The sacral plexus

The sacral plexus (plexus sacralis) is formed by a part of the anterior branch of the fourth and fifth lumbar (LIV-LV) and first-third sacral (SI-SIII) spinal nerves. The plexus is located in the cavity of the lesser pelvis, directly on the fascia covering the anterior surface of the piriformis muscle. The base of the plexus corresponds to the line connecting the pelvic sacral openings. The branches of the sacral plexus are directed to the greater sciatic opening. There are short and long branches of the sacral plexus. The short branches end in the pelvic girdle. The long branches go to the muscles, joints, bones and skin of the free part of the lower limb.

Short branches. The short branches of the sacral plexus include the internal obturator nerve (from LIV-SII), the piriformis nerve (from SI-SII), the nerve of the quadratus femoris (from LIV-SII, going to the muscles of the same name), as well as the superior and inferior gluteal and pudendal nerves.

The superior gluteal nerve (n. gluteus superior) is formed by the fibers of the anterior branches of the fourth and fifth lumbar (LIV-LV) and first sacral (SI) spinal nerves. Together with the artery of the same name, the nerve leaves the pelvic cavity through the suprapiriform opening. The upper branch of this nerve goes forward to the gluteus minimus muscle and innervates it. The lower branch of the superior gluteal nerve passes between the gluteus minimus and medius muscles, innervates them, and also gives off a branch to the muscle that tenses the broad fascia of the thigh.

The inferior gluteal nerve (n. gluteus inferior) consists of fibers of the anterior branches of the fifth lumbar (LV) and first-second sacral (SI-SII) spinal nerves. The nerve exits the pelvic cavity through the infrapiriform opening together with the artery of the same name. With fan-shaped diverging short branches, the nerve enters the gluteus maximus muscle, innervates it, and also gives off branches to the capsule of the hip joint.

The pudendal nerve (n. pudendus) is formed by the anterior branches of the SIII-SIV, and partially SII spinal roots. It is located below the sacral plexus on the anterior surface of the sacrum at the lower edge of the piriformis muscle. Motor fibers extend from this nerve to the muscle that lifts the anus and to the coccygeus muscle. The largest branch of the pudendal plexus is the nerve of the same name - n. pudendus. This nerve exits the pelvic cavity under the piriformis muscle, bends around the ischial tuberosity and passes through the lesser sciatic foramen to the lateral wall of the ischiorectal fossa. Here it divides into branches:

1. inferior rectal nerves (pass to the muscle that constricts the anus and to the skin of the anterior part of the anus);
2. The perineal nerve passes to the superficial transverse perineal muscle, the bulbocavernous muscle, and also to the skin of the posterior side of the scrotum or labia majora.

The dorsal nerve of the penis/clitoris, n. dorsalis penis (clitoridis), also branches off from the pudendal nerve. Its branches supply the deep transverse muscle of the perineum and the compressive initial part of the urethra, as well as the skin of the penis/clitoris and the urethra.

In the sciaticorectal fossa, the pudendal nerve gives off the inferior rectal and perineal nerves. The inferior rectal nerves (nn. rectales inferiores) penetrate the sciaticorectal fossa, innervate the external sphincter of the anus and the skin of the anal area. The perineal nerves (nn. perineales) innervate the muscles and skin of the perineum of the scrotum in men and the labia majora in women. The terminal branch of the pudendal nerve is the dorsal nerve of the penis or clitoris (n. dorsalis penis, s. clitoridis). This nerve passes through the urogenital diaphragm next to the artery of the same name on the dorsal surface of the penis (clitoris), gives off branches to the cavernous bodies, the head of the penis (clitoris), the skin of the penis in men, the labia majora and minora in women, as well as branches to the deep transverse perineal muscle and the urethral sphincter.

Long branches of the sacral plexus. The long branches of the sacral plexus include the posterior femoral cutaneous nerve and the sciatic nerve.

