The brachial plexus is formed from the ventral roots C5 to Th1, with small additions from C4 and Th2. The 5th and 6th cervical nerves unite to form the upper trunk, which continues on to become the major component of the lateral cord. The ventral root of the 7th cervical nerve becomes the middle trunk and together with portions of the upper and lower trunk continues on to become the posterior cord. Ultimately, the 8th cervical nerve and the 1st thoracic nerve unite to form the lower trunk, which together with portions of the middle trunk becomes the medial cord.
Innervation of the leg takes place in the lumbar and sacral plexuses. After exiting the intervertebral foramina (Th 12-L4), the lumbar plexus runs within a fascial sheath between the psoas major muscle and the quadratus lumborum muscle. The lumbar vertebrae and the transverse processes are located mediad.