The Median Nerve

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The median nerve is one of the three major nerves in the forearm and hand. It plays a key role in the function of the upper limb by carrying both sensory and motor information between the forearm and hand and the brain. Likewise, the median nerve passes through the carpal tunnel and is the primary nerve affected by carpal tunnel syndrome.

Anatomy

The median nerve arises from the brachial plexus, a network of nerves formed by the fusion of the C5 through T1 spinal nerves that innervate the upper limb. Two brachial plexus branches (the lateral and medial branches) unite anteriorly to the brachial artery to form the median nerve. From there, the median nerve runs alongside the brachial artery as it passes through the axillary and brachial regions. It doesn’t branch in these regions and instead continues directly along the medial side of the arm to the elbow. In the elbow, the median nerve passes through the medial side of the antecubital fossa and enters the forearm anterior to the ulna.

In the forearm, the median nerve runs between the ulna and radius and forms several smaller branches. Some of these branches innervate the superficial muscles of the anterior compartment of the forearm. The anterior interosseous branch of the median nerve splits off from the main nerve to innervate the deep muscles of the anterior forearm, distal to the pronator teres muscle. Just proximal to the wrist, the palmar cutaneous branch arises from the median nerve and passes through the superficial tissues of the wrist to innervate the skin of the lateral palm.

The median nerve next crosses the wrist joint, passing through the carpal tunnel as it enters the palmar region of the hand. In the palm, it divides into four common palmar digital branches, which extend toward the digits before dividing into the proper palmar digital branches that enter the first four digits of the hand. The recurrent branch of the median nerve turns toward the thenar muscles that control the thumb while a pair of smaller branches innervate the first and second lumbrical muscles in the palm.

Physiology

The median nerve plays a vital role in the function of the upper limb by innervating many of the anterior muscles of the forearm. Through its branches in the forearm, it innervates the palmaris longus and flexor carpi radialis muscles that flex the hand at the wrist, and the pronator teres muscle that pronates the hand at the wrist. The median nerve also innervates the flexor digitorum superficialis muscle that flexes the digits of the hand. Its interosseous branch innervates the deep muscles of the forearm, including the flexor digitorum profundus, which also flexes the digits of the hand; the flexor pollicis longus, which flexes the thumb; and the pronator quadratus, which pronates the hand at the wrist.

In the hand, the median nerve plays another vital role by innervating the three thenar muscles that control the thumb and two of the lumbrical muscles. The thenar muscles include the flexor pollicis brevis, which flexes the thumb at the metacarpophalangeal joint, and the abductor pollicis brevis which abducts the thumb. Playing an especially critical role in the function of the hand is the third thenar muscle (the opponens pollicis) which opposes the thumb against the other digits and allows objects to be held in the hand. In the palm, a pair of small branches extend to the first and second lumbrical muscles, which flex the index and middle fingers at the metacarpophalangeal joints and extend the same fingers at the interphalangeal joints.

In addition to its control of the muscles of the forearm and hand, the median nerve also carries important sensory information from the skin through its cutaneous branches. The palmar cutaneous branch sends signals from sensory receptors in the skin on the lateral side of the palm to the brain. In the hand, the digital cutaneous branches and their smaller branches provide sensory information from the palmar surface of the first three digits and the lateral side of the fourth digit (ring finger) and the dorsal surface of the distal segments of these digits surrounding the fingernails.

A common condition that affects the median nerve is carpal tunnel syndrome. The median nerve passes through the carpal tunnel of the wrist alongside the flexor tendons of the wrist and hand. Carpal tunnel syndrome occurs when the soft tissues of the carpal tunnel swell, compressing the median nerve. The symptoms of carpal tunnel syndrome include pain, numbness, and tingling in the lateral hand due to a compressed median nerve being unable to send sensory information to the brain. Resting the affected wrist, cooling it with ice, and taking anti-inflammatory medications like non-steroidal anti-inflammatory drugs (NSAIDs) often improves this condition by reducing the swelling of the carpal tunnel and freeing the median nerve to resume its function.