The lower limbs have the tremendous responsibility of mobilizing and stabilizing the human body. Muscles of the pelvic region, posterior abdominal wall, and the fifty-nine muscles of the lower limb as well as their corresponding joints are innervated by branches of the lumbosacral plexus. These nerve fibers originate from the first lumbar intervertebral foramen through to the fourth sacral foramen. The fibers from L1 – L4 merge in the substance of psoas major to form the lumbar plexus. These nerves are primarily responsible for innervating the lower anterior abdominal wall and supporting the sacral plexus. The anterior and posterior divisions of the lumbar plexus, recipients of their nerve fibers along with the relationship of their branches to adjacent structures will be discussed in this article. The lumbosacral trunk (point of union between the lumbar and sacral plexuses) and the sacral plexus will be discussed separately.
Branches & Relations
The iliohypogastric nerve is the first branch of the lumbar plexus. It is formed primarily by the L1 nerve roots, with contributions from the subcostal (T12) nerve. This nerve is inferior to the subcostal nerve and courses inferolaterally towards the anterior aspect of the iliac crest. It gives anterior and lateral branches that provide cutaneous supply over the inguinal region as well as the scrotum (or the labia in females). Additionally, motor innervation is provided to the inguinal parts of internal oblique and transversus abdominis muscles.
Similarly, the ilioinguinal nerve is formed from thoracolumbar outflow of primarily from L1 with T12 contributions. It runs inferior to the iliohypogastric nerve in a similar inferolateral direction. At the level of the iliac crest onwards, the nerve lies medial to the iliohypogastric nerve. The ilioinguinal nerve assists the iliohypogastric nerve in supplying the aforementioned structures. Fibers from the posterior division of L1 leave the first branch of the lumbar plexus to innervate the superior part of psoas major muscle.
The genitofemoral nerve is formed by neuronal fibers from L1 – L2 spinal segments. As the name suggests, the nerve splits to supply their respective segments. Fibers of the genital portion arise from L2 and innervate the region of the external and internal spermatic fasciae as well as tunica vaginalis. Motor innervation is provided for the cremaster muscle and cutaneous supply to the anterior scrotal and labial skin. The L1 fibers form the femoral component and provides cutaneous innervation at the midinferior point above the inguinal ligament. The genitofemoral nerve also gives a motor branch to the upper part of psoas minor.
The peritoneum of the iliac fossa, iliac fascia and the lateral side of the thigh (along the iliotibial tract) to the knee is supplied by an entirely sensory nerve called the lateral cutaneous femoral nerve. Proximally, the nerve is formed by fibers of the posterior division of L2 – L3. It then travels inferolaterally to enter the iliac fossa. The nerve fibers gain access to the thigh by passing deep to the lateral portion of the inguinal ligament.
The anterior lumbar outflow from L2, L3 and L4 unite at the level of the iliosacral joint, medial to psoas major, to form the obturator nerve. It passes along the posterior wall of the pelvic cavity; then at the point of bifurcation of the common iliac vessels, it travels toward the obturator foramen where it supplies the pelvic parietal peritoneum. The nerve then divides into an anterior and a posterior division after coursing through the obturator foramen. The former gives motor supply to adductor brevis, adductor longus, gracilis, occasionally pectineus, and the hip joint and cutaneous supply to the medial side of the thigh. The latter gives motor innervation to obturator externus and the pubic division of adductor magnus, in addition to a thin branch that supplies the knee joint.
Occasionally, nerve fibers of the anterior division of L3 and L4 merge at the iliosacral joint (lateral to the obturator nerve) to form the accessory obturator nerve. It travels anteroinferiorly, deep to the inguinal ligament (over the superior pubic ramus) to supply pectineus.
Finally, the femoral nerve is formed from the posterior division of L2, L3 and L4. It gives two cutaneous branches to the iliopsoas muscles; one immediately after the nerve fibers of L2 and L3 meet at about the level of the transverse process of L4, and the other as it courses along the medial surface for ilium. The nerve continues its inferior course deep to the lateral part of the inguinal ligament and enters the femoral canal lateral to the femoral vessels. As it enters the canal, it supplies pectineus, and then divides into multiple parts. The branches of the femoral nerve are mostly named for the muscles they innervate. In addition to the nerve to pectineus, the femoral nerve gives two nerves to rectus femoris (one of which also innervates the hip), a nerve to vastus lateralis, vastus intermedius, and vastus medius. There are also two nerves to sartorius, one of which becomes the intermediate femoral cutaneous nerve and the other, the medial femoral cutaneous nerve. The former gives cutaneous innervation to the fascia lata covering the anterior thigh to the knee as well as the skin in this region. The latter innervates the medial aspect of the thigh. The last noteworthy branch of the femoral nerve is the saphenous nerve. The nerve is responsible for giving cutaneous supply to the skin of the anteromedial part of the knee, over the medial malleolus and all the way down to the distal end of the first metatarsal bone.
Remembering the branches of the lumbar plexus may initially seem daunting. However, it can be made easier using the following trick:
- 2 nerves arise from one root: iliohypogastric and ilioinguinal from L1
- 2 nerves arise from 2 roots: genitofemoral from L1 & L2, and lateral femoral cutaneous from L2 & L3
- 2 nerves arise from 3 roots: femoral and obturator from L2, L3 and L4