Upper Limb - Nerve Anatomy
Upper Extremity - Nerve Anatomy
Major Terminal Nerves: Median, Ulnar, Radial
Relevant Upper Extremity Anatomical Landmarks
- Humerus, proximally.
- Radius in the lateral forearm.
- Ulna in the medial forearm.
- Brachial artery in the medial arm.
- Flexor digitorum profundus (FDP) in the anterior forearm
- Pronator teres (PT) muscle:
- Humeral head originates from the humerus and inserts along the radius.
- Ulnar head originates from the ulna and also inserts along the radius.
- Abductor pollicis longus (APL)
- Passes along the medial arm, anterior to the brachial artery.
- Exits the cubital fossa between the heads of pronator teres, meaning deep to the humeral head and superficial to the ulnar head.
- Thus it can be compressed between the pronator teres heads.
- Runs superficial to flexor digitorum profundus (FDP) along a midline course into the hand.
- Passes beneath the flexor retinaculum through the carpal tunnel (wrist).
- Here it can be compressed (so-called, "carpal tunnel syndrome").
- In the proximal forearm, the median nerve provides the anterior interosseous nerve branch, superficial to the flexor digitorum profundus.
- In the distal forearm, it provides the median palmar cutaneous nerve, which enters the hand superficial to the carpal tunnel.
- So it's spared in carpal tunnel syndrome.
- In hand, the median nerve provides the recurrent motor branch to the thumb, as well as digital sensory and motor branches.
- Clinical Correlation: Median Neuropathy
- Passes medial to the median nerve and brachial artery through the arm, then along the medial aspect of the ulna and into the hand.
- Posterior to the medial epicondyle of the humerus
- Anterior to the olecranon of the ulna (If you flex and extend your arm, you may be able to feel it pop in and out of the ulnar groove.),
- Through the cubital tunnel, which is the tendinous arch that joins the heads of flexor carpi ulnaris (FCU), which it runs deep to.
- The cubital tunnel is the most common ulnar nerve entrapment site.
- All of the motor and sensory components of the ulnar nerve lie distal to the cubital tunnel; therefore, in a cubital tunnel syndrome, all of the components of the ulnar nerve are affected.
- They can additionally be affected by medial epicondyle compression or supracondylar fracture (fracture of the distal humerus proximal to the epicondyles).
- Passes through Guyon's canal (aka Guyon's tunnel) at the wrist formed from the palmar carpal ligament, which runs transverse across the wrist.
- This is the other major ulnar nerve entrapment site.
- Variations of Guyon's canal entrapments exist affecting a variety of different distal ulnar components.
- In the simplest terms, the vast majority of Guyon canal entrapments only affect the deep branch, because the canal it is thickest between the hook of the hamate and the pisiform bone: the superficial sensory branch runs medial to this part of the canal.
- The ulnar nerve derives two ulnar cutaneous nerves:
- Dorsal, which passes posteriorly
- Palmar, which passes superficial to Guyon's canal.
- At the hand, it splits into the superficial sensory branch, which is a purely sensory nerve that passes medial to the hook of the hamate bone and the deep branch, which passes through Guyon's canal to innervate muscle groups across the hand.
- Both nerves run deep to flexor digitorum superficialis (FDS) within the proximal forearm.
- Clinical Correlation: Ulnar Neuropathy
- Wraps around the posterior humerus, down the lateral forearm and into the hand.
- Runs deep to the triceps muscle
- Where it wraps around the posterior mid-humerus is the spiral groove, an important radial nerve compression site.
- It passes anterior to the lateral epicondyle of the humerus.
- Distal to the lateral epicondyle, it divides into the:
- Deep branch, which transitions into the posterior interosseous nerve and and the superficial radial sensory branch.
- The posterior interosseous nerve passes through the arcade of Frohse, an arch within the supinator muscle, which is a key posterior interosseous nerve entrapment site.
- The superficial radial sensory branch, which runs deep to brachioradialis into the lateral hand.
- Clinical Correlation: Radial Neuropathy
Sensory Maps of the Hand
Median Nerve
- The median nerve digital sensory branches cover the lateral palm, lateral half of the ring finger, middle and index fingers, and the palmar thumb and covers the dorsal tips of these fingers
- The median palmar cutaneous nerve covers the palmar portion.
Ulnar Nerve
- The dorsal ulnar cutaneous nerve covers the medial one-third of the hand and the medial half of the fourth digit and fifth digit.
- The superficial sensory division covers the same region on the palmar surface, except where the palmar ulnar cutaneous nerve covers the hypothenar eminence (analogous to the median palmar cutaneous nerve).
Radial Nerve
- The superficial sensory radial nerve covers the remainder of the dorsum of the hand (the lateral two-thirds of the dorsum of the hand, proximal thumb, proximal second and third digits, and proximal lateral half of the fourth digit), and wraps around to the proximal palmar thumb.