The parts of the brachial plexus, from medial to lateral, are roots, trunks, divisions, and cords. All major nerves that innervate the upper limb originate from the brachial plexus, mostly from the cords. Proximal parts of the brachial plexus are posterior to the subclavian artery in the neck, while more distal regions of the plexus surround the axillary artery.
The roots of the brachial plexus are the anterior rami of C5 to C8, and most of T1. Close to their origin, the roots receive gray rami communicantes from the sympathetic trunk (Fig. 7.52). These carry postganglionic sympathetic fibers onto the roots for distribution to the periphery. The roots and trunks enter the posterior triangle of the neck by passing between the anterior scalene and middle scalene muscles and lie superior and posterior to the subclavian artery.
The three trunks of the brachial plexus originate from the roots, pass laterally over rib I, and enter the axilla (Fig. 7.52):
The superior trunk is formed by the union of C5 and C6 roots.
The middle trunk is a continuation of the C7 root.
The inferior trunk is formed by the union of the C8 and T1 roots.
The inferior trunk lies on rib I posterior to the subclavian artery; the middle and superior trunks are more superior in position.
Each of the three trunks of the brachial plexus divides into an anterior and a posterior division (Fig. 7.52):
The three anterior divisions form parts of the brachial plexus that ultimately give rise to peripheral nerves associated with the anterior compartments of the arm and forearm.
The three posterior divisions combine to form parts of the brachial plexus that give rise to nerves associated with the posterior compartments.
No peripheral nerves originate directly from the divisions of the brachial plexus.
The three cords of the brachial plexus originate from the divisions and are related to the second part of the axillary artery (Fig. 7.52):
The lateral cord results from the union of the anterior divisions of the upper and middle trunks and therefore has contributions from C5 to C7—it is positioned lateral to the second part of the axillary artery.
The medial cord is medial to the second part of the axillary artery and is the continuation of the anterior division of the inferior trunk—it contains contributions from C8 and T1.
The posterior cord occurs posterior to the second part of the axillary artery and originates as the union of all three posterior divisions—it contains contributions from all roots of the brachial plexus (C5 to T1).
Most of the major peripheral nerves of the upper limb originate from the cords of the brachial plexus. Generally, nerves associated with the anterior compartments of the upper limb arise from the medial and lateral cords and nerves associated with the posterior compartments originate from the posterior cord.
Branches (Table 7.7)
Branches of the roots
In addition to small segmental branches from C5 to C8 to muscles of the neck and a contribution of C5 to the phrenic nerve, the roots of the brachial plexus give rise to the dorsal scapular and long thoracic nerves (Fig. 7.53).
The dorsal scapular nerve: originates from the C5 root of the brachial plexus, passes posteriorly, often piercing the middle scalene muscle in the neck, to reach and travel along the medial border of the scapula (Fig. 7.54), and innervates the rhomboid major and minor muscles from their deep surfaces.
The long thoracic nerve: originates from the anterior rami of C5 to C7, passes vertically down the neck, through the axillary inlet, and down the medial wall of the axilla to supply the serratus anterior muscle (Fig. 7.54), and lies on the superficial aspect of the serratus anterior muscle.
Branches of the trunks