The median nerve is a branch of the brachial plexus that supplies most of the superficial and deep flexors in the forearm, thenar and lumbrical muscles. It also gives sensation to certain areas of the skin of the hand.
Due to its innervation field, the median nerve enables us to perform both coarse and fine movements of the upper limb. One example is thumb opposition, which is important for precision handling and performing lots of activities like writing, threading a needle or winding a watch.
In this article we will discuss the anatomy and function of the median nerve, as well as the clinical points related to it.
|Origin||Brachial plexus (C6-T1)|
|Branches||Muscular branches, anterior interosseus nerve, articular branches, cutaneous nerve of palm, common palmar digital nerves, reccurent branch
|Motor supply||- Flexor muscles in the forearm (except flexor carpi ulnaris and the ulnar head of flexor digitorum profundus)
- Muscles of the thenar eminence
- Radial two lumbricals
|Sensory supply||The skin of the:
- Palmar and distal dorsal aspects of the lateral three-and-the-half digits and adjacent palm
- Palmar and distal dorsal aspects of the thumb and radial half of 2nd digit
- Palmar and distal dorsal aspects of the adjacent sides of 2nd–4th digits
- Central palm
- Origin and nerve roots
- Branches and innervation
- Clinical notes
Origin and nerve roots
The median nerve arises in the axillary region and it is formed by the unification ofthe medial and lateral cords of the brachial plexus. It contains fibres from roots of spinal nerves C6-T1, but in some individuals it can also contain fibers from C5.
After its formation from 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.
Just before it enters the forearm, the median nerve passes between the tendons of biceps brachii and brachialis. At this point it again 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 flexor digitorum superficialis.
The median nerve then passes through the carpal tunnel beneath the flexor retinaculum, and terminates by dividing into two terminal branches, the common palmar digital nerves.
Branches and innervation
The median nerve gives off numerous branches in the forearm and hand regions. The branches in the forearm region include:
- The muscular branches to pronator teres, palmaris longus, flexor digitorum superficialis and flexor carpi radialis; these branches innervate the corresponding muscles.
- The anterior interosseus nerve,which supplies the flexor pollicis longus and radial part of flexor digitorum profundus. This branch runs on the interosseous membrane with the anterior interosseous artery, and passes deep to the pronator quadratus to supply it. It terminates by giving articular branches to the distal radioulnar, radiocarpal and carpal joints.
The major branches in the hand include:
- The cutaneous nerve of the palm, which supplies the proximal aspect of the palm. This branch does not enter the carpal tunnel and is hence spared in carpal tunnel syndrome.
- The two common palmar digital nerves, the first of which supplies the radial two lumbricals. The second runs between the ring and middle finger, and divides to give the proper digital nerves that provide sensation to certain areas of the hand.
- The 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.
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To summarize, the median nerve provides the motor supply to 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 thenar muscles as well as the radial two lumbricals.
The sensory supply of the median nerve includes:
- The skin of the palmar and distal dorsal aspects of the lateral three-and-the-half digits and adjacent palm.
- The skin of the palmar and distal dorsal aspects of the thumb and radial half of 2nd digit.
- The skin of palmar and distal dorsal aspects of the adjacent sides of 2nd–4th digits.
- The skin of the central palm.
Learn more about the median nerve with our video tutorial and quiz:
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.
To help you remember the hand sign associated with a median nerve injury, use the following mnemonic.
- Drop = Radial nerve
- Claw = Ulnar nerve
- Median nerve = Ape (Apostol's) hand
The median nerve can become trapped between the two heads of pronator teres, resulting in wrist flexor wasting.
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