbar sympathetic trunk, descends against the ventral surface of the sacrum, converging in front of the coccyx to form the unpaired ganglion impar. Three or four sacral trunk ganglionic enlargements are found on each side of the midline, just medial to the ventral sacral foramina. No white rami communicantes are present in this region; however, the postsynaptic gray rami communicantes from each ganglion join the corresponding sacral or coccygeal nerves for distribution to sweat glands, blood vessels, and erector pilori muscles. In addition, the sacral sympathetic trunks provide fine branches to the superior hypogastric plexus. The superior hypogastric plexus is the caudal continuation of the periaortic sympathetic plexus; it lies on the anterior surface of the fifth lumbar vertebra and upper sacrum in the retroperitoneal tissue. Fibers of the superior hypogastric plexus diverge into right and left hypogastric nerves opposite the first sacral vertebra. The term hypogastric nerve may be a misnomer, because the structure is really a narrow plexus of fibers. The hypogastric nerves represent the principal sympathetic inputs to the inferior hypogastric plexus.

The parasympathetic contributions to the pelvic plexi arise from the ventral S2 to S4 nerve roots. These preganglionic fibers form the pelvic splanchnic nerves (nervi erigentes). The parasympathetic system provides motor innervation to the detrusor muscle of the bladder and is primarily responsible for the vascular reflexes that sustain erectile function. The sympathetic system plays less of a role in normal voiding reflexes but is important for male fertility by promoting timely transport of spermatozoa from the testes to the seminal vesicles and by coordinating reflexes responsible for ejaculation.

Blood supply

The internal pudendal artery is the arterial trunk supplying blood to all of the perineal structures inferior to the pelvic diaphragm. It begins as a branch of the internal iliac, which is located subperitoneally in the lateral pelvis. It exits the bony pelvis, crosses the sacrospinous ligament, and enters the ischiorectal fossa. At this point the artery, along with the internal pudendal vein and nerve, becomes enclosed by the obturator fascia, forming the pudendal canal. As the artery enters the pudendal canal it gives off an inferior rectal artery which supplies the anorectal junction. The remaining portion of the internal pudendal artery reaches the base of the urogenital diaphragm and gives off a series of perineal branches. These supply the contents of both superficial and deep perineal spaces, including the vagina, urethra, and clitoris.

The venous drainage of both perineal triangles parallels the arterial supply. There is also a rich submucosal venous plexus in the distal vagina. Distension of these submucosal veins can produce vaginal or vulvar varices. The inferior rectal veins join the internal pudendal vein just as it leaves the ischiorectal fossa at the lesser sciatic foramen. Both the rectal and the vaginal submucosal plexuses penetrate the pelvic diaphragm to communicate with the endopelvic space. Here, vaginal veins may anastomose with uterine veins and inferior rectal veins with middle rectal veins.

The blood supply of the uterus and the upper vagina is via a single arterial trunk, the internal iliac artery. This arises from the division of the common iliac artery at the junction of the sacrum and the ilium. Descending in the lateral pelvis subperitoneally, it gives off a series of visceral branches, including rectal, uterine, and vesicle; these course medially to enter the endopelvic space at the base of the broad ligament. Before reaching the isthmus of the uterus, the uterine artery crosses superior to the ureter and gives branches to the vaginal fornix and cervix. Turning superiorly in the parametrial space of the broad ligament, a series of arterial branches is given to the body of the uterus until the artery anastomoses with the ovarian artery at the ureterotubal junction.

The uterine vein is usually plexiform, coursing laterally in the base of the broad ligament before reaching the lateral pelvic wall. Here the plexus of veins forms a series of tributaries entering the internal iliac vein, which in turn empties into the inferior vena cava. Other veins in the endopelvic space include the middle recto veins draining the rectum.

The normal route of rectal venous flow is into the internal iliac vein. During pregnancy the fetus may partially occlude the inferior vena cava when the woman is recumbent, increasing venous resistance and diminishing pelvic venous flow into the inferior vena cava. Because the middle rectal veins also communicate with the superior rectal branches of the inferior mesenteric vein, there is the potential for pelvic blood to ascend via the portal circulation. None of the pelvic veins contains valves, which allows blood to take the path of least resistance. Middle rectal veins also communicate with inferior rectal veins; these are tributaries of the internal pudendal vein, which drains into the iliac veins before entering the inferior vena cava. Increased bloodflow in these vessels, particularly in the last trimester of pregnancy, is a well-known cause of hemorrhoids.

