The lumbar plexus is formed from the anterior rami of L2 through L4. The latter also joins with L5 to form the lumbosacral trunk. The fibers pass through the psoas muscle emerging as the genitofemoral, femoral, and obturator nerves. The sacral plexus forms from the anterior rami of L4 through S3 in front of the pyriformis muscle after passing in front of the sacroiliac joint. The major branches pass through the greater sciatic foramen, emerging as the sciatic and pudendal nerves. Smaller branches include the superior and inferior gluteal nerves, the posterior cutaneous nerve of the thigh, and muscular branches to pelvic muscles. Lumbar plexus studies require stimulation of L2, L3, and L4 roots with a needle two to three cm lateral and cephalad to the L4 process; recording is from the vastus medialis; a distal stimulation site is the femoral nerve below the inguinal ligament and lateral to the femoral pulse.
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