Course
The median nerve runs a straight path down the anterior compartments of the arm and forearm, from the axilla to the palm, and has no branches in the arm.
Origin in the axilla
The lateral root, from the lateral cord, and the medial root, from the medial cord, unite in front of or just lateral to the third part of the axillary artery. The two roots and the cords they come from make the letter M seen on every axilla dissection, with the artery lying in the notch between the roots.
In the arm
The nerve descends with the brachial artery along the medial border of biceps brachii, first lateral to the artery. Around the level of the coracobrachialis insertion it crosses the artery, usually in front of it, so that it lies medial to the artery by the time it reaches the elbow.
At the elbow and in the forearm
In the cubital fossa the nerve lies medial to the brachial artery, on brachialis, under the bicipital aponeurosis. It leaves the fossa between the humeral and ulnar heads of pronator teres; the ulnar (deep) head separates it from the ulnar artery. It then passes under the tendinous arch of flexor digitorum superficialis (FDS) and runs down the forearm attached to the deep surface of that muscle, lying on flexor digitorum profundus (FDP).
At the wrist and in the hand
A few centimetres above the wrist the nerve emerges from the lateral edge of FDS and becomes superficial, lying between the tendons of flexor carpi radialis (FCR) and FDS, deep to or just lateral to palmaris longus. It enters the carpal tunnel beneath the flexor retinaculum, in front of the flexor tendons, and divides at the distal edge of the retinaculum into its motor and digital branches.
Branches and supply
The median nerve supplies the pronators, the wrist and finger flexors apart from the ulnar-sided ones, and the thenar muscles that oppose the thumb.
| Level | Branch | Supplies |
|---|---|---|
| Elbow | Muscular branches | Pronator teres, FCR, palmaris longus, FDS; articular twigs to the elbow |
| Upper forearm | Anterior interosseous nerve (AIN) | Flexor pollicis longus (FPL), lateral half of FDP (index and middle), pronator quadratus; wrist joint |
| Distal forearm | Palmar cutaneous branch | Skin of the lateral palm and thenar eminence; passes superficial to the flexor retinaculum |
| Hand | Recurrent (thenar motor) branch | Abductor pollicis brevis, flexor pollicis brevis (superficial head), opponens pollicis |
| Hand | Palmar digital nerves | First and second lumbricals; palmar skin of the lateral three and a half digits and the dorsal skin over their distal phalanges |
The AIN leaves the back of the median nerve as it passes between the heads of pronator teres. It runs down on the interosseous membrane with the anterior interosseous artery, between FPL and FDP, and ends deep to pronator quadratus. It has no cutaneous branch.
Relations
The median nerve is always close to the main artery of the limb above the elbow and close to the long flexor tendons below it.
- Axilla: anterior or lateral to the third part of the axillary artery, with the musculocutaneous nerve lateral and the ulnar nerve medial.
- Arm: in the medial bicipital groove with the brachial artery and its venae comitantes, the basilic vein medial and superficial.
- Cubital fossa: the most medial of the main contents, medial to the brachial artery, which is medial to the biceps tendon.
- Forearm: between FDS in front and FDP behind, accompanied by a small median artery from the anterior interosseous artery.
- Carpal tunnel: the most superficial structure in the tunnel, in front of the nine flexor tendons (four FDS, four FDP, one FPL).
Clinical relevance
Median nerve injuries are graded by level: the higher the lesion, the more forearm muscles are lost, while the hand signs are common to all levels.
| Level | Typical cause | Findings |
|---|---|---|
| Above the elbow | Supracondylar fracture, ligament of Struthers | Weak pronation; loss of flexion of the thumb, index and middle interphalangeal joints; index and middle stay extended on attempting a fist; thenar wasting; sensory loss including the lateral palm |
| Proximal forearm | Pronator syndrome (compression by pronator teres, the FDS arch or the bicipital aponeurosis) | Aching forearm with paraesthesia in the median digits and palm |
| AIN | Compression, fracture, neuritis | Cannot make an OK sign: tip pinch collapses to pulp pinch; no sensory loss |
| Carpal tunnel | Carpal tunnel syndrome | Night-time paraesthesia in the lateral three and a half digits; palm spared; later weak abduction and wasting of the thenar eminence |
| Wrist laceration | Glass or knife injury | Loss of LOAF muscles and digital sensation; thumb lies adducted in the plane of the palm (ape hand) |
Carpal tunnel syndrome is the commonest entrapment neuropathy. The palm keeps its sensation because the palmar cutaneous branch runs over the retinaculum, not through the tunnel. At decompression the retinaculum is divided on the ulnar side of the line of the ring finger to keep clear of the recurrent branch, whose course varies: it usually leaves the nerve beyond the retinaculum but can arise within the tunnel or pierce the retinaculum.
Variants that change the examination
A Martin–Gruber anastomosis carries fibres from the median nerve or AIN to the ulnar nerve in the forearm, so a median lesion above it can weaken some ulnar-type hand muscles and an ulnar lesion may spare them. A supracondylar process on the humerus, with the ligament of Struthers running from it to the medial epicondyle, can trap the nerve above the elbow. A persistent median artery or a bifid median nerve may crowd the carpal tunnel.