The fibers from the lumbar nerves L1 to L4 unite in the substance of the psoas major muscle to form the lumbar plexus.
The nerves that arise from the lumbar plexus include: the iliohypogastric nerve which is formed from the roots of the first lumbar nerve L1, with contributions from the subcostal nerve T12, and supplies motor fibers to the transversus abdominis and internal oblique muscles. It also gives off cutaneous branches supply to the skin over the pubis and part of the gluteal region. The ilioinguinal nerve, which also provides motor innervation to the transverse abdominis and internal oblique muscles, is formed mainly from the roots of the first lumbar nerve L1. This nerve also gives off cutaneous branches that supply the skin over the root of the penis and the upper part of the testes in males, and the mons pubis and labia majora in females.
The genitofemoral nerve, which is formed from the fibers from the spinal nerves L1 and L, gives off a genital branch and a femoral branch as the name suggests. The genital branch of this nerve provides motor innervation to the cremaster muscle in males and gives off cutaneous branches that supply the skin of the anterior scrotum and labia. The femoral branch of the nerve gives cutaneous fibers that supply a small portion on the upper medial thigh.
The lateral femoral cutaneous nerve is formed from fibers from the spinal nerves L2 and L3. It supplies the skin on the lateral aspect of the thigh. The obturator nerve, which is formed from the spinal nerves L2, L3 and L4, divides to give an anterior and a posterior division. This nerve provides motor innervation to the muscles of the adductor compartment of the lower limb and the obturator externus muscle.
The final nerve which arises from the lumbar plexus is the femoral nerve, which is formed from the posterior division of spinal nerve L2 to L4, innervates the muscles of the anterior compartment of the thigh. It also gives cutaneous fibers to supply the skin over the thigh.
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