All of the following statements regarding the brachial plexus are correct EXCEPT:

Explanation

## Brachial Plexus Organization ### Nerve Root to Trunk Formation **Key Point:** The brachial plexus arises from nerve roots C5, C6, C7, C8, and T1, which form three trunks: - **Upper trunk**: C5 + C6 - **Middle trunk**: C7 alone - **Lower trunk**: C8 + T1 ### Trunk to Cord Anatomy Each trunk divides into anterior and posterior divisions. Three cords form from these divisions: - **Lateral cord**: anterior divisions of upper and middle trunks - **Medial cord**: anterior division of lower trunk - **Posterior cord**: all three posterior divisions (upper, middle, lower) ### Terminal Branches and Their Origins | Terminal Branch | Origin | Correct? | |---|---|---| | Radial nerve | Posterior cord | ✓ | | Median nerve | Lateral + Medial cords | ✓ | | Ulnar nerve | Medial cord | — | | Musculocutaneous nerve | Lateral cord | — | | Axillary nerve | **Posterior cord** | ✗ | **High-Yield:** The axillary nerve arises from the **posterior cord**, NOT from the anterior division of the upper trunk. This is a common source of confusion. ### Why the Distractor Is Planted The axillary nerve does branch early in the plexus course, but it originates from the posterior cord after it has been formed from the three posterior divisions. It is NOT a direct branch of any trunk division. **Clinical Pearl:** Axillary nerve injury (e.g., from shoulder dislocation or axillary artery puncture) causes loss of shoulder abduction (deltoid paralysis) and loss of sensation over the lateral shoulder (regimental badge area).