Chapter 131. Brachial Plexus

Denis Jochum, MD

Two independent layers from the roots to terminal branches:
- A simple and constant dorsal layer: extensor and supinator muscles
- A complex and variable ventral layer: flexor and pronator muscles:
  - This explains the variations and the relationships between the median, musculocutaneous, and ulnar nerves
The brachial plexus can be:
- “Prefixed” with a contribution from C4 (two thirds of cases), or
- Normal, or
- “Postfixed,” with the participation of T2
- Or even spread in both directions:
  - Clinical significance of these variations because motor and sensory distributions are modified
Origin of the trunks:
- Upper trunk: confluence of the ventral branches of C5 and C6 (C4 participation, prefixed plexus)
- Middle trunk: ventral division of C7
- Lower trunk: confluence of the ventral branches of C8 and T1
Constitution of the cords (most common configuration):
- Lateral cord: confluence of ventral divisions of upper and middle trunks (80% of cases)
- Medial cord: ventral division of lower trunk (95% of cases)
- Posterior cord: confluence of dorsal divisions of the three trunks (70% of cases)
Constitution of terminal nerves = branches (at distal edge of the pectoralis minor muscle):
- Posterior cord: posterior plane of the brachial plexus, with mainly radial and axillary nerves
- Lateral and medial cords: anterior plane of the brachial plexus with mainly the median nerve, as well as musculocutaneous and ulnar nerves