The plexus formation allows rearrangements of the efferent and afferent somatic and autonomic fibers (the latter reach the roots of the plexus through sympathetic rami communicantes) so that they are redirected through the various trunks, divisions, and cords into the most appropriate channels—the terminal branches—for distribution to the muscles, skin, vessels, and glands in the upper limbs.
Trunks. The upper roots (C5, C6) unite to form the superior trunk, the C7 root continues alone as the middle trunk, and the lower roots (C8, T1) constitute the inferior trunk of the plexus. The trunks lie in the lower part of the posterior cervical triangle.
Divisions and Cords. Each trunk divides into three ventral (anterior) and three dorsal (posterior) divisions, which supply the ventral (flexor) and dorsal (extensor) structures in the upper limb. In the axilla, the divisions become regrouped as follows: the ventral division of the inferior trunk continues as the medial cord (C8, T1), the ventral divisions of the superior and middle trunks unite to form the lateral cord (C5, C6, C7), and all three dorsal divisions of the trunks join to produce the posterior cord (C5 to C8, T1). (The terms “medial,” “lateral,” and “posterior” indicate the relationships of the cords to the second part of the axillary artery.)
Branches. Most of the branches of the plexus originate in the axilla from the cords located below the level of the clavicle—infraclavicular branches. However, several branches arise from the roots and trunks in the posterior cervical triangle above the clavicle—supraclavicular branches. Nerves derived from a cord do not necessarily contain fibers from all its constituent roots; for instance, the axillary nerve arising from the posterior cord (C5 to C8, T1) contains fibers from only C5 and C6.