Brachial Plexus (Left)

Contributing Nerves

Typically, the brachial plexus is formed by the anterior rami of the fifth to eighth cervical and first thoracic nerves (C5-T1). All fibers from these anterior rami become part of the brachial plexus with two exceptions. The anterior ramus of the fifth cervical nerve (C5) contributes to the phrenic nerve, while the anterior ramus of the first thoracic nerve (T1) branches to become the first intercostal nerve. These aforementioned nerves are not considered part of the brachial plexus.

In a large percentage of cases, up to 50%, a variation to the C5-T1 root basis of the brachial plexus is seen. In postfixed brachial plexuses, a large branch of the anterior ramus of the fourth cervical nerve (C4) will communicate with the anterior rami of the fifth and sixth cervical nerves (C5 and C6), while the first thoracic nerve (T1) contribution is small or absent (Yan and Horiguchi, 2000; Guday, Bekele and Muche, 2016). Less common is the postfixed anatomical variant, in which the C5 root of the brachial plexus is absent or minimal, but a well-developed T2 root is present and contributes to the brachial plexus (Loukas, Louis and Wartmann, 2007).

Organization

The brachial plexus is typically organized into five parts: roots, trunks, divisions, cords, and branches.

There are five roots, which correspond to the spinal segment origins of the brachial plexus. These are essentially the anterior rami of the fifth cervical through first thoracic nerves. Typically, three nerves emerge from the brachial plexus at the level of the roots: the dorsal scapular (C5), the long thoracic (C5, C6, and C7), and the subclavian nerves (C5 and C6).

There are three trunks, which are the area where fibers from different spinal segments first mix. The C5 and C6 roots merge to form the superior trunk, the C7 root remains independent and becomes the middle trunk, and the C8 and T1 roots combine to form the inferior trunk. Typically, the only nerve that emerges from the trunks is the suprascapular nerve, which branches off the superior trunk.

The trunks then divide, each into posterior and anterior divisions, to form the six divisions of the brachial plexus. Thus, the anterior and posterior divisions of the superior trunk both contain fibers from the C5 and C6 roots. The anterior and posterior divisions of the middle trunk only contain fibers from the C7 root, while the anterior and posterior divisions of the inferior trunk contain fibers from the C8 and T1 roots. Typically, there are no nerves that branch off at the level of the divisions. Variation does exist, however, with some reports suggesting the medial pectoral nerve is more likely to arise from the anterior division of the inferior trunk than from the medial cord.

There are three cords that represent the next site at which fibers from different spinal origins mix. The lateral cord is formed by the merger of the anterior divisions of the superior and middle trunks. This has fibers from C5, C6, and C7, and gives rise to the lateral pectoral nerve. The medial cord is formed by the continuation of the anterior division of the inferior trunk which has fibers from C8 and T1. It gives rise to three nerves: the medial pectoral nerve, the medial cutaneous nerve of the arm, and the medial cutaneous nerve of the forearm. The third cord is the posterior cord. This cord is formed by a merger of the posterior divisions of all three trunks. The posterior cord contains fibers from all levels, from C5 through T1, and gives rise to three nerves: the upper subscapular nerve, the thoracodorsal nerve, and the lower subscapular nerve.

There are five branches of the brachial plexus, which represent the termination of the cords into named nerves. The lateral cord gives rise to the musculocutaneous nerve and half of the median nerve. The medial cord gives rise to the ulnar nerve and the second half of the median nerve. All three of these nerves serve the anterior compartment of the arm, forearm, and palmar surface of the hand. The posterior cord gives rise to two nerves, the axillary and radial nerves. These serve the shoulder and posterior compartment of the upper limb respectively.

Course

The brachial plexus originates as the anterior rami of the C5-T1 spinal nerves. These are located just lateral to the intervertebral foramen between the C4-C5 vertebrae (C5 spinal nerve) through the T1-T2 vertebrae (T1 spinal nerve).

These rami run first as the roots of the brachial plexus inferolaterally, then as trunks between the anterior and middle scalene muscles of the neck and into the posterior triangle. Posterior to, and roughly at the mid-clavicular level, the trunks split into anterior and posterior divisions. Continuing inferolaterally, the divisions and then cords of the plexus run into the axilla. Here they meet up with and travel along the axillary artery, to which the cords of the brachial plexus orientate and receive their names.

The cords of the brachial plexus travel within the axillary sheath, a fascial layer that encompasses the cords of the brachial plexus, axillary artery, and axillary vein within the axilla. This sheath is a fascial layer that is an extension of the prevertebral fascia of the neck, within which is found the roots, trunks, and divisions of the brachial plexus.

In the lower part of the axilla, the brachial plexus ends as the terminal branches enter the arm.

Branches

The branches of the brachial plexus refer to the terminal nerves that run from the cords and into the arms.

—Musculocutaneous nerve formed from the terminal branch of the lateral cord, this nerve supplies motor innervation to the muscles of the anterior arm and sensation to the anterolateral forearm.

—Median nerve is formed by branches from both the lateral and medial cords, this nerve supplies motor innervation to most forearm flexors and the thenar muscles of the hand. It conveys sensory innervation from the skin of the anterior surface of the lateral hand and fingers.

