Lumbar plexus

Lumbar plexus(plexus lumbalis) is formed by the anterior branches of the three upper lumbar (LI-LIII), part of the anterior branch of the twelfth thoracic (ThXII) and part of the fibers of the anterior branch of the fourth lumbar (LIV) spinal nerves. The remaining parts of the anterior branches of the fourth and fifth lumbar spinal nerves join together under the name of the lumbosacral trunk, which descends into the pelvic cavity. The lumbar plexus in the form of the anterior branches of the spinal nerves connecting with each other is located in front of the transverse processes of the lumbar vertebrae and on the anterior surface of the square muscle of the lower back, in the thickness of the large psoas muscle. The branches of the lumbar plexus appear from under the lateral edge of this muscle or pierce it and innervate part of the muscles and skin of the abdominal wall, the skin of the external genitalia, the skin and muscles of the medial side of the thigh, the skin of the medial surface of the leg. The branches of the lumbar plexus are the muscular branches, the iliohypogastric nerve, the ilioinguinal, the genitofemoral nerves, the lateral cutaneous nerve of the thigh, the obturator and femoral nerves.

Muscular branches (rr. musculares) originate from all the anterior branches that form the lumbar plexus, even before they join together. These branches go to the quadratus lumborum, the small and large lumbar muscles, and the intertransverse lateral muscles of the lumbar region.

Lumbar plexus, its branches and the organs they innervate

Nerves (branches) of the lumbar plexus	Spinal cord segments	Innervated organs
Muscular branches	ThXII-LI-LIV	Quadratus lumborum, psoas major and minor muscles, lateral intertransverse muscles of the lumbar region
Iliohypogastric nerve	ThXII-LI	Transverse abdominal muscle, external and internal oblique abdominal muscles, pyramidal muscle. Skin of the upper lateral part of the gluteal region, upper lateral region of the thigh, skin of the pubic region
Ilioinguinal nerve	ThXII-LI	Transverse abdominal muscle, external and internal oblique abdominal muscles. Skin of the pubis, inguinal area, penis, anterior side of the scrotum (labia majora)
Genitofemoral nerve	LI-LII	The muscle that lifts the testicle. Skin of the upper medial side of the thigh, scrotum (labia majora), area of the subcutaneous ring of the femoral canal
Lateral femoral cutaneous nerve	LI-LII	Skin of the lateral thigh (up to the level of the knee joint)
Obturator nerve	LII-LIV	Long, short and large adductor muscles, pectineus muscle, gracilis muscle, external obturator muscle. Skin of the medial thigh, capsule of the knee joint
Femoral nerve	LI-LIV	Sartorius, pectineus muscles; quadriceps femoris. Skin of the anterior thigh, anteromedial leg, dorsum and medial edge of the foot (to the big toe)

The iliohypogastric nerve (n. iliolipogastricus, THXII-LI) pierces the upper lateral part of the psoas major muscle, passes behind the kidney (along the anterior surface of the quadratus lumborum muscle). Then the nerve goes forward and downward and, before reaching the iliac crest, pierces the transverse abdominal muscle, where it is located between this muscle and the internal oblique muscle of the abdomen, giving them muscle branches. Then the ilioscapular nerve at the level of the middle of the iliac crest gives off a lateral cutaneous branch (r. cutaneus lateralis), which pierces the muscles of the abdominal wall and branches in the skin of the lateral region of the thigh above the greater trochanter. The anterior cutaneous branch (r. cutaneus anterior) passes through the aponeurosis of the external oblique muscle of the abdomen, innervates the skin next to the outer ring of the inguinal canal. The terminal branch of the iliohypogastric nerve branches in the skin of the lower abdomen above the pubic symphysis.

The ilioinguinal nerve (n. ilionguinalis) is formed by the fibers of the twelfth thoracic and anterior branch of the first lumbar spinal nerves (ThXII-LI). The nerve emerges from under the lateral edge of the psoas major muscle and runs below the iliohypogastric nerve along the anterior surface of the quadratus lumborum muscle, almost parallel to the iliac crest. In its initial section, the nerve is covered by the transverse fascia of the abdomen, then goes between the transverse and internal oblique muscles of the abdomen, innervating them. Then it passes into the inguinal canal, appears through its outer ring and branches in the skin of the pubis, scrotum in men (anterior scrotal nerves, nn. scrotales anteriores) or the labia majora in women (anterior labial nerves, nn. labiales anteriores).