The ovarian arteries arise as lateral branches from the abdominal aorta, descend in the retroperitoneal space, cross the ala of the sacrum, and enter the suspensory ligament of the ovary. As the ovarian artery enters the lateral edge of the broad ligament, it courses medially between the two layers of the ligament, giving branches to the ovary and the uterine tube.

The venous drainage of the structures in the superior part of the broad ligament is via the ovarian vein, which parallels the ovarian artery as the vein ascends in the retroperitoneal space. On the right side of the ovarian vein is the tributary of the inferior vena cava, whereas on the left side it drains into the left renal vein.

Branches to the Buttock and Lower Limb.

These are seven in number. The superior gluteal nerve (L4, L5, S1) passes backward through the greater sciatic notch, above the piriformis. An upper branch supplies the gluteus medius, and a lower branch supplies the gluteus minimus, tensor fasciae latae, and hip joint. The inferior gluteal nerve (L5, S1, S2) passes through the greater sciatic foramen below the piriformis and supplies the gluteus maximus. The nerve to the obturator internus (L5, S1, S2) leaves the pelvis below the piriformis, supplies the superior gemellus, and then passes through the lesser sciatic foramen to the obturator internus. The nerve to the quadratus femoris (L4, L5, S1) leaves the pelvis below the piriformis in front of the sciatic nerve. It supplies the inferior gemellus, the quadratus femoris, and the hip joint. The posterior femoral cutaneous nerve (S1 to S3) leaves the pelvis below the piriformis. It descends in company with the sciatic nerve, becomes superficial near the popliteal fossa, and accompanies the small saphenous vein to the middle of the calf. Its branches are inferior clunial nerves (gluteal branches) to the skin of the buttock, perineal branches to the skin of the genitalia, and femoral and sural branches to the skin on the back of the thigh and calf. The perforating cutaneous (inferior medial clunial) nerve (S2, S3) pierces the sacrotuberous ligament and supplies the skin over the lower part of the buttock.

The sciatic nerve (L4, L5, S1 to S3), the largest nerve in the body, consists of peroneal and tibial parts, which are usually bound together, leaving the pelvis through the greater sciatic foramen, below the piriformis (see Fig. 2-12).2 Sometimes they leave separately, the peroneal portion piercing the piriformis and the tibial portion passing below it. The exit of the sciatic nerve from the pelvis is indicated by the superior point of trisection of a line from the posterior superior iliac spine to the ischial tuberosity. Its downward course is indicated by a line down the middle of the back of the thigh (from the midpoint of a line between the greater trochanter and ischial tuberosity). The sciatic nerve descends under cover of the gluteus maximus, between the greater trochanter and ischial tuberosity. In the thigh it lies anteriorly on the adductor magnus and is accompanied by the posterior femoral cutaneous nerve and the companion artery. In the lower third of the thigh the nerve separates into its two components, the tibial and common peroneal nerves (the separation may occur at any level in the gluteal region or thigh). Its branches arise mostly on the medial side and supply the semitendinosus, semimembranosus, long head of the biceps, adductor magnus (all by the tibial nerve), and short head of the biceps (by the common peroneal nerve).

The tibial (medial popliteal) nerve (L4 to S3) descends separately through the popliteal fossa (see Fig. 2-12). It then lies on the popliteus muscle, under cover of the gastrocnemius, and at the lower border of the popliteus passes deep to the fibrous arch of the soleus to reach the back of the leg. Here it descends first on the tibialis posterior and flexor digitorum longus and then on the tibia. Then, becoming more superficial and crossing the posterior tibial artery posteriorly to gain its lateral side, it ends by dividing into medial and lateral plantar nerves under cover of the flexor retinaculum.

In the thigh, muscular branches arise as listed with the sciatic nerve. In the popliteal fossa, branches are given to the knee joint and muscular branches to the gastrocnemius, soleus, plantaris, popliteus, and tibialis posterior. A branch of the nerve to the popliteus, the interosseous nerve of the leg, descends on the interosseous membrane. The medial sural cutaneous nerve joins the peroneal communicating branch of the common peroneal nerve to form the sural nerve. The sural nerve descends in company with the small saphenous vein, gives lateral calcaneal branches to the skin of the back of the leg and lateral aspect of the foot and heel, gives twigs to the ankle joints, and continues forward along the lateral side of the little toe as the lateral dorsal cutaneous nerve. In the leg the tibial nerve gives muscular branches to the soleus, tibialis posterior, flexor hallucis longus, and flexor digitorum longus. Medial calcaneal branches supply the skin of the heel and sole, and a twig is given to the ankle joint. The course of the tibial nerve in the leg is indicated on the surface by a line from about the level of the tibial tuberosity downward to the midpoint between the medial malleolus and the heel.

