anatomy

The median nerve (n. medianus) extends along the middle of the arm and forearm to the hand, and it is the most studied nerve during the daily routine activities in each clinical neurophysiopathological laboratory not only for the frequency of the pathologies of which it is affected like the carpal tunnel syndrome (CTS) but also for the simplicity to investigate that makes it a preferred nerve in multiple disease studies. Brachial PlexusIt is formed by a combination of the lateral cord (C6-C7 roots) and medial cord (C8-T1 roots) of the brachial plexus. The lateral cord supplies the median sensory fibers to the thenar eminence, digit I (thumble), digit II (index), and digit III (middle finger); and motor fibers to the proximal median forearm muscles such as pronator teres (PT) and flexor carpi radialis (FCR) muscles. The medial cord supplies motor fibers to the median muscles of the distal forearm, such as pronator quadratus (PQ) muscle, and hand, such as abductor pollicis brevis (APB) muscle, and sensory fibers to the lateral half (radial side) of the digit IV (ring finger). As it descends through the arm, it lies at first lateral to the brachial artery; about the level of the insertion of the coracobrachialis it crosses the artery, usually in front of, but occasionally behind it, and lies on its medial side at the bend of the elbow, where it is situated behind the lacertus fibrosus (bicipital fascia), and is separated from the elbow-joint by the brachialis. In the antecubital fossa, the nerve lies medial to the brachial artery, which lies medial to the biceps tendon (order from lateral to medial is TAN). The supracondylar process (supracondyloid, supraepitrochlear, epicondylic) is a rare, anomalous, beak-like bony process arising from the anteromedial surface of the humerus 5-7 cm above the medial epicondyle. The process generally projects downward, forward and medially. Found in 2% of the population, more common in caucasians than blacks. Motor branches are given off to the pronator teres, the flexor carpi radialis and the palmaris longus. The median nerve travels deep in the antecubital fossa to its apex and passes between the heads of the pronator teres. The antecubital fossa is by definition the triangular area bounded by the pronator teres, brachioradialis, and a line drawn horizontally betwen the humeral epicondyles. The deep fascia of the forearm, with the bicipital aponeurosis, forms the roof. The floor is mainly the brachialis muscle, with a bit of supinator. The contents are the biceps tendon, brachial artery, median nerve, posterior interosseous nerve, and radial nerve. In most cases, a fibrous ligament (Struther’s) extends from the supracondylar process to the medial epicondyle, enclosing a fibro-osseous foramen through which the median nerve and brachial artery pass. The coracobrachialis and the pronator teres are attached to this fibro-osseous bridge. Struther’s ligament may represent lower part of the tendon of a vestigial muscle (latissimocondyloideus), present in climbing animals. The supracondyloid canal is present in many lower animals. Deep to the pronator teres the anterior interosseus nerve (AIN) is given off which follows the anterior interosseous membrane and supplies the flexor pollicis longus, the flexor digitorum profundus and the pronator quadratus. The main nerve passes beneath the flexor digitorum sublimis but on top of the flexor digitorum profundus directly to the carpal tunnel. In the forearm, the median nerve supplies flexors to digits II-V. The median crosses anterior to the brachial artery in the midarm from the lateral to the medial side of the artery. In the antecubital fossa, the median nerve lies behind the bicipital aponeurosis (lacertus fibrosus) and in front of the brachialis. The nerve enters the forearm between the two heads of the pronator teres. The AIN is given off from the posterior surface of the median nerve as it passes between the two heads of the pronator teres. The median nerve then passes under the fibrotendinous arcade that represents the origin of the flexor digitorum sublimis muscle. The median nerve descends in the forearm adherent to the undersurface of the flexor digitorum sublimis and lying superficial to the flexor digitorum profundus.

In the forearm it passes between the two heads of the pronator teres and crosses the ulnar artery, but is separated from this vessel by the deep head of the pronator teres. It descends beneath the flexor digitorum sublimis, lying on the flexor digitorum profundus, to within 5 cm. of the transverse carpal ligament; here it becomes more superficial, and is situated between the tendons of the flexor digitorum sublimis and flexor carpi radialis. In this situation it lies behind, and rather to the radial side of, the tendon of the palmaris longus, and is covered by the skin and fascia. It then passes behind the transverse carpal ligament into the palm of the hand. In its course through the forearm it is accompanied by the median artery, a branch of the volar interroseous artery.

BRANCHES

MOTOR BRANCH:

With the exception of the nerve to the pronator teres, which sometimes arises above the elbow-joint, the median nerve gives off no branches in the arm. As it passes in front of the elbow, it supplies one or two twigs to the joint. In the forearm its branches are: muscular, volar interosseous, and palmar.

The muscular branches (rami musculares) are derived from the nerve near the elbow and supply all the superficial muscles on the front of the forearm, except the flexor carpi ulnaris.SYNOPTIC MOTOR 2

Synoptic MOTOR tableThe volar interosseous nerve (n. interosseus volaris; anterior interosseous nerve) supplies the deep muscles on the front of the forearm, except the ulnar half of the flexor digitorum profundus. It accompanies the volar interosseous artery along the front of the interosseous membrane, in the interval between the flexor pollicis longus and flexor digitorum profundus, supplying the whole of the former and the radial half of the latter, and ending below in the pronator quadratus and wrist-joint.

The palmar branch (ramus cutaneus palmaris n. mediani) of the median nerve arises at the lower part of the forearm. It pierces the volar carpal ligament, and divides into a lateral and a medial branch; the lateral branch supplies the skin over the ball of the thumb, and communicates with the volar branch of the lateral antibrachial cutaneous nerve; the medial branch supplies the skin of the palm and communicates with the palmar cutaneous branch of the ulnar.

In the palm of the hand the median nerve is covered by the skin and the palmar aponeurosis, and rests on the tendons of the flexor muscles. Immediately after emerging from under the transverse carpal ligament the nerve becomes enlarged and flattened and splits into a smaller, lateral, and a larger, medial portion. The lateral portion supplies a short, stout branch to certain of the muscles of the ball of the thumb, viz., the abductor brevis, the opponens, and the superficial head of the flexor brevis, and then divides into three proper volar digital nerves; two of these supply the sides of the thumb, while the third gives a twig to the first lumbricalis and is distributed to the radial side of the index finger. The medial portion of the nerve divides into two common volar digital nerves. The first of these gives a twig to the second lumbricalis and runs toward the cleft between the index and middle fingers, where it divides into two proper digital nerves for the adjoining sides of these digits; the second runs toward the cleft between the middle and ring fingers, and splits into two proper digital nerves for the adjoining sides of these digits; it communicates with a branch from the ulnar nerve and sometimes sends a twig to the third lumbricalis.

SENSORY BRANCH:

Each proper digital nerve, opposite the base of the first phalanx, gives off a dorsal branch which joins the dorsal digital nerve from the superficial branch of the radial nerve, and supplies the integument on the dorsal aspect of the last phalanx. At the end of the digit, the proper digital nerve divides into two branches, one of which supplies the pulp of the finger, the other ramifies around and beneath the nail. The proper digital nerves, as they run along the fingers, are placed superficial to the corresponding arteries.

sensory distribution 1Synoptic SENSORY 2

Synoptic SENSORY table

NOTES

This page incorporate adapted text from a public domain edition of Gray’s anatomy of the human body (the complete 20th U.S. edition published in 1918).

Captions and tables were taken and adapted from the volumes:

G. Gentili, M. Di Napoli. The Median Nerve: sensory conduction studies (SPRINGER, 2015);

G. Gentili, M. Di Napoli. The Median Nerve: motor conduction studies (SPRINGER, 2015).