Fig. 10.1
Anatomy of the brachial plexus
After exiting the intervertebral foramina, the roots traverse through the interscalene groove or triangle (scalene hiatus) before forming the trunks and entering the floor of the posterior triangle of the neck (supraclavicular fossa). In the posterior triangle, the plexus is covered only by platysma, deep fascia, and skin and is palpable, especially in thin individuals. The anterior and middle scalene muscles lie immediately anterior and posterior respectively to the plexus in the interscalene region, forming the so-called interscalene groove or triangle. The plexus at this level consists of upper (superior), middle, and lower (inferior) trunks and is enclosed within the interscalene fascial sheath. Most commonly, the upper trunk is formed by the convergence of C5 and C6, while C7 becomes the middle trunk, and C8 and T1 converge to make the lower trunk. The trunks are crossed by the external jugular vein, the superficial (transverse) cervical and suprascapular arteries (branches of the thyrocervical trunk that can function as external collaterals of the subclavian artery), the inferior belly of the omohyoid, and the supraclavicular nerves as they course inferolaterally.
At and above the level of the interscalene groove (C6), the plexus lies posterolateral to the internal jugular vein and common carotid artery. As the plexus emerges between the scalene muscles, its proximal part is cephalad to the third part of the subclavian artery and its lower trunk posterior to it. The plexus then courses behind the medial two thirds of the clavicle where it is also posterior to the subclavius muscle and suprascapular vessels. Near the midpoint of the clavicle, the plexus and the subclavian artery are separated from the subclavian vein by the tendinous insertion of the anterior scalene muscle (Fig. 10.2).
Fig. 10.2
Course of the brachial plexus at the interscalene groove and under the clavicle
Coursing distally, each of the three trunks then branches into anterior and posterior divisions deep to the clavicle (subclavian location), following which the divisions reunite to form three cords in the axilla (axillary location) and five major terminal nerves in the upper arm (brachial location) (Fig. 10.3). The three cords of the plexus, namely, lateral, medial, and posterior, are named for their relationship to the second part of the axillary artery. Within the axilla, the brachial plexus is enclosed together with the first part of the axillary artery and axillary vein within the fibrous axillary sheath, a continuation of the prevertebral layer of deep cervical fascia. The major terminal nerves and their branches can been seen in Fig. 10.4.
Fig. 10.3
Schematic diagram of the brachial plexus
Fig. 10.4
A “map” of the brachial plexus showing sensory and motor supply, terminal nerves, and targets of interscalene, supraclavicular, and infraclavicular blocks
10.2 Branches of the Brachial Plexus
From proximal to distal, the plexus is distributed regionally and consists of five roots (paravertebral), three trunks (interscalene), two divisions per trunk (subclavian), three cords (axillary), and five major terminal nerves (brachial) (Figs. 10.3 and 10.4).
Branches arising from the roots and trunks of the brachial plexus are generally classified as “supraclavicular” branches, while those arising from the cords are referred to as “infraclavicular” branches.
Root branches (supraclavicular):
Nerve to longus cervicis muscle (C5–C8)
Nerve to longus colli and scalene muscles (C5–C8)
Contribution to phrenic nerve (C5)
Dorsal scapular nerve [nerve to rhomboids] (C5)
Trunk branches (supraclavicular):
Nerve to subclavius (superior trunk, C5, C6)
Suprascapular nerve (superior trunk, C5, C6)
Long thoracic nerve [posterior thoracic nerve] (C5–C7)
Cord branches (infraclavicular):
Lateral cord
Lateral pectoral nerve (C5–C7); musculocutaneous nerve (C5–C7); lateral root (head) of median nerve (C5–C6, C7)
Medial cord
Medial pectoral nerve (C8, T1); medial cutaneous nerve of arm (medial brachial cutaneous nerve, C8, T1); medial cutaneous nerve of forearm (medial antebrachial cutaneous nerve, C8, T1); medial root (head) of median nerve (C8, T1); ulnar nerve (C7–C8, T1)
Posterior cord
Upper subscapular nerve (C5, C6); thoracodorsal nerve (C6–C8); lower subscapular nerve (C5, C6); axillary nerve (C5, C6); radial nerve (C5–C8, T1)
Selective clinically relevant nerves will be described in more detail below with respect to their origins, course, and function as related to clinical practice.
