Radial Nerve — Course and Relations in the Arm
Overview of Course
The radial nerve has a characteristic winding course through the arm, passing from the axilla → posterior compartment (radial groove) → anterior compartment → cubital fossa.
Segment 1: Upper Part of the Arm (Posterior to Brachial Artery)
After arising from the posterior cord in the axilla, the radial nerve:
- Lies posterior to the brachial artery in the upper arm
- Passes posterolaterally with the profunda brachii artery
- Exits the anterior compartment through the lower triangular space (between teres major above, long head of triceps medially, and humerus laterally)
Segment 2: Radial (Spiral) Groove
The radial nerve winds around the posterior surface of the humerus in the radial (spiral/musculospiral) groove:
- Passes between the lateral head of triceps (above) and medial head of triceps (below)
- Accompanied by the profunda brachii artery
- Runs obliquely downwards and laterally from medial to lateral side of the humerus
Relation to humerus: The nerve lies directly on the bone in the radial groove — this explains its vulnerability in mid-shaft humeral fractures.
Segment 3: Lateral Side of Arm (Below Radial Groove)
About 5 cm above the lateral epicondyle (at the level of the deltoid tuberosity), the nerve:
- Pierces the lateral intermuscular septum
- Enters the anterior compartment of the arm
Segment 4: Anterior Compartment / Cubital Fossa
In the lower arm and cubital fossa:
- Runs between brachialis (medially) and brachioradialis + extensor carpi radialis longus (laterally)
- Lies deep in the lateral part of the cubital fossa
Order of structures in cubital fossa (medial to lateral): Median nerve → Brachial artery → Biceps tendon → Radial nerve
Mnemonic: MBBR — My Bottom Bleeds Red
Termination
The radial nerve terminates just below the level of the lateral epicondyle of the humerus (in the cubital fossa) by dividing into:
- Superficial branch (superficial terminal branch) — sensory continuation
- Deep branch (deep terminal branch = posterior interosseous nerve) — motor branch to back of forearm
Summary of Course
| Segment | Location | Key Companion |
|---|---|---|
| Axilla | Posterior to axillary artery | Profunda brachii artery |
| Upper arm | Posterior to brachial artery | Profunda brachii artery |
| Radial groove | Between heads of triceps on back of humerus | Profunda brachii artery |
| Lower arm | Pierces lateral intermuscular septum | Radial collateral artery |
| Anterior compartment | Between brachialis and brachioradialis | — |
| Cubital fossa | Deep, lateral to biceps tendon | — |