Upper Limb - Median Nerve
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Median Nerve
- Innervates the lateral flexors (below the elbow), derived from C6 – T1
- C6 and C7 are mostly sensory / C8 and T1 are mostly motor.
- Does not innervate any of the muscles of the upper arm or provide any of its sensory coverage.
Superficial forearm group
- C6 and C7 roots -> innervate pronator teres (PT) and flexor carpi radialis (FCR).
- C7, C8 roots -> innervate flexor digitorum superficialis (FDS) and palmaris longus (PL).
- Anterior interosseous nerve branch innervates the anterior interosseous group:
- C7–T1 roots -> innervate pronator quadratus (PQ), flexor pollicis longus (FPL), and flexor digitorum profundus 2 and 3 (FDP 2,3).
- When the anterior interosseous nerve is injured in isolation (anterior interosseous syndrome or neuropathy), patients exhibit the "OK" sign (or more aptly, the not "OK" sign); wherein, they are unable to form a circle with the thumb and index finger. Neither the thumb's interphalangeal joint nor the index finger's distal interphalangeal joints will flex.
Recurrent motor branch of the thumb innervates the thenar group.
- C8, T1 roots -> innervate abductor pollicis brevis (APB), opponens pollicis (OP), and flexor pollicis brevis (FPB).
The terminal motor group.
- C8, T1 roots -> innervate first and second lumbricals.
- The digital sensory branches provide distal sensory innervation via digital sensory branches.
Median neuropathies
Proximal
- Along the distal humerus, supracondylar fracture can injure the median nerve (and also the ulnar nerve) and affect all of the downstream components.
- The humeral and ulnar heads of the pronator teres muscle can ensnare the median nerve as it exits the cubital fossa. All of the downstream components are affected except the superficial forearm group because innervation to the superficial forearm group lies proximal to the entrapment site.
- Anterior interosseous syndrome (see above)
Distal
- Carpal tunnel at the wrist
- Most common entrapment site of the median nerve.
- The median palmar cutaneous nerve, enters the hand superficial to the carpal tunnel, so it's spared in carpal tunnel syndrome
- A laceration of the palm can injure the recurrent branch of the thumb in isolation.
Clinical Effect of Median Neuropathies
- Median nerve injury produces lateral flexor weakness and thenar eminence (thumb pad) wasting, so-called "ape hand".