“Labourer’s nerve” (controls most long flexor muscles of forearm) + “Eye of the hand” (sensory to palmar lateral 3½ digits; critical for fine tactile discrimination). More disabling than ulnar nerve injury — compromises pincer grip.

Origin

Formed in axilla by union of two roots:

Two roots embrace the 3rd part of the axillary artery and unite on its lateral or anterior aspect.

Root value: C6, C7, C8, T1 (occasionally C5)

Course

In the Arm

• Initially lateral to brachial artery
• At level of coracobrachialis insertion (mid-arm): crosses in front of brachial artery → medial side
• Continues medial to brachial artery to cubital fossa
• No branches in the arm

In the Cubital Fossa

• Medial to brachial artery; behind bicipital aponeurosis; in front of brachialis
• Leaves by passing between two heads of pronator teres (superficial humeral + deep ulnar heads)
• While between heads: crosses in front of ulnar artery (separated by deep head of pronator teres)

In the Forearm

• After pronator teres: deep to fibrous arch of FDS
• Runs on posterior surface of FDS, lying on FDP
• Accompanied by median artery (branch of anterior interosseous artery)
• ~5 cm above flexor retinaculum: emerges from behind lateral border of FDS → superficial; between FCR (lateral) and FDS (medial), overlapped by palmaris longus

Entry into the Hand

• Enters palm deep to flexor retinaculum through carpal tunnel (most anterior/superficial structure in tunnel)
• Immediately distal to retinaculum: divides into lateral and medial divisions

Relations — Summary Table

Branches

In the Cubital Fossa and Forearm

1. Muscular branches (cubital fossa):

• Pronator teres, flexor carpi radialis, palmaris longus, FDS

2. Anterior Interosseous Nerve (AIN)

• Arises upper forearm; descends on front of interosseous membrane between FPL and FDP; with anterior interosseous artery
• Passes deep to pronator quadratus; ends on front of carpus
• Supplies: FPL + lateral half of FDP (index and middle) + pronator quadratus
• Articular branches to distal radioulnar and wrist joints
• Pure motor — no sensory component

3. Palmar cutaneous branch

• Arises just above flexor retinaculum
• Passes superficial to (not through) flexor retinaculum
• Supplies skin over thenar eminence and central palm
• Spared in CTS → thenar eminence sensation preserved

4. Articular branches (elbow + proximal radioulnar joint); vascular branches; communicating branch to ulnar nerve

Terminal Branches — In the Hand

Lateral division:

• Recurrent (muscular) branch — curls upward around distal border of flexor retinaculum → thenar muscles (abductor pollicis brevis, opponens pollicis, flexor pollicis brevis); superficially placed → vulnerable in carpal tunnel surgery
• Digital branches → both sides of thumb, lateral side of index finger (+ 1st lumbrical)

Medial division:

• Two common palmar digital branches → adjoining sides of index, middle, ring fingers (2nd and 3rd interdigital clefts)
• Lateral common digital branch → 2nd lumbrical

Muscles Supplied — Summary

LOAF — Lumbricals 1 & 2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis

Cutaneous (Sensory) Distribution in the Hand

• Palmar: Lateral 3½ digits (thumb both sides, index, middle, lateral half of ring) + palm proximal to these
• Dorsal: Distal phalanges + nail beds of lateral 3½ digits (dorsal proximal/middle phalanges = radial nerve)

The Carpal Tunnel

Flexor retinaculum bridges concave anterior surface of carpus.

Attachments:

Contents:

• Median nerve (most anterior)
• FDS and FDP tendons (in ulnar bursa)
• FPL tendon (in radial bursa)

Palmar cutaneous branch arises proximal to retinaculum and travels superficial to it — does NOT pass through carpal tunnel.

Applied Anatomy

1. High Median Nerve Injury (at the Elbow)

Common cause: Supracondylar fracture of humerus

2. Anterior Interosseous Nerve Syndrome (Mid-Forearm)

• Isolated AIN injury (pure motor)
• Weakness of FPL, lateral half of FDP, pronator quadratus
• Same pointing index finger sign on fist
• No sensory loss

3. Injury at the Wrist / Distal Forearm

• Thenar paralysis + lateral two lumbricals paralysis
• Sensory loss palmar lateral 3½ digits including thenar eminence (palmar cutaneous branch also cut — unlike CTS)
• “Hand of benediction” — index and middle fingers lag in MCP flexion (paralysis of 1st and 2nd lumbricals)

4. Carpal Tunnel Syndrome (CTS)

Most common entrapment neuropathy — compression of median nerve within carpal tunnel.

Key Differentiating Points — Levels of Injury

The median nerve is called the “labourer’s nerve” (controls most long flexor muscles of the forearm) and the “eye of the hand” (sensory to the palmar aspect of the lateral 3½ digits, critical for fine tactile discrimination). Injury is more disabling than ulnar nerve injury because it compromises the pincer grip essential for precision activities.

Origin

Formed in the axilla by union of two roots:

The two roots embrace the third part of the axillary artery and unite on its lateral or anterior aspect.

Root value: C6, C7, C8, T1 (occasionally C5).

Course

In the Arm

• Descends through the arm initially lateral to the brachial artery.
• At the level of insertion of coracobrachialis (mid-arm), crosses in front of the brachial artery to reach its medial side.
• Continues medial to the brachial artery to the cubital fossa.
• No branches given in the arm (apart from an occasional vascular twig).

