Five roots contribute to the formation of the plexus for the upper limb. They are the fibers that remain in the anterior rami of C5, 6, 7, 8 and T1 after these have given their segmental supply to the prevertebral and scalene muscles. They are to divide into anterior and posterior divisions to supply the flexor and extensor compartments respectively, but before doing so they unite to form three trunks in the following manner. Of the five roots of the plexus the upper two unite to form the upper trunk, the lower two unite to form the lower trunk, and the central root runs on as the middle trunk. The five roots lie behind scalenus anterior muscle and emerge between it and scalenus medius to form the trunks which cross the lower part of the posterior triangle of the neck. Each of the three trunks divides into an anterior and a posterior division behind the clavicle. Here, at the outer border of the first rib, the upper two anterior divisions unite to form the lateral cord, the anterior division of the lower trunk runs on as the medial cord, while all three posterior divisions unite to form the posterior cord. These three cords enter the axilla above the first part of the artery, approach and embrace its second part, and give off their branches around its third part.
It is important to appreciate the situation of the constituents of the plexus, i.e. roots between scalene muscles, trunks in the triangle, divisions behind the clavicle, and cords in the axilla. The various branches of the plexus are derived from the roots, trunks and cords; there are none from the divisions. The roots and trunks with their branches form the supraclavicular part of the plexus, with the cords and their branches forming the infraclavicular part.
When the basic pattern has been learned it is a simple matter to put in the branches of the plexus. They consist of 3 branches from the roots and 3, 5 and 5 from the lateral, medial and posterior cords respectively. The only exception to the 3, 5, 5 rule is in the branch from
Branches from the roots of brachial plexus:
Three in number, i.e. the dorsal scapular nerve, the nerve to subclavius, and the long thoracic nerve, they arise successively from C5, C5, 6 and C5, 6, 7, and pass downwards behind, in front of, and behind the roots in that order.
The dorsal scapular nerve (nerve to the rhomboids) arises from the posterior aspect of C5, enters scalenus medius, appears at its posterior border, and courses downwards beneath levator scapulae, lying on serratus posterior superior. It is accompanied by the dorsal scapular vessels. It supplies both rhomboids on their deep surfaces and usually gives a branch to levator scapulae.
The nerve to subclavius arises from the roots of C5 and 6 just as they combine to form the upper trunk (and is often classified as arising from the upper trunk). It passes down in front of the trunks and the subclavian vessels to enter the posterior surface of subclavius. It may carry some aberrant phrenic nerve fibers.
The long thoracic nerve (nerve to serratus anterior) arises from the posterior aspects of C5, 6 and 7. Branches of C5 and 6 enter scalenus medius, unite in the muscle, emerge as a single trunk from its lateral border and pass down into the axilla. On the surface of serratus anterior (the medial wall of the axilla) this is joined by the branch from C7 which has descended in front of scalenus medius. The nerve passes down posterior to the midaxillary line and supplies serratus anterior muscle segmentally.
Branch from the trunks of brachial plexus:
The solitary branch from the trunks (unless the nerve to subclavius is also considered to arise from the upper trunk) is the suprascapular nerve, which arises from the upper trunk in the lower part of the posterior triangle. It can be seen above the clavicle as a large nerve leaving the upper trunk and passing back and laterally to disappear beneath the border of trapezius. It passes through the suprascapular foramen (beneath the transverse scapular ligament) and supplies supraspinatus, descerids lateral to the scapular spine with the suprascapular vessels and supplies infraspinatus and gives a twig to the shoulder joint.
Branches from the lateral cord of brachial plexus:
Three in number, they are the lateral pectoral, musculocutaneous and lateral head of the median nerve.
The lateral pectoral nerve pierces the clavipectoral fascia to supply pectoralis major with fibres from C5, 6 and 7. It communicates across the axillary artery with the medial pectoral nerve and through this communication supplies pectoralis minor. It has no cutaneous branch.
The musculocutaneous nerve leaves the lateral cord, runs obliquely downwards and sinks into coracobrachialis, giving a twig of supply to it (C5 and 6) before entering the muscle. Lower down it supplies biceps and brachialis and becomes the lateral cutaneous nerve of the forearm.
