10.1 Introduction

The brachial plexus is a confluence of nerves that control all function, motor and sensory, for the entire upper extremity. Knowledge of this anatomy and the surrounding structures is crucial for anyone caring for the upper limb or performing surgeries in the neck and shoulder region. Understanding of which muscles are innervated by which nerves, and subsequently where those nerves come from, is needed to perform an appropriate physical examination or interpret electrodiagnostic studies. This chapter will describe the relevant anatomy and provide case examples.

10.2 Neural Anatomy

The brachial plexus is typically made up of the ventral rami of the C5–T1 nerve roots. This “normal” pattern occurs in 75% of the population. ¹ In 20 to 25% of the population, there is an additional contribution from the C4 nerve root, termed a prefixed cord. Approximately 1% of the population has a postfixed cord, where there is an additional contribution from the T2 nerve root. ¹

The brachial plexus has five distinct components: the roots, trunks, divisions, cords, and terminal branches. As this branching pattern is somewhat complicated, the mnemonic “Randy Travis Drinks Cold Beer” (Roots Trunks Divisions Cords Branches) may be helpful for remembering the order of branching from proximal to distal. The five roots, C5–T1, combine to form the upper, middle, and lower trunks (▶Fig. 10.1). C5 and C6 combine to form the upper trunk, C7 continues to form the middle trunk, and C8 and T1 combine to form the lower trunk. Subsequently, each trunk divides into anterior and posterior divisions, for a total of three anterior divisions and three posterior divisions. The anterior divisions supply innervation to the flexor compartments of the arm and forearm, whereas the posterior divisions supply innervation to the extensor compartments.

The divisions combine to form three cords, termed the lateral, medial, and posterior cords, based on their relationship to the axillary artery (▶Fig. 10.2). The lateral cord is formed by the combination of the anterior divisions from the upper and middle trunk, whereas the medial cord is a continuation of the anterior division of the lower trunk. All three posterior divisions combine to form the posterior cord.

The terminal branches of the brachial plexus are the major peripheral nerves of the upper extremity: the axillary nerve, the musculocutaneous nerve, the median nerve, the ulnar nerve, and the radial nerve. The musculocutaneous nerve is a terminal branch from the lateral cord, and the ulnar nerve is a terminal branch from the medial cord. Contributions from both the lateral and medial cords combine to form the median nerve terminal branch. The posterior cord terminal branches are the axillary and radial nerves.

Numerous smaller branches arise from the brachial plexus to provide motor and/or sensory innervation throughout the upper extremity. We think it best to segregate these branches based on the distinct components of the brachial plexus in order to make it easier to remember each one. There are three branches from the root level. The dorsal scapular nerve, which innervates the rhomboids, arises from the C5 nerve root. The phrenic nerve, which innervates the diaphragm, is composed of contributions from the C3, C4, and C5 nerve roots. Last, the long thoracic nerve, which innervates the serratus anterior, is composed of contributions from the C5, C6, and C7 nerve roots.

Only two branches arise from the level of the trunks, both of which come off the upper trunk. In the proximal portion of the upper trunk, the nerve to subclavius arises and travels along the anterior aspect of the plexus to innervate the subclavius muscle. The suprascapular nerve, which innervates the supraspinatus and infraspinatus muscles, has a very distal takeoff. We have noted that there is often a trifurcation of the upper trunk into the anterior and posterior divisions and suprascapular nerve (▶Fig. 10.3). No branches arise from the level of the divisions.

Numerous smaller branches come off the level of the cords; however, these can be remembered by breaking things down into the individual cord branches. Only one branch arises from the lateral cord, the lateral pectoral nerve, which innervates the pectoralis major. The posterior and medial cords have three branches each. Arising from the medial cord from proximal to distal is the medial pectoral nerve, which innervates the pectoralis major and minor; the medial cutaneous nerve of the arm, which supplies sensation to the medial arm; and the medial cutaneous nerve of the forearm, which supplies sensation to the medial forearm. The branches of the posterior cord from proximal to distal are the upper subscapular nerve, which innervates the subscapularis; the thoracodorsal nerve, which innervates the latissimus dorsi; and the lower subscapular nerve, which innervates the subscapularis and teres major.

All of the aforementioned peripheral nerves are part of the parasympathetic nervous system. The sympathetic nervous system travels as the sympathetic chain and synapses in the inferior cervical and upper thoracic ganglia. Injury to the sympathetic fibers in the region of the brachial plexus, particularly affecting the superior cervical ganglion, can result in Horner’s syndrome on the ipsilateral side.