At which anatomical location does the radial nerve emerge from the axilla to enter the radial groove of the humerus?

Explanation

## Radial Nerve Course in the Upper Arm **Key Point:** The radial nerve emerges from the axilla posterior to the axillary artery and enters the radial groove (spiral groove) of the humerus by passing between the medial and lateral heads of the triceps brachii. ### Anatomical Sequence 1. Radial nerve arises from the posterior cord of the brachial plexus (C5–C8, T1) 2. Descends posteriorly in the axilla 3. Passes between the medial and lateral heads of triceps 4. Enters the radial groove of the humerus 5. Spirals around the posterior and lateral aspects of the shaft 6. Emerges in the anterior compartment between brachialis and brachioradialis (above the elbow) ### Clinical Significance **High-Yield:** The radial nerve is vulnerable to injury in the radial groove during mid-shaft humeral fractures. This is the most common site of radial nerve injury in the upper arm. **Clinical Pearl:** Compression in the radial groove ("Saturday night palsy") occurs when prolonged pressure is applied to the posterior arm, classically from sleeping with the arm draped over a chair or partner's shoulder. ### Branches in the Upper Arm - **Posterior cutaneous nerve of arm** — sensory - **Branches to triceps** — motor (medial, lateral, long heads) - **Branch to anconeus** — motor - **Posterior cutaneous nerve of forearm** — sensory