Lumbar plexus is one of the main nervous pathways supplying the lower limb of respective side. It is formed by the anterior rami of upper four lumbar spinal nerves and from contribution of subcostal nerve (T12) in the lumbar region, within the psoas major muscle.
As described above, the lumbar plexus is formed in the lumbar region, within the psoas major muscle. It is present lateral to the intervertebral foramina of lumbar region.
The plexus receives contributions from subcostal nerve (T12). Additionally, the 4th lumbar nerve gives communicating branches, called the lumbosacral trunk, to the sacral plexus.
The plexus is also connected to the sympathetic nervous system. The anterior rami receive gray rami communicans (efferent nerve of sympathetic ganglion chain) from the sympathetic trunk. The upper anterior rami give off white rami communicans (afferent nerve of sympathetic ganglion chain) to the sympathetic trunk.
The small motor branches of the plexus are distributed directly to the psoas major muscle. The large motor branches leave the muscle along its lateral and medial borders and along the anterior surface. Consequently, they are categorized as follows;
Branches leaving the lateral border of psoas:
These include the following branches (in order from above downward)
- Iliohypogastric nerve:
It supplies the skin of lower part of anterior abdominal wall.
- Ilioinguinal nerve:
It passes through the inguinal canal to supply the skin of groin and scrotum (in males) or labium majus (in females).
- Lateral cutaneous nerve of thigh:
It crosses the iliac fossa in front of the iliacus muscle and enters the lateral aspect of thigh behind the lateral end of inguinal ligament. It supplies the skin over the lateral surface of thigh.
- Femoral nerve:
It is the largest branch of lumbar plexus. It enters the thigh lateral to the femoral sheath and supplies various muscles. In the abdomen, it supplies the iliacus muscle.
Branches leaving the medial border of psoas:
- Obturator nerve:
It crosses the pelvic brim in front of the sacroiliac joint and then leaves the pelvis by passing into the thigh through the obturator foramen.
- Fourth lumbar root of lumbosacral trunk:
It emerges from the medial border of the psoas at the pelvic brim. It takes part in the formation of sacral plexus.
Branches leaving the anterior surface of psoas:
- Genitofemoral nerve:
After emerging from the anterior surface of the muscle, it runs down in front of it and divides into a genital branch and a femoral branch. The former supplies the cremaster muscle and the latter supplies a small area of skin of thigh.