Brachial Plexus — Roots, Trunks, Cords, Branches

Brachial plexus consists of Roots (C5–T1), Trunks (upper, middle, lower), Divisions (each trunk splits into anterior/posterior), Cords (lateral, posterior, medial), and terminal Branches (musculocutaneous, axillary, radial, median, ulnar). Anatomical course: roots emerge between scalene muscles; trunks in posterior triangle; divisions under clavicle; cords encircle axillary artery and give off named nerves (e.g. lateral cord → musculocutaneous). Key relationships: the long thoracic nerve (C5-7) arises from roots (winged scapula if injured); axillary nerve from posterior cord (risk in shoulder dislocation); radial nerve from posterior cord (mid-shaft humerus fracture → wrist drop). Injury patterns: Upper plexus (Erb’s palsy, C5-6) causes arm adducted/internally rotated ('waiter’s tip'); Lower plexus (Klumpke’s, C8-T1) causes hand paralysis and Horner syndrome if sympathetic chain involved. Examination: look for motor deficits by peripheral nerve distribution (e.g. loss of shoulder abduction suggests axillary nerve/C5 injury) and sensory deficits (e.g. lateral forearm numbness indicates musculocutaneous nerve). Imaging: MRI or nerve conduction studies help localize root avulsions vs stretch injuries. Management ranges from physical therapy for neuropraxia to nerve grafts/transfers for severe root avulsions.