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26. 2. 2013.

Plexus brachialis - part III

The radial nerve

The radial nerve is the major nerve from the posterior cord, root value C5, 6, 7, 8 (T1), being one of its terminal branches. In the axilla, the radial nerve lies behind the axillary and upper part of the brachial arteries, passing anterior to the tendons of subscapularis, latissimus dorsi and teres major. The radial nerve, together with the profunda brachii artery, enters the posterior compartment of the arm by passing through the lower triangular space, formed by the humerus laterally, the long head of triceps medially and teres major above. In passing through this space, the nerve enters the spiral(or radial) groove of the humerus, descending obliquely between the lateral border of the humerus in the distal third of the arm. The nerve pierces the lateral and medial heads of triceps, reaching the lateral border of the humerus in the distal third of the arm. The nerve pierces the lateral intermuscular septum to enter the anterior compartment where it lies in a muscular groove between brachialis and brachioradialis. In front of the lateral epicondyle of the humerus, the radial nerve divides into its terminal superficial and deep branches.

In the arm, the radial nerve gives a supply to all three heads of triceps, anconeus, the lateral part of brachialis, brachioradialis and extensor carpi radialis longus. The branches to triceps all arise before the radial nerve enters the spiral groove; anconeus is supplied by a branch to the medial head of triceps.
The radial nerve also gives articular branches to the elbow joint and has three cutaneous branches which supply skin on the back of the arm and forearm(figure a).
The posterior cutaneous nerve of the arm arises in the axilla, piercing the deep fascia near the posterior axillary fold. It supplies skin on the posterior surface of the proximal third of the arm.
The lower cutaneous nerve of the arm arises before the radial nerve pierces the lateral intermuscular septum, and becomes cutaneous just below deltoid. It supplies the skin over the lower lateral part of the arm and a small area on the forearm.
The posterior cutaneous nerve of the forearm arises just below the previous nerve, and supplies a variable area of skin on the dorsum of the forearm as far as the wrist, or occasionally beyond.
The superficial branch is the direct continuation of the radial nerve, beginning in front of the lateral epicondyle and descending along the anterolateral side of the forearm. It is entirely sensory. It lies on supinator, pronator teres, flexor digitorum superficialis and flexor pollicis longus covered by brachioradialis with the radial artery medial to it. In the distal third of the forearm, the nerve passes below the tendon of brachioradialis and pierces the deep fascia to become superficial. It supplies the skin on the dorsum of the wrist, the lateral dorsal surface of the hand and dorsum of the thumb, and then divides into four or five digital nerves. The digital nerves supply the skin on the dorsum of the thumb, index, middle and adjacent half of the ring finger as far as the distal interphalangeal joint. The digital branches also give articular branches to the metacarpophalangeal and proximal interphalangeal joints of all five digits.
The deep branch, more often called the posterior interosseus nerve, is entirely muscular and articular. It begins in front of the lateral epicondyle of the humerus and enters the posterior compartment of the forearm by passing between the two heads of supinator, thereby curving around the lateral and posterior surfaces of the radius. During its course, the nerve supplies both extensor carpi radialis brevis and supinator. It then descends between the deep and superficial groups of extensor muscles, accompanied by the posterior interosseus artery, supplying all the muscles in the extensor compartment of the forearm: extensor digitorum, extensordigiti minimi, extensor carpi ulnaris, extensor pollicis longus, extensorindicis, abductor pollicis longus and extensor pollicis brevis.
In the lower part of the forearm, the posterior interosseus nerve lies on the interosseus membrane and ends in a flattened expansion, which gives articular branches to the intercarpal joints.

Applied anatomy

The radial nerve is often injured as it crosses the humerus, either as the result of a fracture or by pressure from a direct blow or incorrect use of a crutch. Triceps usually escapes denervation as it derives its supply from branches given off high in the arm, but a total paralysis of the extensors of the wrist and digits leads to the deformity of a “dropped wrist”(figure a). As a result, any attempt to grip or make a fist leads to increased flexion of the wrist and an inability to carry out effective movement. This is due to the loss of the synergic action of the wrist extensors which usually prevent the unwanted flexion of the wrist produced by the continued action of the finger flexors.