—Radial nerve is one of the two terminal branches of the posterior cord, this nerve supplies motor innervation to the muscles of the posterior compartments of the arm and forearm. It conveys sensory innervation from the skin of the posterior surfaces of the arm, forearm, hand, and fingers.

—Axillary nerve is the second and smaller of the two terminal branches of the posterior cord, this nerve supplies motor innervation to the deltoid and teres minor muscles. It conveys sensory innervation from the skin of the lateral arm.

—Ulnar nerve is the terminal branch of the medial cord, this nerve supplies motor innervation to most of the muscles of the hand, the flexor carpi ulnaris, and the medial flexor digitorum profundus. It conveys sensory innervation from the skin of the medial surface of the hand and fourth digit as well as the fifth digit.

Supplied Structures

The brachial plexus carries all sensory and motor innervation associated with the axilla and upper limb, as well as some of the superficial muscles of the back. Supplied structures are best described for each nerve that arises from the brachial plexus.

—Dorsal scapular nerve (C5) is a motor nerve that serves the levator scapulae, rhomboid minor, and rhomboid major muscles of the neck and back.

—Long thoracic nerve (C5- C7) is a motor nerve that serves the serratus anterior muscle.

—Subclavian nerve (C5 and C6) is a motor nerve that serves the subclavius muscle.

—Suprascapular nerve (C5 and C6) arises from the superior trunk. It is a mixed nerve, containing sensory fibers from the glenohumeral and acromioclavicular joints, and motor fibers serving the supraspinatus and infraspinatus muscles.

—Lateral pectoral nerve (C5-C7) arises from the lateral cord. It is a motor nerve that serves primarily the pectoralis major muscle, with some potential innervation of the pectoralis minor muscle via the ansa pectoralis, its communication with the medial pectoral nerve.

—Musculocutaneous nerve (C5-C7) arises as the terminal branch of the lateral cord. It is a mixed nerve containing sensory fibers from the skin of the anterolateral forearm, and motor fibers that supply the muscles of the anterior compartment of the arm.

—Medial pectoral nerve (C8-T1) arises from the medial. It is a motor nerve that supplies the pectoralis minor muscle and pectoralis major muscle. It receives a communicating branch from the lateral pectoral nerve via the ansa pectoralis, and thus also carries fibers from the C5, C6, and C7 spinal segment levels.

—Medial brachial cutaneous nerve (C8-T1) arises from the medial cord. It is a sensory nerve that serves the skin of the medial surface of the distal half of the arm.

—Medial antebrachial cutaneous nerve (C8-T1) arises from the medial cord. It is a sensory nerve that serves skin of the anterior surface of the distal arm, the cubital fossa, and the anteromedial surface of the forearm.

—Median nerve (C5-T1) arises from branches of both the medial and lateral cords. It is a mixed nerve that contains sensory fibers from the skin of the lateral to mid-palmar surfaces of the hand and the lateral three and a half digits. It supplies motor innervation to all the muscles in the anterior compartment of the forearm except the flexor carpi ulnaris and the medial half of the flexor digitorum profundus. It also innervates the three thenar muscles and the two lateral lumbricals.

—Ulnar nerve (C7-T1) arises as the terminal branch of the medial cord. It is a mixed nerve carrying sensory fibers from the skin of the medial surface of the hand, the medial surface of the fourth digit, the fifth digit, and the hypothenar region. Its motor fibers innervate the intrinsic muscles of the hand except for the three thenar and two lateral lumbrical muscles innervated by the median nerve. Other muscles innervated by the ulnar nerve include the flexor carpi ulnaris and the medial half of the flexor digitorum profundus.

—Upper subscapular nerve (C5 and C6) arises from the posterior cord and is a motor nerve that serves the subscapularis muscle.

—Thoracodorsal muscle (C6-C8) arises from the posterior cord and is a motor nerve that serves the latissimus dorsi muscle.

—Lower subscapular nerve (C5 and C6) arises from the posterior cord and is a motor nerve hat serves the subscapularis muscle.

—Axillary nerve (C5-C6) arises as one of two terminal branches of the posterior cord. It is a mixed nerve, containing sensory fibers from the glenohumeral joint and skin of the lateral shoulder, and motor fibers supplying the deltoid and teres minor muscles.

—Radial nerve (C5-T1) arises as one of two terminal branches of the posterior cord. It is a mixed nerve, containing sensory fibers from the skin of the dorsolateral surface of the hand, posterior surface of the forearm, and inferior posterolateral surface of the arm. The motor fibers supply all muscles of the posterior compartment of the arm and forearm.

List of Clinical Correlates

—Erb’s palsy

—Thoracic outlet syndrome

References

Guday, E., Bekele, A. and Muche, A. (2016) 'Anatomical study of prefixed versus postfixed brachial plexuses in adult human cadaver', ANZ Journal of Surgery, 87(5), pp. 399-403.Loukas, M., Louis, R. G., Jr. and Wartmann, C. T. (2007) 'T2 contributions to the brachial plexus', Neurosurgery, 60, pp. ONS13-8.Yan, J. and Horiguchi, M. (2000) 'The communicating branch of the 4th cervical nerve to the brachial plexus: the double constitution, anterior and posterior, of its fibers', Surgical and Radiologic Anatomy, 22(3), pp. 175-179.