Corpus: Lumbar plexus

The lumbar plexus is a network of nerves formed by the anterior branches of the spinal nerves from segments L1 to L3, along with contributions from T12 and L4. Together with the sacral plexus, the lumbar plexus supplies the nerves of the pelvis and legs, and both combine to form the lumbosacral plexus.

The lumbar plexus provides motor innervation to the lower abdominal wall muscles, sensory innervation to the skin of the lower abdomen and genital region, and both motor and sensory innervation to the anterior thigh. Some sensory fibers also extend to the lower leg.

3.1. Main branches

The main branches of the lumbar plexus are:

• Iliohypogastric nerve (T12-L1):
  • Motor: Supplies the obliquus internus muscle, transversus abdominis, and pyramidal muscles.
  • Sensory: Innervates the skin over the hip region and lower abdominal area (hypogastrium).
• Ilioinguinal nerve (T12-L1):
  • Motor: Supplies the obliquus internus muscle, transversus abdominis, and pyramidal muscles.
  • Sensory: Provides sensation to the front of the scrotum in males or labia majora in females.
• Genitofemoral nerve (L1-L2):
  • Motor: Innervates the cremaster muscle in males.
  • Sensory: Supplies the skin on the front of the scrotum or labia majora and the femoral triangle region.
• Lateral femoral cutaneous nerve (L2-L3):
  • Sensory: Supplies the skin on the lateral side of the anterior and posterior thigh.
• Femoral nerve (L1-L4):
  • Motor: Innervates the iliopsoas, pectineus, sartorius, and quadriceps femoris muscles.
  • Sensory: Supplies the skin of the anterior thigh (via anterior cutaneous branches), the skin of the knee, and, via the saphenous nerve, the medial side of the lower leg and foot.
• Obturator nerve (L2-L4):
  • Motor: Innervates the adductor muscles (adductor longus, adductor brevis, adductor magnus, adductor minimus, gracilis, and pectineus) and the external obturator muscle.
  • Sensory: Supplies the skin on the inner (medial) thigh.

Additionally, fibers from the ventral part of the L4 spinal nerve, along with L5 fibers, form the lumbosacral trunk, which connects to the sacral plexus.

3.2. Secondary branches

Apart from the main branches, smaller muscular branches arise from the lumbar plexus. These branches supply the quadratus lumborum muscle and portions of the iliopsoas muscle.

The lumbar plexus is located on the side of the spine, behind the psoas major muscle. All its branches are associated with this muscle. Most branches run behind the psoas major muscle, over the quadratus lumborum or iliacus muscle, and then through the fascia of the iliopsoas to reach the abdominal wall or travel to the leg through the muscular compartment under the inguinal ligament.

The genitofemoral nerve pierces through the psoas major muscle. Medially, the obturator nerve passes through the lesser pelvis, and exits via the obturator canal to supply the adductor muscles.

The iliohypogastric and ilioinguinal nerves run behind the kidney to the groin area, making them susceptible to injury during kidney surgeries. Irritation of these nerves in the groin can also mimic kidney pain.

The obturator nerve lies near the ovary. Inflammation of the ovary (oophoritis) may irritate this nerve, causing pain that radiates to the inner thigh.