The interphalangeal joints of the fingers can be extended by the lumbricals and interossei which have an attachment to the dorsal digital expansion, but proper use of the hand requires effective form of “lively” splint which compensates for the paralysed muscles. Even though the sensory distribution of the radial nerve on the dorsum of the hand appears extensive, overlap by adjacent cutaneous nerves means that the area of exclusive radial nerve supply is a small patch on the dorsum of the thumb web.

The median nerve

The median nerve is complex in that it arises partly from the lateral cord (C5, 6, 7) and partly from the medial cord (C8, T1) of the brachial plexus. These two contributing heads of the median nerve unite by embracing the third part of the axillary artery. Once formed, the nerve descends under cover of biceps passing at first laterally to the brachial artery and then medially, having crossed it anteriorly. In the lower part of the arm the median nerve lies on brachialis, and in the cubital fossa is protected by the bicipital aponeurosis which crosses it.
The median nerve enters the forearm by passing between the two heads of pronator teres, and then runs below the tendinous arch connecting the heads of flexor digitorum superficialis to gain access to its deep surface. Closely bound to the deep surface of flexor digitorum superficialis, it descends on flexor digitorum profundus until just above the wrist where it becomes superficial by passing between the tendons of flexor digitorum superficialis and flexor carpi radialis, deep to palmaris longus. The median nerve enters the hand deep to the flexor retinaculum, passing anteriorly to the long flexor tendons. Consequently, it is one fo the structures found within the carpal tunnel.
During its course the median nerve gives articular branches to the elbow joint and supplies pronator teres, flexor carpi radialis, palmaris longus and flexor digitorum superficialis.
The palmar cutaneous nerve arises in the distal third of the forearm. It pierces the deep fascia and enters the palm by passing superficial to the flexor retinaculum. It supplies a small area of skin on the lateral side of the palm and thenar eminence.
In the cubital fossa, the anterior interosseus nerve arises from the median nerve and descends, with the anterior interosseus artery, on the anterior surface of the interosseus membrane between flexor pollicis longus and flexor digitorum profundus. It then runs deep to pronator quadratus eo end at the wrist by giving articular branches to the radiocarpal and intercarpal joints. The anterior interosseus nerve supplies flexor pollicis longus, the lateral half of flexor digitorum profundus and pronator quadratus.
Once the median nerve has passed through the carpal tunnel to enter the hand, it divides into lateral and medial terminal branches. The lateral branch passes laterally and proximally to enter the thenar eminence and supply abductor pollicis brevis, flexor pollicis brevis, opponens pollicis and the first lumbrical. It gives sensory branches to the adjacent sides of the thumb and index finger.
The medial branch of the median nerve divides into a variable number of branches, the palmar digital nerves, the most lateral of which supplies the second lumbrical. These nerves are sensory to the palmar surface of adjacent sides of the index and middle, and middle and ring fingers(figure a). Each of these digital nerves produces a dorsal branch which passes backwards to supply the dorsal aspect of the distal phalanx and nail bed, and a variable amount of the middle phalanx of the same digits.

The digital nerves lie deep to the palmar aponeurosis and superficial palmar arch, but superficial to the long flexor tendons. As well as the sensory innervation, they also give articular branches to the interphalangeal and metacarpophalangeal joints.

Applied anatomy

The median nerve can be injured in the forearm by deep cuts with a resultant loss of flexion at all interphalangeal joints, except the distal ones in the ring and little fingers. The metacarpophalangeal joints of these same fingers can still be flexed by the lumbricals and interossei but the movement of pronation is severely restricted. In the hand, the thumb is held in extension and adduction thus losing its ability to oppose and abduct. This, combined with the sensory loss, proves a major disability. More commonly the nerve is damaged just proximal to the flexor retinaculum by laceration, or deep to it in the carpal tunnel where compression gives rise to the carpal tunnel syndrome. In this instance only the thenar muscles, lateral two lumbricals and sensation in the hand will be affected. 

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