The brachial plexus starts in the neck from the ventral rami of spinal nerves C5 - T1 (5th cervical to 1st thoracic spinal cord segments). These rami are called roots. The roots will continue through the neck and, some of them merge, to form trunks. C5 and C6 form the upper trunk, C7 continues as the middle trunk and C8 and T1 for the lower trunk. While still in the neck, the trunks divide into anterior and posterior divisions. The divisions then reunite in different patterns. The anterior divisions of the upper and middle trunks merge to form the lateral cord. The anterior division of the lower trunk continues as the medial cord. The posterior divisions of all trunks merge to form the posterior cord. At this point, the cords are in the axilla. The cords are named according to their relationship with the axillary artery. Medial to it, lateral to it or posterior to it. Finally, the cords give rise to various branches that supply the upper limb structures. I want to point out that although most of the branches to the upper limb muscles arise from the plexus in the axilla, some arise from the cervical (neck) part of the plexus. These nerves are the dorsal scapular, nerve to subclavius, long thoracic, and suprascapular. Needless to say, the brachial plexus is a very important structure in the axilla and can be injured here through various types of trauma (athletic injuries, humeral dislocations, crutch injuries, surgical injuries), carcinomas and other pathological problems

Branches of Brachial Plexus