Median Nerve
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Direct answer
Two roots that clasp the third part of the axillary artery (lateral C5-C7 and medial C8-T1 roots) unite to form the median nerve anterior to the vessel. It descends lateral to the brachial artery, crosses it at mid-arm, and enters the cubital fossa medial to the biceps tendon, giving no branches above the elbow. It then passes between the two heads of pronator teres, runs between flexor digitorum superficialis and profundus giving the anterior interosseous branch, and sends a palmar cutaneous branch across the wrist before entering the carpal tunnel. In the hand it supplies the thenar muscles via a recurrent motor branch, the lateral two lumbricals, and the skin of the lateral three and a half digits.
What you must remember
- Formation: lateral (C5-C7) and medial (C8-T1) roots straddling the axillary artery — the only nerve formed by two roots from two cords, a favourite identification point.
- Forearm motor territory: all flexors of the forearm except flexor carpi ulnaris and the ulnar half of flexor digitorum profundus; the anterior interosseous branch additionally supplies flexor pollicis longus, the radial half of profundus and pronator quadratus.
- Anterior interosseous syndrome: pure motor weakness of pinch — the patient cannot make an "OK" ring because flexor pollicis longus and the radial profundus fail.
- Palmar cutaneous branch: leaves the nerve in the distal forearm and passes superficial to the flexor retinaculum, so it is spared in carpal tunnel syndrome but vulnerable to wrist lacerations — the classic dissociation asked in vivas.
- Hand motor supply: recurrent motor branch to the thenar trio, plus lateral two lumbricals; all other intrinsic hand muscles are ulnar.
- Carpal tunnel syndrome: night pain and paraesthesia in the lateral three and a half digits, thenar wasting late, positive Phalen and Tinel signs — the palm itself is spared because the palmar cutaneous branch bypasses the tunnel.
- High lesion signs: hand of benediction when the patient clenches a fist (loss of flexion at the interphalangeal joints of the index and middle fingers), ape-hand resting posture with thenar wasting.
- Martin-Gruber anastomosis: median-to-ulnar crossing in the forearm, reported in roughly one in six people by common estimates, which can mask or mimic ulnar lesions.
Localising three lesions along one nerve
A woman waking with numb thumb and index fingers, shaking her hands to restore sensation, has carpal tunnel syndrome: the compression is at the wrist, the palm is sensate, and thenar abduction-opposition weaken only late. A man stabbed just above the wrist loses everything the nerve carries distal to the cut — thumb-to-finger sensory loss, thenar palsy, and a numb palm because the palmar cutaneous branch was divided. A third patient, after a supracondylar fracture, cannot flex the index interphalangeal joints or the thumb's distal joint, cannot pronate against resistance, and shows benediction of the hand on attempted fist clench — the lesion is above the elbow.
The localising logic is the branch list read backwards: intact thenar power with lost flexor pollicis longus points to the anterior interosseous branch alone; a numb palm points to a wrist lesion including the palmar cutaneous branch; forearm weakness points above the elbow.
Where students slip
The palm question fails most candidates: they state the palm is numb in carpal tunnel syndrome, forgetting the palmar cutaneous branch leaves above the retinaculum. The second slip is the forearm flexor exceptions — flexor carpi ulnaris and the ulnar half of profundus belong to the ulnar nerve, and pronation with a flexed elbow (pronator quadratus intact) may survive a high lesion tested incorrectly. Third, the hand of benediction is described as a resting posture; it appears only on attempted fist clenching, whereas the ape hand with a flat, wasted thenar eminence is the resting picture. Indian viva boards add the carpal tunnel contents: the nerve lies deep to the flexor retinaculum, and the recurrent motor branch hooks around its distal edge, vulnerable in palmar lacerations. Candidates who draw the tunnel in cross-section with contents in order close the topic with a distinction.
Frequently asked questions
How is the median nerve formed?
By union of lateral (C5-C7) and medial (C8-T1) roots that clasp the third part of the axillary artery, in front of the vessel.
Why is the palm spared in carpal tunnel syndrome?
The palmar cutaneous branch leaves the nerve proximal to the wrist and passes superficial to the flexor retinaculum, escaping compression inside the tunnel.
What is hand of benediction and when does it appear?
The inability to flex the interphalangeal joints of the index and middle fingers when clenching a fist, seen in high median nerve lesions — not a resting posture.
What deficit characterises anterior interosseous syndrome?
Pure motor weakness of flexor pollicis longus and the radial flexor digitorum profundus, so the pinch fails and the OK sign cannot be completed.
Which muscles does the median nerve supply in the hand?
Abductor pollicis brevis, opponens pollicis, the superficial head of flexor pollicis brevis through the recurrent branch, and the lateral two lumbricals.