Plexus brachialis - part I

Introduction

The nerves supplying the structures in the arm are all derived from the brachial plexus, a complex of intermingling nerves originating in the neck.

The brachial plexus is formed by the ventral rami of the lower four cervical nerves and the first thoracic nerve to give it a root value of C5, 6, 7 and 8 and T1. Occasionally, there may be a contribution from C4 or T2 or both.

The ventral rami are found as the anterior division of the spinal nerve, just outside the intervertebral foramen, lying between scalenus anterior and medius. They are collectively termed the roots of the plexus. Each spinal nerve receives an autonomic contribution, C5 and 6 receiving grey rami communicates from the middle cervical ganglion, while C7, 8, and T1 receive them from the inferior or cervicothoracic ganglion.

Commonly, the upper two roots(C5 and 6) unite to form the upper trunk, the lower two roots(C8 and T1) unite to form the lower trunk, and the C7 root continues as the middle trunk. These three trunks are found running between the scalene muscles and the upper border of the clavicle in the posterior triangle of the neck. The lower trunk may groove the upper surface of the first rib behind the subclavian artery; the T1 root is always in contact with the rib.

Just above the clavicle, each of the three trunks divides into an anterior and posterior division which supply the flexor and extensor compartments of the arm respectively. The three posterior divisions unite to form the posterior cord, while the anterior divisions of the upper and middle trunks unite to form the lateral cord, and the anterior division of the lower trunk continues as the medial cord. These three cords pass downwards into the axilla, running firstly posterolateral to the axillary artery, but then in their named positions with respect to the second part of the axillary artery posterior to pectoralis minor, that is medial, posterior and lateral. The cords and axillary artery are bound together in an extension of the prevertebral fascia which protrudes into the axilla, the axillary sheath.

Applied anatomy

The brachial plexus itself is subject to direct injury, consequently a knowledge of its formation is of help in determining in exactly which parts, and at what levels, the damage has occurred. Traction injuries occur when the roots of the plexus are torn from the spinal cord, or when the constituent parts are partially or completely torn. In extreme cases, the whole plexus may be disrupted to produce a completely denervated arm. If the upper roots are completely torn, and Erb’s paralysis affecting the musculature of the upper arm is produced; whereas if the lower roots are completely torn, a Klumpikes’ paralysis affecting the hand and forearm will result.

Nerves arising from the brachial plexus and their distribution

The simplest way in which to describe the nerves of the brachial plexus is to put them into terminological order relating to the part of the plexus from which they originate(and to indicate their root value).

Branches from the roots

1. Nerves to the scalene and longus colli muscles(C5, 6, 7, 8).
2. A branch to the phrenic nerve(C5).
3. The dorsal scapular nerve(C5).
4. The long thoracic nerve(C5,6,7).

Branches from the trunks

1. The nerve to subclavius muscle(C4, 5, 6).
2. Suprascapular nerve(C4, 5, 6).

There are no nerves arising from the divisions.

Branches from the cords of the plexus

Medial cord:

1. Medial pectoral nerve(C8, T1).
2. Medial cutaneous nerve of the forearm(C8, T1).
3. Medial cutaneous nerve of the arm(T1).
4. Ulnar nerve(C7, 8, T1).
5. Medial part of the median nerve(C8, T1).

Posterior cord:

1. Upper subscapular nerve(C4, 5, 6, 7).
2. Thoracodorsal nerve(C6, 7, 8).
3. Lower subscapular nerve(C5, 6).
4. Axillary nerve(C5, 6).
5. Radial nerve(C5, C6, C7, C8, T1).

Lateral cord:

1. Lateral pectoral nerve(C5, 6, 7).
2. Musculocutaneous nerve(C5, 6, 7).
3. Lateral part of the median nerve(C5, 6, 7).

Branches from the roots

The muscular supply to the scalene and longus colli muscles arises by twigs from the upper surface of the anterior primary rami as they emerge from the intervertebral foramina, directly entering the muscles. The C5 contribution to the phrenic nerve arises as the lateral border of scalenus anterior.

The dorsal scapular nerve(C5) passes through scalenus medius to gain the deep surface of the levator scapulae muscle, from which it runs onto the anterior surface of the rhomboid muscles. It supplies both rhomboid major, minor and levator scapulae.

The C5 and 6 roots of the long thoracic nerve unite after piercing scalenus medius and are joined by the C7 root on the anterior surface of the muscle. The nerve passes behind the trunks of the plexus between the first rib and the axillary artery to gain the outer(axillary) surface of serratus anterior which it supplies. The upper two digitations of the muscle are supplied by C5, the next two by C6, and the remaining four by C7.

The long thoracic nerve may be damaged by direct pressure on it from above the shoulder.

The resultating paralysis of serratus anterior causes the characteristic “winged” scapula, with the inability to perform activities, such as abduction of the arm, where the scapula is stabilized or laterally rotated.

Branches from the trunks

The nerve to subclavius(C4, 5, 6) is a small branch from the upper trunk. It descends anterior to the subclavian artery to supply subclavius. It may communicate with the phrenic nerve.

The suprascapular nerve(C4, 5, 6) is a large branch from the upper trunk. It passes inferolaterally above and parallel to the trunks through the suprascapular notch deep to trapezius to enter the supraspinous fossa of the scapula. It then runs deep to the supraspinatus muscle and enters the infraspinous fossa via the spinoglenoid notch. The suprascapular nerve supplies both supraspinatus and infraspinatus, and also gives articular filaments to the shoulder and acromioclavicular joints.

All of the above branches arise from the plexus above the clavicle, whereas those considered below all arise below the level of the clavicle in the axilla.

Branches from the cords

The lateral pectoral nerve arises from the lateral cord of the plexus with a root value of C5, 6, 7. It crosses anteromedially in front of the axillary artery, giving a branch to the medial pectoral nerve before piercing the clavipectoral fascia to gain access to the deep surface of pectoralis major which it supplies.

The medial pectoral nerve arises from the medial cord with a root value C8, T1. It receives a contribution from the lateral pectoral nerve and passes between the axillary artery and vein to gain access to the deep surface of pectoralis minor which it supplies. It then pierces this muscle to end in pectoralis major which it also supplies.

The upper and lower subscapular nerves both arise from the posterior cord, root values C4, 5, 6, 7 respectively. From behind the axillary artery they descend towards the subscapular fossa where they both supply the subscapularis muscle. In addition, the lower subscapular nerve enters and supplies teres major.

The thoracodorsal nerve also arises from the posterior cord(C6, 7, 8), between the two subscapular nerves. The nerve passes inferomedially along the posterior wall of the axilla, and the anterolateral surface of latissimus dorsi before entering its deep surface to supply it.

The medial cutaneous nerve of the arm is a small nerve arising from the medial cord(root value T1). It descends through the axilla on the medial side of the axillary vein, and then along the medial side of the brachial artery. It pierces the deep fascia to supply the skin and fascia on the medial side of the proximal half of the arm, extending onto both the anterior and posterior surfaces. It may be partly or entirely replaced by the intercostobrachial nerve(root value T2, 3).

The medial cutaneous nerve of the forearm arises directly from the medial cord with a root value of C8, T1. It descends on the medial side of the axillary and brachial arteries. It pierces the deep fascia, together with basilic vein, in the middle of the arm, and descends with it to the elbow, where it then divides into anterior and ulnar branches. The medial cutaneous nerve of the forearm supplies the skin over the lower part of biceps, the medial side of the forearm as far as the wrist, and part of the medial side of the posterior surface of the forearm.