The medial plantar nerve, the larger of the two terminal branches of the tibial nerve, at first lies deep to the abductor hallucis. It then runs forward in the sole between the abductor and the flexor digitorum brevis. It supplies these muscles and the skin on the medial side of the sole. Its terminal branches are four plantar digital nerves for muscles (flexor hallucis and first lumbrical) and for the medial side of the big toe and the adjacent sides of the first and second, second and third, and third and fourth toes. The nerves extend on to the dorsum and supply the nail beds and tips of the toes.

The lateral plantar nerve runs forward and laterally between the quadratus plantae and the flexor digitorum brevis and divides into superficial and deep branches. During its course it supplies the quadratus plantae and abductor digiti minimi and the skin of the lateral side of the sole. The superficial branch supplies the flexor digiti minimi brevis and the lateral side of the sole and little toe and, by plantar digital nerves, the adjacent sides of the fourth and fifth toes. The deep branch turns medially; supplies the interossei, the second, third, and fourth lumbricals, and the adductor hallucis; and gives off articular twigs.

The common peroneal (lateral popliteal) nerve (L4 to S2) descends through the popliteal fossa, following the medial edge of the biceps closely (see Fig. 2-12). It crosses the lateral head of the biceps, gains the back of the head of the fibula, and winds around the neck of that bone (where it is often palpable, and where it is susceptible to injury) under cover of the peroneus longus. Here it divides into its terminal branches, the superficial and deep peroneal nerves.

While a part of the sciatic nerve, it supplies the short head of the biceps and sometimes the knee joint also. In the popliteal fossa, it supplies the knee joint and gives rise to a branch that divides into the lateral sural cutaneous nerve (for the skin on the lateral side of the leg) and the peroneal communicating branch (which joins the medial sural cutaneous nerve to form the sural nerve). At the neck of the fibula, it gives off a small recurrent branch that supplies the knee and tibiofibular joints and the tibialis anterior. The common peroneal nerve sometimes supplies the peroneus longus or extensor digitorum longus or both.

The superficial peroneal (musculocutaneous) nerve, one of the two terminal branches of the common peroneal nerve, descends in front of the fibula, between the peronei and the extensor digitorum longus. Muscular branches supply the peroneus longus and peroneus brevis; the branch to the latter is often prolonged to the extensor digitorum brevis and adjacent joints and is termed the accessory deep peroneal nerve.42,73 In the lower part of the leg, the superficial peroneal nerve divides into medial and intermediate dorsal cutaneous nerves. These pass in front of the extensor retinacula, the medial branch supplying the skin and joints of the medial side of the big toe and (by dorsal digital nerves) the adjacent sides of the second and third toes; the intermediate nerve (by dorsal digital nerves) supplies the adjacent sides of the third and fourth and the fourth and fifth toes.

As in the hand, the territories of distribution of the cutaneous nerves of the foot show considerable variation in size and overlap and reciprocal changes in size.33

The deep peroneal nerve continues the winding course of the common peroneal nerve around the neck of the fibula, then pierces the anterior intermuscular septum and extensor digitorum longus, and descends on the interosseous membrane. It meets the anterior tibial artery, and both pass deep to the extensor retinacula. Branches are given to the tibialis anterior, extensor hallucis longus, extensor digitorum longus, peroneus tertius, and ankle joint. In the foot, where it lies about midway between the malleoli, the nerve divides into its terminal branches, medial and lateral. The medial branch gives dorsal digital nerves for the adjacent sides of the first and second toes, and the lateral branch supplies the extensor digitorum brevis and adjacent joints. It may also send twigs (probably afferent) to the first three dorsal interossei.

What goes through greater and lesser sciatic foramen?

What structure forms the boundary between the greater and lesser sciatic foramen?

What is the greater sciatic notch between?

What muscle divides the greater sciatic foramen into two parts?