The brachial plexus is a network of nerves (nerve plexus) formed by the anterior rami of the lower four cervical nerves and first thoracic nerve (C5, C6, C7, C8, and T1). This plexus extends from the spinal cord, through the cervicoaxillary canal in the neck, over the first rib, and into the armpit, it supplies afferent and efferent nerve fibers to the chest, shoulder, arm, forearm, and hand.
Structure
The brachial plexus is divided into five roots, three trunks, six divisions (three anterior and three posterior), three cords, and five branches. There are five "terminal" branches and numerous other "pre-terminal" or "collateral" branches, such as the subscapular nerve, the thoracodorsal nerve, and the long thoracic nerve, that leave the plexus at various points along its length. A common structure used to identify part of the brachial plexus in cadaver dissections is the M or W shape made by the musculocutaneous nerve, lateral cord, median nerve, medial cord, and ulnar nerve.
Roots
The five roots are the five anterior primary rami of the spinal nerves, after they have given off their segmental supply to the muscles of the neck. The brachial plexus emerges at five different levels: C5, C6, C7, C8, and T1. C5 and C6 merge to establish the upper trunk, C7 continuously forms the middle trunk, and C8 and T1 merge to establish the lower trunk. Prefixed or postfixed formations in some cases involve C4 or T2, respectively. The dorsal scapular nerve comes from the superior trunk and innervates the rhomboid muscles which retract and downwardly rotate the scapula. The subclavian nerve originates in both C5 and C6 and innervates the subclavius, a muscle that involves lifting the first ribs during respiration. The long thoracic nerve arises from C5, C6, and C7. This nerve innervates the serratus anterior, which draws the scapula laterally and is the prime mover in all forward-reaching and pushing actions.
Trunks
These roots merge to form the trunks: "superior" or "upper" (C5-C6) "middle" (C7) "inferior" or "lower" (C8, T1)
Divisions
Each trunk then splits in two, to form six divisions: anterior divisions of the upper, middle, and lower trunks posterior divisions of the upper, middle, and lower trunks when observing the body in the anatomical position, the anterior divisions are superficial to the posterior divisions
Cords
These six divisions regroup to become the three cords or large fiber bundles. The cords are named by their position with respect to the axillary artery. The posterior cord is formed from the three posterior divisions of the trunks (C5-C8, T1) The lateral cord is formed from the anterior divisions of the upper and middle trunks (C5-C7) The medial cord is simply a continuation of the anterior division of the lower trunk (C8, T1)
Diagram
Branches
The branches are listed below. Most branches arise from the cords, but few branches arise (indicated in italics) directly from earlier structures. The five on the left are considered "terminal branches". These terminal branches are the musculocutaneous nerve, the axillary nerve, the radial nerve, the median nerve, and the ulnar nerve. Due to both emerging from the lateral cord the musculocutaneous nerve and the median nerve are well connected. The musculocutaneous nerve has even been shown to send a branch to the median nerve further connecting them. There have been several variations reported in the branching pattern but these are very rare.
Bold indicates primary spinal root component of nerve. Italics indicate spinal roots that frequently, but not always, contribute to the nerve.
Function
The brachial plexus provides nerve supply to the skin and muscles of the arms, with two exceptions: the trapezius muscle (supplied by the spinal accessory nerve) and an area of skin near the axilla (supplied by the intercostobrachial nerve). The brachial plexus communicates through the sympathetic trunk via gray rami communicantes that join the plexus roots.
The terminal branches of the brachial plexus (musculocutaneous n., axillary n., radial n., median n., and ulnar n.) all have specific sensory, motor and proprioceptive functions.