Must Know
- Continuation of the posterior cord, roots C5 to T1, largest branch of the plexus.
- Lies on the latissimus dorsi tendon, then leaves the axilla through the triangular interval.
- Branches to long and medial heads of triceps and the posterior cutaneous nerve of arm arise in the axilla.
- Spirals across the back of the humerus with the profunda brachii vessels in the radial groove.
- Lateral head of triceps arises above the groove and medial head below it.
- Pierces the lateral intermuscular septum to enter the anterior compartment.
- Runs between brachialis medially and brachioradialis then ECRL laterally.
- Supplies brachioradialis, ECRL and the lateral part of brachialis.
- Divides into superficial radial and posterior interosseous nerves at the level of the lateral epicondyle.
| Level | Branches |
|---|---|
| Axilla | Long head and medial head of triceps, posterior cutaneous nerve of arm |
| Radial groove | Lateral head, medial head and anconeus, lower lateral cutaneous nerve of arm, posterior cutaneous nerve of forearm |
| Anterior arm | Brachioradialis, ECRL, lateral brachialis, elbow joint |
| Cubital fossa (PIN) | ECRB and supinator before entering supinator |
More on the course and surface marking
- Surface line runs from where the posterior axillary wall meets the arm to two thirds along acromion to lateral epicondyle.
- From there it turns to the front of the lateral epicondyle.
- Superficial radial nerve runs under brachioradialis and lies lateral to the radial artery in the mid forearm.
- It passes deep to the brachioradialis tendon and can be rolled on the tense EPL tendon.
Injury
- Most often injured high, by humeral shaft fracture.
- Wrist drop with loss of wrist and MCP extension.
- IP joints still extend through the interossei and lumbricals.
- Triceps is spared in shaft fractures because its long and medial head branches leave in the axilla.
- Sensory loss is often limited to a patch over the first dorsal interosseous.
- Crutch palsy and Saturday night palsy compress the nerve in the axilla or arm and are usually transient.
Surgical Exposure
- Posteriorly, open the interval between long and lateral heads of triceps.
- The nerve crosses the upper medial head before entering the radial groove.
- At the elbow, retract brachioradialis and ECRL laterally to see the bifurcation.
- Incise superficial supinator to follow the deep branch.
- When stripping the distal lateral humeral shaft, protect the nerve.
Numbers to Remember
- Roots. C5 to T1.
- Surface marking. Two thirds along acromion to lateral epicondyle line.
Memory Aids
- Triceps escapes. Long and medial head branches leave before the groove.
Exam Traps
- The nerve pierces the lateral intermuscular septum, not the medial.
- It divides at the lateral epicondyle, not in the mid forearm.
- Elbow extension usually survives a humeral shaft fracture palsy.
Source
Sinnatamby CS. Last’s Anatomy. Regional and Applied. 12th ed. Edinburgh, Churchill Livingstone Elsevier, 2011. Chapter 2, Upper limb.
Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 17 October 2026.