The posterior cutaneous nerve of the thigh (n. cutaneus femoris posterior) is formed by the fibers of the anterior branches of the first through third sacral spinal nerves (SI-SIII). The nerve exits the pelvic cavity through the infrapiriform foramen and descends next to the sciatic nerve. Then the posterior cutaneous nerve of the thigh goes down in the groove between the semitendinosus and biceps femoris muscles. Its branches pass through the broad fascia of the thigh, branch out in the skin of the posteromedial surface of the thigh down to the popliteal fossa and the upper part of the leg. Near the lower edge of the gluteus maximus muscle, the lower nerves of the buttocks (nn. clunium inferiores) and perineal branches (rr. perineales) depart from the posterior cutaneous nerve of the thigh to the skin of the perineum. The lower nerves of the buttocks innervate the skin of the lower part of the gluteal region.

The sciatic nerve (n. ischiadicus) is the largest nerve in the human body. It is formed by the fibers of the anterior branches of the fourth and fifth lumbar (LIV-LV), first and second sacral (SI-II) spinal nerves. The nerve exits the pelvic cavity through the infrapiriform opening together with the inferior gluteal and genital nerves, the arteries of the same name and the posterior cutaneous nerve of the thigh. The sciatic nerve then runs approximately in the middle between the ischial tuberosity and the greater trochanter of the femur along the posterior surface of the gemellus muscles, the internal obturator and the quadratus femoris muscles. Under the lower edge of the gluteus maximus, the sciatic nerve runs along the posterior surface of the adductor magnus and in front of the long head of the biceps femoris. At the level of the upper angle of the popliteal fossa, and sometimes higher, it divides into the tibial and common peroneal nerves.

In the pelvic region and on the thigh, the sciatic nerve sends muscular branches to the obturator internus, gemelli muscles, to the quadratus femoris, semitendinosus and semimembranosus muscles, to the long head of the biceps femoris and the posterior part of the adductor magnus.

The tibial nerve (n. tibialis) is significantly thicker than the common peroneal nerve. It descends vertically in the popliteal fossa, passes between the heads of the gastrocnemius muscle, posteriorly and slightly lateral to the popliteal artery and vein. Together with the posterior tibial artery, the nerve is directed under the soleus muscle into the tibiofibular canal. On the leg, the tibial nerve is located between the long muscle that flexes the big toe laterally and the long muscle that flexes the fingers medially. In the lower parts of the tibiofibular canal, the tibial nerve passes more superficially. In the groove on the posterior edge of the medial malleolus, the tibial nerve divides into its terminal branches - the medial and lateral plantar nerves.

The tibial nerve along its length gives numerous muscle branches to the triceps surae muscle, long flexors of the fingers and big toe, to the plantar and popliteal muscles. The sensory branches of the tibial nerve innervate the capsule of the knee joint, the interosseous membrane of the leg, the capsule of the ankle joint, and the bones of the leg. The largest sensory branch of the tibial nerve is the medial cutaneous nerve of the calf (n. cutaneus surae medialis). It departs from the tibial nerve at the level of the popliteal fossa, then as a long and thin branch passes first under the fascia of the leg, between the heads of the gastrocnemius muscle. At the level of the beginning of the distal tendon of the gastrocnemius muscle, this nerve pierces the fascia and exits under the skin and connects with the lateral cutaneous nerve of the calf (from the common peroneal nerve). When these two nerves merge, the sural nerve (n. suralis) is formed, which passes first behind the lateral malleolus, then along the lateral edge of the foot under the name lateral dorsal cutaneous nerve (n. cutaneus dorsalis lateralis). This nerve innervates the skin of the areas adjacent to the nerve, and near the calcaneus it gives off cutaneous lateral calcaneal branches (rr. calcanei laterales).

The medial plantar nerve (n. plantaris medialis), which is one of the terminal branches of the tibial nerve, in the foot runs along the medial edge of the tendon of the short flexor of the fingers in the medial plantar groove, next to the medial plantar artery. In the foot, the nerve gives off muscular branches to the short flexors of the fingers and big toe, to the muscle that abducts the big toe, and also to the two medial lumbrical muscles. At the level of the base of the metatarsal bones, the medial plantar nerve gives off the first proper plantar digital nerve (n. digitalis plantaris proprius) to the skin of the medial edge of the foot and big toe, as well as three common plantar digital nerves (nn. digitales plantares communes). These digital nerves pass under the plantar aponeurosis together with the plantar metatarsal arteries. Each common plantar digital nerve at the level of the metatarsophalangeal joints divides into two proper plantar digital nerves (nn. digitales plantares proprii), which innervate the skin of the first through fourth toes facing each other.

