The median nerve (C, 6, 7, 8; T1) is formed by the union of medial and lateral roots arising from the corresponding cords of the brachial plexus (see Plate 3-14).
COURSE IN ARM
The median nerve runs from the axilla into the arm, lateral to the brachial artery. At about the level of the insertion of the coracobrachialis muscle, the nerve inclines medially over the brachial artery and then descends along its medial side to the cubital fossa. Here, it lies behind the bicipital aponeurosis and the median cubital vein and in front of the insertion of the brachialis muscle and the elbow joint. (The close proximity of the vein, artery, and nerve should be remembered when performing venipuncture in this area.) The only branches given off in the arm are filaments to the brachial vessels and an inconstant twig to the pronator teres muscle.
COURSE IN FOREARM
The nerve passes into the forearm between the humeral and ulnar heads of the pronator teres muscle, the latter separating it from the ulnar artery. It then runs deep to the aponeurotic arch between the humeroulnar and radial heads of the flexor digitorum superficialis muscle and continues downward between this muscle and the flexor digitorum profundus muscle. In the forearm, the nerve supplies branches to the pronator teres, flexor digitorum superficialis, flexor carpi radialis, and palmaris longus muscles and articular twigs to the elbow and proximal radioulnar joints.
The longest branch is the anterior interosseous nerve, which, accompanied by the corresponding artery, runs downward on the interosseous membrane between the flexor pollicis longus and the flexor digitorum profundus muscles; it supplies the former muscle and the lateral part of the latter and ends under the pronator quadratus, supplying this muscle and the distal radioulnar, radiocarpal, and carpal joints. Vascular filaments help to innervate the ulnar and anterior interosseous vessels and the nutrient vessels of the radius and ulna. A palmar branch arises 3 to 4 cm above the flexor retinaculum and descends over it to supply the skin of the median part of the palm and the thenar eminence. In the forearm, the median and ulnar nerves are occasionally interconnected by strands, which may explain certain anomalies in the nerve supply of the hand.
In the lower forearm, the median nerve becomes more superficial between the tendons of the palmaris longus and the flexor carpi radialis muscles. Together with the tendons of the digital flexor muscles, it enters the palm through the carpal tunnel that is bound anteriorly by the tough flexor retinaculum and posteriorly by the carpal bones. Emerging from the tunnel, the nerve splays out into its terminal muscular and palmar digital branches. The muscular branch arises close to, or is initially united with, the common palmar digital nerve to the thumb; it curves outward over or through the flexor pollicis brevis muscle to supply it before dividing to supply the abductor pollicis brevis and opponens pollicis muscles. The muscular branch may also supply all or part of the first dorsal interosseous muscle. In rare instances, it arises in the carpal tunnel and pierces the flexor retinaculum an arrangement of potential clinical concern.
The common and proper palmar digital nerves vary in their origins and distributions, but the usual arrangement is that shown in Plates 3-13 and 3-14. The proper palmar digital nerves give off dorsal twigs, which innervate the skin (including the nail beds) over the distal and dorsal aspects of the lateral three and one-half digits. Occasionally, they supply only two and one-half digits. The proper palmar digital branches to the radial side of the index finger and to the contiguous sides of the index and middle fingers also carry motor fibers to supply the first and second lumbrical muscles, respectively. There-fore, the digital nerves are not concerned solely with cutaneous sensibility. They contain an admixture of efferent and afferent somatic and autonomic fibers, which transmit impulses to and from sensory endings, vessels, sweat glands, and arrectores pilorum muscles and between fascial, tendinous, osseous, and articular structures in their areas of distribution.