10.2.1 Branches from the Roots (Ventral Rami)
10.2.1.1 Phrenic Nerve
Strictly speaking, the phrenic nerve is not part of the brachial plexus; however, due to its close relationship to the plexus, the nerve has to be considered from a clinical perspective since it plays an important role and has significant clinical implications when performing brachial plexus block. The phrenic nerve arises from the anterior rami of spinal nerves C3, C4, and C5 (C3, C4 and C5 keep the diaphragm alive!) and normally descends in a superolateral to inferomedial direction on the anterior surface of the scalenus anterior muscle (subject to anatomic variation) deep to the prevertebral fascia, before passing under the clavicle and through the superior thoracic aperture into the superior mediastinum, passing just medial to and in front of the internal thoracic (mammary) artery. Once within the thoracic cavity, both right and left phrenic nerves descend anterior to the pulmonary hila together with the pericardiophrenic vessels (branches of the internal thoracic or mammary vessels) in the plane between the fibrous pericardium medially and the mediastinal pleura laterally on their way to the diaphragm which they pierce to supply. The right phrenic pierces the diaphragm’s central tendon near the caval orifice, while the left phrenic traverses the muscular portion of the left hemidiaphragm and lies somewhat more anterior than the right.
Innervation | Motor: diaphragm (the phrenic is the sole motor supply to the diaphragm) |
Sensory: fibrous pericardium, parietal layer of serous pericardium, mediastinal parietal pleura, diaphragmatic parietal pleura, and diaphragmatic parietal peritoneum |
10.2.1.2 Dorsal Scapular Nerve
Originating from the ventral ramus of C5, this nerve passes through the scalenus medius and courses behind the levator scapulae before running with the deep branch of the dorsal scapular artery to terminate in the rhomboids, which it supplies.
Innervation | Motor: rhomboid major, rhomboid minor, levator scapulae (occasionally) |
Sensory: none |
10.2.2 Branches from the Trunks
10.2.2.1 Long Thoracic Nerve
This nerve has contributions from the roots of C5–C7. Contributions from C5 and C6 pierce the scalenus medius and join within or lateral to the muscle; the nerve then descends dorsal (posterior) to the brachial plexus and first part of the axillary artery. The C7 contribution, when present, emerges between the scalenus medius and scalenus anterior muscles before joining with the C5 and C6 contributions near the superior border of the serratus anterior muscle. The nerve then crosses the superior border of the serratus anterior to reach its lateral surface before descending on the muscle to supply each of its digitations and to terminate at its lower border.
Innervation | Motor: serratus anterior |
Sensory: none |
10.2.2.2 Nerve to the Subclavius
Small but deserving of mention is the nerve to the subclavius. Arising from where the ventral rami of C5 and C6 join, this small filament travels anterior to the main plexus, passing anterior to the third part of the subclavian artery, and is usually connected in some capacity to the phrenic nerve before passing above the subclavian vein to supply the subclavius muscle.
Innervation | Motor – subclavius |
Sensory – none |
10.2.2.3 Suprascapular Nerve
A large branch of the superior trunk, this nerve arises from the anterior rami of C5 and C6; travels laterally, deep to the trapezius and omohyoid muscles; and enters the supraspinous fossa through the suprascapular notch beneath the superior transverse scapular ligament. Coursing deep to the supraspinatus and supplying it, the nerve then curves around the lateral border of the spine of the scapula (spinoglenoid notch) and, with the supraclavicular artery, reaches the infraspinous fossa, where it supplies the infraspinatus and provides articular branches to the capsule of the glenohumeral (shoulder) and acromioclavicular joints. A cutaneous branch, though rare, may pierce the deltoid close to the tip of the acromion and become superficial to supply the skin of the shoulder and upper arm within the region supplied by the axillary nerve.
Innervation | Motor: supraspinatus, infraspinatus |
Sensory: when present, supplies skin on lateral aspect of shoulder, proximal third of arm; articular innervation to shoulder and acromioclavicular joints |
10.2.3 Branches from the Cords
10.2.3.1 Lateral Pectoral Nerve
With contributions from the C5 to C7 anterior rami, the lateral pectoral nerve can either arise from the anterior divisions of the upper and middle trunks or have a single origin from the lateral cord. It passes anterior to the axillary artery and vein before piercing the clavipectoral fascia and terminating on the deep surface of the pectoralis major muscle, which it supplies. A filament from the nerve loops in front of the axillary artery (ansa pectoralis) and joins with the medial pectoral nerve to send fibers to the pectoralis minor muscle.
Innervation | Motor: pectoralis major, some to pectoralis minor |
Sensory: none |
10.2.3.2 Medial Pectoral Nerve
Arising from the medial cord, with contributions from C8 and T1, this nerve travels from its origin posterior to the axillary artery and then swings anteriorly between the axillary artery and vein to join with the filament from the lateral pectoral nerve anterior to the axillary artery. It enters the deep surface of the pectoralis minor and supplies it; usually some branches will travel around the inferior border of the pectoralis minor to terminate in the pectoralis major.
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