The genitofemoral nerve (n. genitofemoralis) is formed by the fibers of the anterior branches of the first and second lumbar spinal nerves (LI-LII). The nerve passes through the thickness of the psoas major muscle to its anterior surface, where it soon divides into two branches - the genital and femoral. The genital branch (r. genitalis), or external spermatic nerve, goes down the anterior surface of the psoas major muscle lateral and anterior to the external iliac artery, pierces the posterior wall of the inguinal canal slightly medial to the deep ring. Together with the spermatic cord, the genital branch goes in the inguinal canal, innervates the muscle that lifts the testicle, and the skin of the scrotum, the dartos and the skin of the superomedial region of the thigh. In women, this branch goes together with the round ligament of the uterus in the inguinal canal and ends near its superficial ring and in the skin of the labia majora. The femoral branch (r. fioralis) is directed downwards along the anterior surface of the psoas major muscle. This branch then passes lateral to the external iliac artery under the inguinal ligament and branches in the skin of the thigh just below this ligament.

The lateral cutaneous nerve of the thigh (n. cutaneus fioris lateralis) is formed by the fibers of the anterior branches of the first and second lumbar spinal nerves (LI-LII). The nerve emerges from under the lateral edge of the psoas major muscle (or from the thickness of the muscle). Then the nerve goes along the anterior surface of the iliac muscle (under its fascia), reaches the anterior superior iliac spine. Lateral to the origin of the sartorius muscle, the nerve passes under the inguinal ligament onto the thigh. On the thigh, the nerve goes down first under the broad fascia of the thigh, then divides into branches that pierce this fascia and branch out in the skin of the lateral side of the thigh all the way to the knee joint.

The obturator nerve (n. obturatorius) is formed by the fibers of the anterior vegways of the second through fourth lumbar spinal nerves (LII-LIV). The nerve emerges from under the medial edge of the psoas major muscle, and lies below the border line on the lateral surface of the lesser pelvis. Together with the artery and vein of the same name, the nerve passes through the obturator canal to the medial side of the thigh. Before entering the canal, the nerve gives off a branch to the external obturator muscle. In the obturator canal or immediately after exiting it, the obturator nerve divides into anterior and posterior branches. The anterior branch (r. anterior) gives off muscle branches to the long and short adductor muscles, to the gracilis muscle, and a cutaneous branch that runs between the gracilis and long adductor muscles to the skin of the medial surface of the thigh. The posterior branch (r. posterior) of the obturator nerve passes through the external obturator muscle and also gives off branches to the capsule of the hip joint, the pectineus muscle and the posterior side of the capsule of the knee joint.

The femoral nerve (n. femoralis) is the largest, thickest nerve of the lumbar plexus, consists of fibers of the anterior branches of the second-fourth lumbar spinal nerves (LII-LIV), which unite into a nerve in the thickness of the large psoas and iliac muscles. From the pelvic cavity, the nerve exits the thigh through the muscular lacuna. In the femoral triangle, the nerve is located lateral to the femoral vessels, being covered by a deep leaf of the broad fascia of the thigh. 3-4 cm below the inguinal ligament, the femoral nerve divides immediately or gradually into muscular, cutaneous branches and the subcutaneous nerve. The muscular branches go to the iliac muscle, quadriceps femoris, sartorius and pectineus muscles, to the capsule of the hip joint.

The number and position of the cutaneous branches are variable. These branches innervate the skin of the anterior surface of the thigh to the level of the patella.

The subcutaneous nerve (n. saphenus) is the longest cutaneous branch of the femoral nerve. It is located in the same fascial sheath with the femoral artery and vein. Initially, the nerve lies lateral to the artery, then passes to its anterior surface. Together with the femoral artery, the nerve enters the adductor canal, and it exits the canal through an opening in its anterior wall (together with the descending artery of the knee).

At the level of the knee joint or slightly below it, the subpatellar branch (r. infrapatellaris) and the medial cutaneous branches of the leg (rr. cutanei cruris mediales) depart from the saphenous nerve. Having given off the subpatellar branch, the saphenous nerve pierces the fascia of the leg at the level of the tibial tuberosity, descends along the medial surface of the leg next to the great saphenous vein of the leg, lateral to it, innervating the skin of the anteromedial side of the leg. Below, the saphenous nerve passes slightly in front of the medial malleolus, goes to the medial side of the foot to the big toe.

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