The brachial plexus is an arrangement of nerve fibres (a plexus) running from the spine (vertebrae C5-T1), through the neck, the axilla (armpit region), and into the arm. All nerves of the arm stem from the brachial plexus (with the exception of the intercostobrachialis nerve which supplies an area of skin near the axilla). Therefore, lesions of the plexus can lead to severe functional impairment.

Diagram of the brachial plexus

The brachial plexus starts from the five ventral rami of the spinal nerves, these are the five roots. These roots merge to form three trunks.

Each trunk then splits to form an anterior and a posterior division. These six divisions will regroup to become the cords. The cords are named by their position in respect to the axillary artery .

• The posterior cord is formed from the three posterior divisions of the trunks.
• The lateral cord is the anterior divisions from the upper and middle trunks.
• The medial cord is simply a continuation of the lower trunk.

From the cords come the nerves of the arm.

• The axillary nerve is a branch off the posterior cord, the rest of the cord becomes the radial nerve.
• Branches from the lateral and medial cord merge to form the median nerve.
• The musculocutaneous nerve is formed from the rest of the lateral cord.
• The ulnar nerve arises from the rest of the medial cord.