Definition: major nerve plexus formed of the ventral primary rami of the fifth cervical to first thoracic spinal nerves for innervation of the upper limb. The ventral primary rami entering into formation of the plexus constitute the roots of the plexus; the roots are located in the posterior triangle of the neck, converging to emerge from the scalenus anterior and medius muscles. As they emerge from the scalene hiatus, the C5 and C6 roots combine to form the superior trunk, C7 remains alone as the middle trunk, and the C8 and T1 roots combine to form the inferior trunk of the plexus. The trunks pass beneath the clavicle, passing from the neck into the axilla through the cervicoaxillary canal. As they cross the first rib, all three trunks divide into anterior and posterior divisions of the plexus. Nerve fibers contained within anterior divisions are destined for the anterior aspect of the limb; those contained within the posterior divisions are destined for the posterior aspect of the limb. Within the axilla, the anterior divisions of the superior and middle trunks merge to form the lateral cord of the plexus; the anterior division of the inferior trunk becomes the medial cord of the plexus, and the posterior divisions of all three trunks become the posterior cord, the cords being named for their position in relation to the axillary artery, to which they run parallel and which they surround. The cords of the brachial plexus give rise to most of the named peripheral nerves that are the products of the plexus formation. The major nerves of the lateral cord are the musculocutaneous nerve and the lateral root of the median nerve. The medial cord gives rise to the ulnar nerve and medial root of the median nerve. The lateral and medial roots of the median nerve merge to form the median nerve. The posterior cord of the plexus gives rise to the radial and axillary nerves.
Synonym(s): plexus brachialisTA