Median Nerve in the Hand and Carpal Tunnel

The Carpal Tunnel

The flexor retinaculum is a strong fibrous band that bridges the anterior concavity of the carpus, converting it into the carpal tunnel.

Attachments of the flexor retinaculum:

• Medially: pisiform bone and hook of hamate
• Laterally: tubercle of scaphoid and crest of trapezium

The carpal tunnel transmits:

• The median nerve (most anterior/superficial structure in the tunnel)
• The tendons of flexor digitorum superficialis and flexor digitorum profundus (within the ulnar bursa)
• The tendon of flexor pollicis longus (within the radial bursa)

The median nerve lies in the narrow space of the carpal tunnel anterior to the ulnar bursa.

Note: The palmar cutaneous branch of the median nerve arises proximal to the retinaculum and passes superficial to it — it does not pass through the carpal tunnel, and is therefore spared in carpal tunnel syndrome.

Termination of the Median Nerve

Immediately distal to the flexor retinaculum, the median nerve divides into a lateral division and a medial division.

Lateral Division

• Gives a muscular (recurrent) branch that curls upward around the distal border of the flexor retinaculum to supply the thenar muscles. This branch is superficial and clinically vulnerable to injury (e.g., in carpal tunnel surgery or superficial palmar lacerations).
• Gives three digital branches:
  • Two to the thumb (its lateral and medial sides)
  • One to the lateral side of the index finger — this branch also supplies the first lumbrical

Medial Division

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

Muscles Supplied in the Hand

The median nerve supplies five intrinsic muscles of the hand:

1. Abductor pollicis brevis
2. Flexor pollicis brevis
3. Opponens pollicis
4. First lumbrical
5. Second lumbrical

(Mnemonic: LOAF — Lumbricals 1 & 2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis)

Cutaneous (Sensory) Distribution in the Hand

The median nerve supplies palmar skin, with their nail beds, over the lateral three and a half digits:

• Thumb (both sides)
• Index finger
• Middle finger
• Lateral half of the ring finger

It also supplies the skin over the dorsum of the distal phalanges of these same digits (the dorsal proximal/middle phalanges of these digits are supplied by the radial nerve, but the nail-bed/distal tip skin is median).

Sensory loss in median nerve lesions is often less severe than expected due to overlap from adjacent cutaneous nerves.