Anatomy of the Brachial Plexus Essay

The brachial plexus (C5, 6, 7, 8, T1)

The brachial plexus is positioned to the side of the last four cervical vertebrae and the first thoracic vertebra. It is formed by the anterior rami of C5 through T1, with occasional contributions from C4 and T2. From its emergence, the brachial plexus extends onward and laterally, passes over the first rib behind the clavicle, and enters the axilla. Each brachial plexus innervates the entire upper extremity of one side, as well as a number of shoulder and neck muscles (Faiz, & Moffat, 2002). Brachial plexus is divided anatomically into roots, trunks, divisions, and cords. The roots of the brachial plexus are simply continuations of the anterior rami of the cervical nerves. The anterior rami of C5 and C6 converge to become the superior trunk, the C7 ramus becomes the middle trunk, and the ventral rami of C8 and T1 converge to become the inferior trunk. Each of the three trunks immediately divides into an anterior division and a posterior division. The divisions then converge to form three cords. The posterior cord is formed by the convergence of the posterior divisions of the upper, middle, and lower trunks; hence, it contains fibers from C5 through C8. The medial cord is an extension of the anterior division of the lower trunk and primarily contains fibers from C8 and T1. The lateral cord is formed by the convergence of the anterior division of the upper and middle trunk and consists of fibers from C5 through C7. In summary, the brachial plexus is composed of nerve fibers from the anterior branches of spinal nerves C5 through T1 and a few fibers from C4 and T2. Roots are continuations of the anterior rami. The roots converge to form trunks, and the trunks branch into divisions. The divisions in turn form cords, and the nerves of the upper extremity arise from the cords (Groff, 2001).

The axillary nerve (C5, 6): This is a mixed sensory and motor nerve that arises from the posterior cord of the brachial plexus. It passes through the quadrangular space with the posterior circumflex humeral artery. It provides: a motor supply to deltoid and teres minor; a sensory supply to the skin overlying deltoid; and an articular branch to the shoulder joint. The axillary nerve is particularly prone to injury from the downward displacement of the humeral head during shoulder dislocations. Motor deficit loss of deltoid abduction with rapid wasting of this muscle. Loss of teres minor function is not detectable clinically. Sensory deficit is limited to the ‘badge’ region overlying the lower half of deltoid (Ogla, 2006, Groff, 2001).

The radial nerve (C5, 6, 7, 8, T1): This is a mixed sensory and motor nerve which arises as a continuation of the posterior cord of the brachial plexus. It runs with the profunda brachii artery between the long and medial heads of triceps into the posterior compartment and down between the medial and lateral heads of triceps. At the midpoint of the arm it enters the anterior compartment by piercing the lateral intermuscular septum. In the region of the lateral epicondyle, the radial nerve lies under the cover of brachioradialis and divides into the superficial radial and posterior interosseous nerves. The branches of the radial nerve include: branches to triceps, brachioradialis and brachialis as well as some cutaneous branches. It terminates by dividing into two major nerves: The posterior interosseous nerve passes between the two heads of supinator at a point three fingerbreadths distal to the radial head thus passing into the posterior compartment. It supplies the extensor muscles of the forearm; the superficial radial nerve descends the forearm under the cover of brachioradialis with the radial artery on its medial side. It terminates as cutaneous branches supplying the skin of the back of the wrist and hand. Injury occurs when humeral shaft gets a fracture resulting in damage to the radial nerve in the spiral groove: Motor deficit loss of all forearm extensors- wrist drop; Sensory deficit usually small due to overlap: sensory loss over the anatomical snuffbox is usually constant (Ogla, 2006, Groff, 2001).

The musculocutaneous nerve (C5, 6, 7): This is a mixed sensory and motor nerve which arises from the lateral cord of the brachial plexus. It passes laterally through the two conjoined heads of coracobrachialis and then descends the arm between brachialis and biceps, supplying all three of these muscles en route. It pierces the deep fascia just below the elbow (and becomes the lateral cutaneous nerve of the forearm). Here it supplies the skin of the lateral forearm as far as the wrist.