In the Cubital Fossa

• Lies medial to the brachial artery, behind the bicipital aponeurosis, in front of brachialis.
• Leaves the cubital fossa by passing between the two heads of pronator teres (superficial humeral head and deep ulnar head).
• While passing between the two heads, it crosses in front of the ulnar artery, separated from it by the deep (ulnar) head of pronator teres.

In the Forearm

• After emerging between the heads of pronator teres, passes deep to the fibrous arch of flexor digitorum superficialis (FDS).
• Runs down the forearm on the posterior surface of FDS, lying on flexor digitorum profundus (FDP).
• Accompanied by the median artery (branch of the anterior interosseous artery).
• About 5 cm above the flexor retinaculum, emerges from behind the lateral border of FDS to become superficial — lies between tendons of flexor carpi radialis (laterally) and FDS (medially), overlapped by palmaris longus.

Entry into the Hand

• Enters the palm deep to the flexor retinaculum, through the carpal tunnel, lying anterior to the ulnar bursa (most superficial/anterior structure in the tunnel).
• Immediately distal to the retinaculum, divides into lateral and medial divisions.

Relations — Summary Table

Branches

In the Cubital Fossa and Forearm

1. Muscular branches (given off in the cubital fossa):

• Pronator teres
• Flexor carpi radialis
• Palmaris longus
• Flexor digitorum superficialis

2. Anterior Interosseous Nerve (AIN)

• Arises in the upper forearm; descends on the front of the interosseous membrane between flexor pollicis longus and FDP, with the anterior interosseous artery.
• Passes deep to pronator quadratus; ends on the front of the carpus.
• Supplies:
  • Flexor pollicis longus
  • Lateral half of flexor digitorum profundus (tendons to index and middle fingers)
  • Pronator quadratus
• Also gives articular branches to the distal radio-ulnar and wrist joints.
• Pure motor nerve — no sensory component.

3. Palmar cutaneous branch

• Arises just above the flexor retinaculum.
• Passes superficial to (not through) the flexor retinaculum.
• Supplies skin over the thenar eminence and central palm.
• Clinically important: spared in carpal tunnel syndrome → thenar eminence sensation is preserved.

4. Articular branches — to the elbow joint and proximal radio-ulnar joint.

5. Vascular branches — to the radial and ulnar arteries.

6. Communicating branch — to the ulnar nerve in the forearm.

Terminal Branches — In the Hand

Immediately distal to the flexor retinaculum, the nerve divides into lateral and medial divisions:

Lateral division:

• Recurrent (muscular) branch — curls upward around the distal border of the flexor retinaculum to supply the thenar muscles (abductor pollicis brevis, opponens pollicis, flexor pollicis brevis). Superficially placed — vulnerable in carpal tunnel surgery and superficial palmar lacerations.
• Digital branches — to both sides of the thumb, lateral side of index finger (this branch also supplies the 1st lumbrical).

Medial division:

• Divides into two common palmar digital branches supplying the adjoining sides of index, middle, and ring fingers (2nd and 3rd interdigital clefts).
• The lateral common digital branch also supplies the 2nd lumbrical.

Muscles Supplied — Summary

Mnemonic (LOAF) for intrinsic hand muscles — Lumbricals 1 & 2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis.

Cutaneous (Sensory) Distribution in the Hand

• Palmar surface: Lateral 3½ digits — thumb (both sides), index, middle, and lateral half of ring finger; and the palm proximal to these digits.
• Dorsal surface: Distal phalanges and nail beds of the same lateral 3½ digits (dorsal proximal/middle phalanges of these digits are supplied by the radial nerve).

The Carpal Tunnel

The flexor retinaculum bridges the concave anterior surface of the carpus, forming the carpal tunnel.

Attachments:

Contents of the carpal tunnel:

• Median nerve (most anterior structure)
• Tendons of FDS and FDP (within ulnar bursa)
• Tendon of flexor pollicis longus (within radial bursa)

The palmar cutaneous branch of the median nerve arises proximal to the retinaculum and travels superficial to it — it does not pass through the carpal tunnel.

Applied Anatomy

1. High Median Nerve Injury (at the Elbow)

Common cause: Supracondylar fracture of the humerus.

Effects:

2. Anterior Interosseous Nerve Syndrome (Mid-Forearm)

• Isolated injury to the AIN (pure motor branch).
• Weakness of FPL, lateral half of FDP, pronator quadratus.
• Same pointing index finger sign on attempted fist-making.
• No sensory loss — AIN is purely motor.

3. Injury at the Wrist / Distal Forearm

• Both the median nerve and palmar cutaneous branch may be cut by lacerations at the wrist.
• Paralysis of thenar muscles + lateral two lumbricals.
• Sensory loss over palmar lateral 3½ digits including the thenar eminence (palmar cutaneous branch is also cut — unlike CTS).
• On attempting a fist: index and middle fingers lag in flexion at MCP joints — “hand of benediction” appearance — due to paralysis of 1st and 2nd lumbricals.

4. Carpal Tunnel Syndrome (CTS)

The most common entrapment neuropathy — compression of the median nerve within the carpal tunnel.

Clinical features:

Key Differentiating Points — Levels of Injury

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