The only branches from the trunks of the brachial plexus are two nerves that originate from the superior trunk (upper trunk): the suprascapular nerve and the nerve to the subclavius muscle (Fig. 7.53).
The suprascapular nerve (C5 and C6): originates from the superior trunk of the brachial plexus, passes laterally through the posterior triangle of the neck (Fig. 7.54) and through the suprascapular foramen to enter the posterior scapular region, innervates the supraspinatus and infraspinatus muscles, and is accompanied in the lateral parts of the neck and in the posterior scapular region by the suprascapular artery.
The nerve to the subclavius muscle (C5 and C6) is a small nerve that: originates from the superior trunk of the brachial plexus, passes anteroinferiorly over the subclavian artery and vein, and innervates the subclavius muscle.
Branches of the lateral cord
Three nerves originate entirely or partly from the lateral cord (Fig. 7.53).
The lateral pectoral nerve is the most proximal of the branches from the lateral cord. It passes anteriorly, together with the thoraco-acromial artery, to penetrate the clavipectoral fascia that spans the gap between the subclavius and pectoralis minor muscles (Fig. 7.55), and innervates the pectoralis major muscle.
The musculocutaneous nerve is a large terminal branch of the lateral cord. It passes laterally to penetrate the coracobrachialis muscle and pass between the biceps brachii and brachialis muscles in the arm, and innervates all three flexor muscles in the anterior compartment of the arm, terminating as the lateral cutaneous nerve of the forearm.
The lateral root of the median nerve is the largest terminal branch of the lateral cord and passes medially to join a similar branch from the medial cord to form the median nerve (Fig. 7.55).
Branches of the medial cord
The medial cord has five branches (Fig. 7.55).
The medial pectoral nerve is the most proximal branch. It receives a communicating branch from the lateral pectoral nerve and then passes anteriorly between the axillary artery and axillary vein. Branches of the nerve penetrate and supply the pectoralis minor muscle. Some of these branches pass through the muscle to reach and supply the pectoralis major muscle. Other branches occasionally pass around the inferior or lateral margin of the pectoralis minor muscle to reach the pectoralis major muscle.
The medial cutaneous nerve of the arm (medial brachial cutaneous nerve) passes through the axilla and into the arm where it penetrates deep fascia and supplies skin over the medial side of the distal third of the arm. In the axilla, the nerve communicates with the intercostobrachial nerve of T2. Fibers of the medial cutaneous nerve of the arm innervate the upper part of the medial surface of the arm and floor of the axilla.
The medial cutaneous nerve of the forearm (medial antebrachial cutaneous nerve) originates just distal to the origin of the medial cutaneous nerve of the arm. It passes out of the axilla and into the arm where it gives off a branch to the skin over the biceps brachii muscle, and then continues down the arm to penetrate the deep fascia with the basilic vein, continuing inferiorly to supply the skin over the anterior surface of the forearm. It innervates skin over the medial surface of the forearm down to the wrist.
The medial root of the median nerve passes laterally to join with a similar root from the lateral cord to form the median nerve anterior to the third part of the axillary artery.