MEDIAN NERVE
The median
nerve (C[5], 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.
