Lumbar plexus

The lumbar plexus is a collection of nerves that lies in the lumbar region within the substance of the psoas major muscle, anterior to the transverse processes of the lumbar vertebrae. It is formed by the ventral rami of the first to fourth lumbar spinal nerves (L1 - L4). Additionally, it usually receives contributions from the ventral ramus of the twelfth thoracic nerve (T12). Please note that the lumbar plexus receives only part of the ventral ramus of the fourth lumbar nerve (L4), as the rest forms the lumbosacral trunk (Read more!).

The lumbar plexus is a complex structure; however, compared to the brachial plexus, there is less merging and formation of trunks. This plexus lies posterior to and within the psoas major muscle. It provides motor and sensory supply to the structures of the lower abdomen, lower limb, and pelvis, including the external genitalia. The nerves arising from the plexus are essential for everyday activities, such as extending the knee or flexing the hip.

The following nerves arise from the lumbar plexus:

• Iliohypogastric nerve (L1) (Read more!) - innervates the skin overlying the iliac crest, gluteal region, and the inguinal and hypogastric regions; also supplies the transversus abdominis and internal abdominal oblique muscles;
• Ilioinguinal nerve (L1) (Read more!) - primarily provides sensory innervation to the skin of the upper medial thigh, external genitalia (root of the penis and the superior portion of the scrotum), and pubic region;
• Genitofemoral nerve (L1 - L2) (Read more!) - divides into the femoral and genital branches; the femoral branch innervates the skin over the femoral triangle and the anterior, upper, and medial thigh; the genital branch innervates the cremaster and dartos muscles, as well as the skin of the scrotum, and provides sensory fibers to the spermatic fascia and tunica vaginalis of the testicle;
• Obturator nerve (L2 - L4) (Read more!) - innervates the adductor muscles of the thigh, gracilis, and obturator externus; it also provides sensory innervation to a small area of the medial thigh and gives articular branches to the hip joint and knee joint;
• Accessory obturator nerve (L3 - L4; not always present) (Read more!) - innervates the hip joint and pectineus muscle;
• Lateral femoral cutaneous nerve (L2 - L3) (Read more!) - supplies the anterolateral thigh down the knee and contributes to the peripatellar plexus; also innervates the lateral thigh from the greater trochanter and sometimes the gluteal region;
• Femoral nerve (L2 - L4) (Read more!) - provides motor innervation to the iliacus, pectineus, sartorius, and quadriceps muscles; provides sensory innervation to the anteromedial thigh and the anteromedial, medial, and posteromedial leg, the medial malleolus, and the medial foot down to the base of the hallux; also supplies the hip joint and knee joint.

The lumbar plexus is divided into two divisions or parts - anterior and posterior. The femoral nerve and lateral femoral cutaneous nerve represent the posterior division of the lumbar plexus, while the rest arise from the anterior division. All the mentioned nerves descend along the posterior or lateral abdominal walls to reach their target structures.