Remember the plexus begins as the ventral rami of five spinal nerves, C5-T1, exiting from their respective intervertebral foramina.

Prior to C5-T1 spinal nerves joining to form the terminal nerves and most of the nerve branches of the brachial plexus, C5 gives off the dorsal scapular branch to the rhomboids and levator scapulae. C5, C6 and C7 each give off branches to form the long thoracic nerve to the serratus anterior.

Figure 11-1
The Brachial Plexus.

Then the five so-called roots of the plexus join to form trunks; the superior trunk is composed of brachial plexus roots C5 and C6 and gives off the suprascapular nerve to the supra and infraspinatus. Actually these "roots" are ventral rami. Sometimes the rookie anatomist confuses the term brachial plexus roots with dorsal and ventral roots of a spinal nerve. Don't be a rookie! (Figure 11-1)

The middle trunk is composed of plexus root C7, the inferior trunk is composed of plexus roots C8 and T1.

Each trunk then splits into anterior and posterior divisions. These divisions will go on to supply the anterior and posterior portions of the upper extremity.

The result of this last split is the formation of the cords of the brachial plexus. The lateral and medial cords are composed of anterior division nerves; the posterior cord is formed from posterior division nerves. You could probably predict where the nerves from each plexus cord will end up. These cords surround the axillary artery and are named according to their relationship to it.

Each cord will give rise to individual nerve branches and to terminal nerves.

The medial cord gives rise to the medial pectoral nerve innervating the pectoralis major/minor, and this cord terminates as the ulnar nerve, which supplies muscles primarily in the hand. The medial cord also gives rise to the medial root of the median nerve.

The lateral cord gives rise to the lateral pectoral nerve innervating the pectoralis major/minor. It terminates as the musculocutaneous nerve, innervating the structures in the anterior arm. The lateral cord also gives rise to the lateral root of the median nerve.

The median nerve is formed by the medial and lateral roots ( follow me on this one) of the medial and lateral cords, serving most of the flexors of the wrist, digits and intrinsic thumb muscles. (Figure 11-1)

The posterior cord gives rise to the upper subscapular nerve to the subscapularis, thoracodorsal nerve to the latissimus, and the lower subscapular nerve to the teres major and subscapularis. It terminates as the axillary nerve to teres minor, and the deltoid, and as the radial nerve to forearm, wrist and digital extensors.