Lumbar Plexus

The lumbar plexus starts from L1 and goes to L4, and has six branches.

The mnemonic "I Twice Got Lunch On Friday" is an easy way to remember the branches of the lumbar plexus. “I twice” means that there are 2 “I”s, which stand for the Illiohypogastric nerve and the Illioinguinal nerve, both stem from L1. The illiogastric nerve is proximal to the illioiguinal nerve, so the end of the illiohypogastric nerve has to come before the illioinguinal. “Got” stands for the Genitofemoral nerve, which stems from L1 and L2. Next, “Lunch” stands for the Lateral femoral cutaneous nerve, stemming from L2 and L3. Then, “On” stands for the Obturator nerve. This nerve stems from L2, L3, and L4. Finally, “Friday” stands for the Femoral nerve, which also stems from L2, L3, and L4.

In this diagram, you can also see the anterior division, represented by yellow, and the posterior division, represented by pink.

The femoral nerve is responsible for knee extension. Extension of the knee is done by muscles that are innervated by the L2, L3, and L4 nerve roots via the femoral nerve. The femoral nerve innervates the muscles of the anterior compartment of the thigh, called the quadriceps muscles. The quadriceps muscles are responsible for knee extension. The femoral nerve provides sensation to the area of the medial lower leg.

The obturator nerve innervates the muscles of the medial compartment of the thigh, which cause hip adduction. The muscles that are responsible for hip adduction are the adductor brevis, adductor longus, and adductor magnus. They are innervated by L2, L3, and L4 nerve roots via the obturator nerve. Sensation to the medial aspect of the lower thigh is supplied by the obturator nerve.

The lateral femoral cutaneous nerve comes from the L2 and L3 nerve roots. The lateral femoral cutaneous nerve of the thigh innervates the skin on the lateral aspect of the thigh. Risk of injury to this nerve can occur with bone graft, with hip or acetabular procedures, or with external fixators of the pelvis.

The genitofemoral nerve comes from the L1 and L2 nerve roots. The genitofemoral nerve supplies sensation to a small area on the front of the thigh and a small area near the groin

The iliohypgastric and the ilioinguinal nerve both come from just the L1 nerve root. The Iliohypogastric nerve supplies sensation above the genitofemoral nerve. The Ilioiguinal nerve supplies sensation to the area between the genitofemoral and ilioiguinal nerve areas.

Two branches come from one nerve root, two branches come from two nerve roots, and two branches come from three nerve roots. The two nerves that arise from one nerve root are the iliohypogastric and ilioinguinal nerves. Both of these arise from the L1 nerve root. The iliohypogastric is superior to the ilioinguinal nerve. The two nerves that arise from two nerve roots at the genitofemoral (L1 and L2) and the lateral femoral cutaneous nerve of the thigh (L2 and L3). Finally, the two nerves that arise from three nerve roots are the obturator nerve and the femoral nerve, both of which arise from the L2, L3, and L4 nerve roots.

There is also contribution from T12, which supplies into the L1 nerve root. Also, the fourth lumbar nerve root passes a branch to the lumbosacral trunk at the fifth lumbar nerve root.

All of the nerve of the lumbar plexus, with exception of the obturator and genitofemoral nerve, emerge from the lateral aspect of the psoas major muscle. The genitofemoral nerve emerges anteriorly, and the obturator nerve emerges medially, in relationship to the psoas major muscle.