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  • 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

Figure 131-2. Brachial Plexus and Muscle Innervation

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