The lateral plantar nerve (n. plantaris lateralis) is thinner than the medial. It is located in the lateral plantar groove between the quadratus plantaris muscle and the short flexor of the fingers. In the proximal part of the fourth intermetatarsal space, the lateral plantar nerve divides into a deep and superficial branch. The deep branch (r. profundus) gives off branches to the quadratus plantaris muscle, the muscle that abducts the little finger, the short flexor of the little finger, the 3rd and 4th lumbrical muscles, the interosseous muscles; to the muscle that adducts the big toe, and to the lateral part of the short flexor of the big toe. The superficial branch (r. superficialis) of the lateral plantar nerve gives off cutaneous branches to the lateral side of the little finger and the sides of the IV and V toes facing each other (common plantar digital nerve, n. digitalis plantaris communis), dividing into two proper plantar digital nerves (nn. digitales plantares proprii).

The common peroneal nerve (n. fibularis [peroneus] communis) is the second large branch of the sciatic nerve, directed obliquely downward and laterally. The nerve occupies the lateral part of the popliteal fossa, giving off branches to the knee and tibiofibular joints, to the short head of the biceps femoris. At the level of the popliteal fossa, the lateral cutaneous nerve of the calf (n. cutaneus siirae lateralis) branches off from the common peroneal nerve, which gives off cutaneous branches to the lateral side of the leg, and at the level of the middle of the back of the leg it pierces the fascia, exits under the skin and connects with the medial cutaneous nerve of the calf (forms the sural nerve).

The common peroneal nerve near the lateral angle of the popliteal fossa bends around the neck of the fibula on the lateral side. The nerve then pierces the initial part of the peroneus longus muscle and divides into the superficial and deep peroneal nerves.

The superficial peroneal nerve (n. fibularis superficialis, s. peroneus superficialis) goes down and laterally in the superior musculofibular canal, innervates the brevis and longus peroneus muscles. At the border of the middle and lower thirds of the leg, the nerve exits the superior musculofibular canal, pierces the fascia of the leg, goes down and medially towards the dorsum of the foot. In the upper lateral region of the foot (or slightly higher) it divides into the medial and intermediate dorsal cutaneous nerves. The medial dorsal cutaneous nerve (n. cutaneus dorsalis medialis) innervates the skin of the dorsum of the foot near its medial edge and the skin of the dorsum of the second and third toes facing each other. The intermediate dorsal cutaneous nerve (n. cutdneus dorsalis intermedius) innervates the skin of the upper lateral surface of the dorsum, as well as the facing sides of the third, fourth and fifth toes (dorsal digital nerves of the foot, nn. digitales dorsales pedis).

The deep peroneal nerve (n. fibularis profundus, s. peroneus profundus) from the place of origin goes in the medial direction, passes through the opening in the anterior intermuscular septum of the leg. Then the nerve goes in the thickness of the long muscle, extending the fingers. Together with the anterior tibial artery and veins, the nerve descends along the anterior surface of the interosseous membrane of the leg. For some distance, the vascular-nerve bundle passes between the anterior tibial muscle medially and the long muscle, extending the fingers, laterally. Then the deep peroneal nerve goes down next to the tendon of the long extensor of the big toe (foot). On the dorsum of the foot, the nerve passes under the short extensor of the big toe, then in the first intermetatarsal groove. At the level of the distal part of the first intermetatarsal space, the deep peroneal nerve divides into two terminal branches - the dorsal digital nerves (nn. digitales dorsales), which innervate the skin of the sides of the first and second toes facing each other.

On the lower leg, the deep peroneal nerve gives off branches to the anterior tibialis muscle, the long extensor of the fingers (of the foot), and also to the long extensor of the big toe. On the dorsum of the foot, the deep peroneal nerve innervates the short extensor of the fingers and the short extensor of the big toe. It gives off sensory branches to the ankle joint, to the joints and bones of the foot.

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