The upper limb is one of the reasons our species has evolved to be so dominant. It enables us to grip, throw and strike. In order to understand the significance of the upper limb, we cannot ignore the median nerve. It gives us one of the most essential functions of life, opposing our thumb which is important to do precision handling and perform lots of activities like writing, threading a needle or winding a watch. In this article we will discuss the anatomy of the median nerve as well as the clinical points related to it.
The median nerve is a major peripheral nerve and supplies the flexor muscles in the forearm except flexor carpi ulnaris and the ulnar head of flexor digitorum profundus, which is supplied by the ulnar nerve. It also supplies the muscles of the thenar eminence as well as the radial two lumbricals.
Origin & Nerve Roots
The nerve roots of the median nerve are from the ventral rami of the C6-T1 nerves. The median nerve is formed by the unification of the medial and lateral cords. It can be seen as the central line of an ‘M Shaped’ brachial plexus, the cords of which surround the axillary artery.
After its formation from the medial and lateral cords of the brachial plexus, the median nerve descends down the centre of the arm in a superficial course. Initially it is lateral to the brachial artery but as it descends it eventually becomes medial. It gives no branches in the arm.
Just before it enters the forearm, the median nerve passes between the tendons of biceps brachii and brachialis. At this point it becomes lateral to the brachial artery. Next, in order to gain access to the forearm, it passes between the deep and superficial heads of the pronator teres muscle. Once it passes this point it dives deeper and runs between the flexor digitorum profundus and superficialis. It gives:
- muscular branches to pronator teres, palmaris longus, flexor digitorum superficialis and flexor carpi radialis
- the anterior interosseus nerve
- the cutaneous nerve of the palm
The anterior interosseous nerve supplies the flexor pollicis longus and radial part of flexor digitorum profundus. This branch runs on the interosseous membrane with anterior interosseous artery passes deep to the pronator quadratus and supplies it. It terminates by giving articular branches to the distal radio ulnar, radiocarpal and carpal joints. It also supplies to the proximal part of the palm, via the palmar cutaneous branch. This branch does not enter the carpal tunnel and is hence spared in carpal tunnel syndrome.
Median nerve passes through the carpal tunnel beneath the flexor retinaculum, and divides to form two common palmar digital nerves, the first of which supplies the radial two lumbricals and a common branch between the index and middle fingers. The second runs between the ring and middle finger, and divides to give the proper digital nerves. The branch also provides sensation to the radial three and a half fingers on their palmar aspect as well as the nail beds of the index, middle and half the ring finger, via the proper digital nerves.
The median nerve also gives off a recurrent branch to the muscles of the thenar eminence (flexor pollicis brevis, abductor pollicis brevis, opponens pollicis). It is also known as the ‘million dollar nerve’ to signify its importance for basic hand function. The reason it is referred to as the recurrent branch is due to the course it takes. It wraps around the flexor pollicis brevis and then arches back to pass into the thenar eminence. This does not include adductor pollicis, which although moves the thumb is not a part of the thenar eminence and is supplied by a deep branch of the ulnar nerve.
Carpal Tunnel Syndrome
The median nerve passes within the carpal tunnel and runs deep to the flexor retinaculum. If the tendons of the long finger flexors become swollen due to overuse (typist), or oedema (pregnancy), then the median nerve becomes compressed. This results in pain, paresthesia and numbness (tingling and pins and needles) of the lateral three and a half fingers. Tapping over the median nerve in the carpal tunnel (‘Tinel’s maneuver’) results in numbness or pain in the median nerve distribution. Flexion of the wrist for 60 seconds will also elicit pain or numbness. Symptoms include thenar muscle wasting and weakness. Treatment is with surgical compression.
Hand of Benediction
The median nerve is vulnerable to be damaged at the elbow, commonly from a supracondylar fracture. This results in the radial head of flexor digitorum profunda being denervated. The forearm is constantly supinated and the lateral two lumbricals have also been denervated, the flexion at the metacarpophalangeal and interphalangeal joints of 2 & 3 digits is lost. There is inability to make a fist as both of these fingers are extended and the hand is in a classic position known as the ‘hand of benediction’ (when the person tries to flex their fingers).
Simian/Ape Hand Deformity
If the recurrent motor branch of the median nerve is damaged, the muscles of the thenar eminence become denervated. This includes abductor pollicis brevis, flexor pollicis brevis, opponens pollicis and the two radial lumbricals. This results in the person becomes unable to oppose the thumb i.e. bring the tip of the thumb to the tip of the other fingers. The appearance is that of an ape’s hand, due to the unopposable thumb. This is distinguished from the hand of benediction as the radial head of flexor digitorum profundus is still functional.
The median nerve can become trapped between the two heads of pronator teres, resulting in wrist flexor wasting.