The lateral head of the median nerve is the continuation of the lateral cord (C5, 6 and 7). It is joined by the medial head of the median nerve (from the medial cord, C8 and Tl); the two heads embrace the artery and, when the arm is pulled down to depress the shoulder, may in some cases compress the vessel. Do not confuse the roots of the plexus with the roots of the median nerve.
Branches from the medial cord of brachial plexus:
Five in number, they are the medial pectoral, medial head of the median nerve, ulnar nerve, and the two cutaneous nerves, to the arm and forearm respectively.
The medial pectoral nerve arises from the medial cord (C8 and Tl) behind the axillary artery and enters the deep surface of pectoralis minor, giving a branch of supply before doing so, perforates the muscle and enters the pectoralis major, in which it ends by supplying the lower costal fibers. It is joined by a communication from the lateral pectoral nerve which passes across the axillary artery. Like the lateral pectoral nerve it has no cutaneous branch.
The medial head of the median nerve is the continuation of the medial cord, with fibers from C8 and T1, and it crosses the axillary artery to join the lateral head.
The medial cutaneous nerve of the arm is the smallest and most medial of all the branches. It runs down on the medial side of the axillary vein and supplies skin over the front and medial side of the upper arm.
The medial cutaneous nerve qf the forearm is a large nerve which runs down between artery and vein in front of the ulnar nerve and supplies skin over the lower part of the arm and the medial side of the forearm.
The ulnar nerve is the largest branch of the medial cord (C8 and T1). It runs down between artery and vein behind the medial cutaneous nerve of the forearm, and is the most posterior of the structures which run down the medial side of the flexor compartment of the arm. It receives a branch from the lateral cord (C6, 7) in over 90%; these fibers are given off in the forearm as the motor branch to flexor carpi ulnaris. The ulnar nerve eventually supplies most of the small muscles of the hand.
Branches from the posterior cord of brachial plexus:
Five in number, they are the upper subscapular, thoracodorsal nerve (nerve to latissimus dorsi), lower subscapular, axillary (circumflex), and radial nerves.
The upper subscapular nerve is a small nerve (C5 and 6) which enters the upper part of subscapularis.
The thoracodorsal nerve (nerve to latissimus dorsi C6, 7 and 8) is a large nerve which runs down the posterior axillary wall, crosses the lower border of teres major and enters the deep surface of latissimus dorsi, well forward near the border of the muscle. It comes from high up behind the subscapular artery, but as it descends to enter the muscle it lays in front of the artery, at this level now called the thoracodorsal artery. It is thrown into prominence in the position of lateral rotation and abduction of the humerus and is thus in danger in operations on the lower axilla.
The lower subscapular nerve (C5 and 6) is larger than the upper subscapular and supplies the lower part of the subscapularis. It gives a separate twig to teres major, which runs in the angle between the subscapular and circumflex scapular arteries.
The axillary nerve is one of the two large terminal branches of the posterior cord (the other is the radial nerve). The axillary nerve (C5 and 6) supplies nothing in the axilla despite its name having been changed from circumflex to axillary. From its origin, which is higher than usually imagined, it passes backwards between subscapularis and teres major, lateral to the long head of triceps, i.e. through the quadrilateral space. Here it lies in contact with the surgical neck of the humerus, just below the capsule of the shoulder joint, with the posterior circumflex humeral vessels below it. Having given a branch to the “shoulder joint, it divides into anterior and posterior branches. The anterior branch runs forward around the humerus in contact with the periosteum and enters the deep surface of the deltoid to supply it; a few terminal twigs pierce the muscle and reach the skin. The posterior branch gives off the motor nerve to teres minor, then winds around the posterior border of deltoid and becomes cutaneous. It is here called the upper lateral cutaneous nerve of the arm. It supplies a few of the posterior fibres of deltoid.
The radial nerve is the continuation of the posterior cord, and is the largest branch of the whole plexus. It crosses the lower border of the posterior axillary wall, lying on the glistening tendon of latissimus dorsi. It passes out of sight through a triangular space below the lower border of teres major, between the long head of triceps and the humerus. Before disappearing it gives nerves of supply to the long head of triceps, to the medial head by a nerve which accompanies the ulnar nerve along the medial side of the arm, and a cutaneous branch which supplies the skin along the posterior surface of the upper arm (the posterior cutaneous nerve of the arm). The radial nerve is important as the nerve of the extensor compartment of the forearm.