Lumbar, Sacral, and Coccygeal Plexuses


The lumbar plexus is produced by the union of the ventral rami of the first three lumbar nerves and the greater part of the fourth, with a contribution from the subcostal nerve. The plexus lies anterior to the lumbar vertebral transverse processes, embedded in the posterior part of the psoas major muscle.


The first lumbar nerve receives a fascicle from the subcostal nerve and divides into upper and lower branches; the upper branch splits into the iliohypogastric and ilioinguinal nerves, whereas the lower branch joins a twig from the second lumbar nerve and becomes the genitofemoral nerve. Except near their terminations, all three nerves run parallel to the lower intercostal nerves and help to supply the transverse and oblique abdominal muscles. The iliohypogastric nerve gives off a lateral cutaneous branch to the skin on the anterolateral aspect of the buttock and ends as the anterior cutaneous branch to the skin above the pubis. The ilioinguinal nerve pierces the internal oblique muscle above the anterior part of the iliac crest and then runs above and parallel to the inguinal ligament to traverse the inguinal canal and supply the skin over the root of the penis, the adjoining part of the femoral triangle, and the upper part of the scrotum (mons pubis and adjacent part of labium majus in the female). The genitofemoral nerve penetrates the psoas major muscle and divides into genital and femoral branches. The genital branch in males passes through the inguinal canal and supplies the cremaster muscle and the skin of the scrotum; in females, it ends in the mons pubis and labia majora. The femoral branch in both males and females supplies the skin over the upper part of the femoral triangle.


The larger part of the second lumbar nerve, the entire third lumbar nerve, and the offshoot from the fourth lumbar nerve to the lumbar plexus split into ventral (anterior) and dorsal (posterior) divisions, which unite to constitute, respectively, the obturator and femoral nerves (see Plates 5-15 and 5-16). The lateral femoral cutaneous nerve (see Plate 5-16) is formed by offshoots from the second and third posterior divisions.


The lower part of the ventral ramus of the fourth lumbar nerve joins the ventral ramus of the fifth to form the lumbosacral trunk. The trunk and the ventral rami of the first three sacral nerves and the upper part of the fourth sacral ramus constitute the sacral plexus.


The sacral plexus, by convergence and fusion of its roots, becomes a flattened band that gives rise to many branches before its largest part passes below the piriformis muscle and through the greater sciatic foramen as the sciatic nerve (see Plate 5-17). The rami forming the sacral plexus divide into ventral (anterior) and dorsal (posterior) divisions, which subdivide and regroup to become branches of the plexus.


Coccygeal Plexus. The lower part of the ventral ramus of the fourth and fifth sacral nerves and the coccygeal nerves form the small coccygeal plexus. It consists of two loops on the pelvic surface of the coccygeus and levator ani muscles. Twigs are given off to these muscles, and fine anococcygeal nerves supply the skin between the anus and coccyx.


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Sep 2, 2016 | Posted by in NEUROLOGY | Comments Off on Lumbar, Sacral, and Coccygeal Plexuses

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