At what level does the radial nerve arise from the posterior cord of the brachial plexus?

Explanation

## Origin and Initial Course of the Radial Nerve **Key Point:** The radial nerve is the largest terminal branch of the posterior cord of the brachial plexus, arising in the lower third of the axilla, posterior to the axillary artery. ### Anatomical Course The radial nerve: 1. Arises from the posterior cord (C5–C8, T1) 2. Emerges in the **lower axilla**, posterior to the axillary artery 3. Descends behind the axillary and proximal humeral artery 4. Enters the radial groove (spiral groove) on the posterior surface of the humerus 5. Passes through the radial groove with the deep brachial artery 6. Emerges in the anterior arm between the brachialis and brachioradialis muscles 7. Divides into superficial and deep branches at the elbow **High-Yield:** The radial nerve's relationship to the **radial groove** is clinically critical — fractures of the mid-shaft humerus can injure the nerve as it traverses this groove. **Clinical Pearl:** The radial nerve is the **most commonly injured nerve** in brachial plexus injuries and humeral shaft fractures because of its intimate anatomical relationship with the humerus. ### Mnemonic: **RADS** - **R**adial nerve arises from **R**ear (posterior cord) - **A**xilla (lower third) - **D**escends in radial groove - **S**pirals around humerus