The median nerve (C6, 7, 8, T1): This is a mixed sensory and motor nerve which arises from the confluence of two roots from the medial and lateral cords lateral to the axillary artery in the axilla. The median nerve initially lies lateral to the brachial artery but crosses it medially in the mid-arm. In the cubital fossa it lies medial to the brachial artery which lies medial to the bicipital tendon. The median nerve passes deep to the bicipital aponeurosis then between the two heads of pronator teres. A short distance below this the anterior interosseous branch is given off. This branch descends with the anterior interosseous artery to supply the deep muscles of the flexor compartment of the forearm except for the ulnar half of flexor digitorum profundus. In the forearm the median nerve lies between flexor digitorum superficialis and flexor digitorum profundus and supplies the remaining flexors except for flexor carpi ulnaris. A short distance above the wrist it emerges from the lateral side of flexor digitorum superficialis and gives off the palmar cutaneous branch which provides a sensory supply to the skin overlying the thenar eminence. At the wrist the median nerve passes beneath the flexor retinaculum (i.e. through the carpal tunnel) in the midline and divides here into its terminal branches: the recurrent branch to the muscles of the thenar eminence (but not adductor pollicis); the branches to the 1st and 2^nd lumbricals; and the cutaneous supply to the palmar skin of the thumb, index, middle and lateral half of the ring fingers (Scanlon, 2007).

Other branches of the brachial plexus

Supraclavicular branches: Suprascapular nerve (C5, 6) which passes through the suprascapular notch to supply supra- and infraspinatus muscles, and long thoracic nerve (of Bell) (C5, 6, 7) which supplies serratus anterior (Eder, 2003).

Infraclavicular branches: Medial and lateral pectoral nerves which supply pectoralis major and minor, medial cutaneous nerves of the arm and forearm, thoracodorsal nerve (C6, 7, 8) which supplies latissimus dorsi, and upper and lower subscapular nerves which supplies subscapularis and teres major.

Brachial Plexus Injuries

Erb–Duchenne paralysis: Excessive downward traction on the upper limb during birth can result in injury to the C5 and C6 roots. This results in paralysis of the deltoid, the short muscles of the shoulder, brachialis and biceps. The combined effect is that the arm hangs down by the side with the forearm pronated and the palm facing backwards. This has been termed the ‘waiter’s tip’ position (Scanlon, 2007).

Klumpke’s paralysis: Excessive upward traction on the upper limb can result in injury to the T1 root. As the latter is the nerve supply to the intrinsic muscles of the hand this injury results in ‘clawing’ (extension of the metacarpophalangeal joints and flexion of the interphalangeal joints) due to the unopposed action of the long flexors and extensors of the fingers. There is often an associated Horner’s syndrome (ptosis, pupillary constriction and ipsilateral anhidrosis) as the traction injury often involves the cervical sympathetic chain (Scanlon, 2007).

Crutch paralysis: The radial nerve is vulnerable to several types of trauma. Crutch paralysis may result when a person improperly supports the weight of the body for an extended period of time with a crutch pushed tightly into the axilla. Compression of the radial nerve between the top of the crutch and the humerus may result in radial nerve damage. Likewise, dislocation of the shoulder frequently traumatizes the radial nerve. Children are particularly at risk as adults yank on their arms. A fracture to the body of the humerus may damage the radial nerve, which parallels the bone at this point. The principal symptom of radial nerve damage is wrist drop, in which the extensor muscles of the fingers and wrist fail to function. As a result, the joints of the fingers, wrist, and elbow are in a constant state of flexion (Scanlon, 2007).

Ulnar Nerve Damage: The ulnar nerve can be palpated in the ulnar sulcus between the medial epicondyle of the humerus and the olecranon of the ulna. This area is commonly known as the “funny bone” or “crazy bone.” Ulnar nerve damage may occur as the medial side of the elbow is banged against a hard object. The immediate perception of this trauma is a painful tingling that extends down the ulnar side of the forearm and into the hand and medial two digits. Although common, ulnar nerve damage is generally not serious (Scanlon, 2007; Guyton, 1991).

References

1. Eder, C. (2003) Laboratory atlas of anatomy and physiology. 4^th ed., New York: McGraw-Hill.
2. Faiz, O., and Moffat, D. (2002) Anatomy at a glance. Oxford: Blackwell Science.
3. Groff, V. D. (2001) Human anatomy. 6^th Ed., New York: McGraw-Hill.
4. Scanlon, V. C., (2007) Essentials of anatomy and physiology. 5^th ed., Philadelphia: E. A. Davis Company.
5. Ogla, H. J. (2006) Principles of anatomy and physiology. 3^rd ed., Nairobi, Kenya: Paloma Publishers.
6. Guyton, A. C., (1991) Textbook of medical physiology. Philadelphia: W. B. Saunders.