Figure. (A) The cutaneous distribution and (B) course of the median nerve
The median nerve arises partly from the lateral cord (C5, 6, 7) and partly from the medial cord (C8, Tl) of the brachial plexus. These two contributing heads unite by embracing the third part of the axillary artery. Once formed, the nerve descends under cover of biceps brachii initially 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 access 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 flexors digitorum superficialis and carpi radialis, deep to palmaris longus. The median nerve enters the hand deep to the flexor retinaculum, passing anteriorly to the long flexor tendons. It is, therefore one of 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 interosseous nerve arises from the median nerve and descends, with the anterior interosseous artery, on the anterior surface of the interosseous membrane between flexor pollicis longus and flexor digitorum profundus. It then runs deep to pronator quadratus to end at the wrist by giving articular branches to the radiocarpal and intercarpal joints. The anterior interosseous 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 (Fig. A). Each digital nerve produces a dorsal branch which passes posteriorly 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.
Figure. Typical median nerve deformity
The median nerve can be injured in die 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 carpal tunnel syndrome. In this instance only the thenar muscles, lateral two lumbricals and sensation in the hand will be affected. Static splinting following median nerve lesions often involves holding the thumb in abduction and some opposition to prevent loss of the thumb webspace.