C5–T1 ventral rami form roots → trunks → divisions → cords → terminal branches. Lesion level predicts the deficit exactly.
Why this is true
The plexus exists because the upper limb bud drags spinal segments with it during development, then rotates. Fibres from several roots must be re-sorted so that one nerve can supply one functional muscle group rather than one embryonic segment. That re-sorting is what the trunks, divisions and cords accomplish: anterior divisions carry flexor-compartment fibres, posterior divisions carry extensor-compartment fibres. This is why the posterior cord — built purely from posterior divisions — supplies every extensor of the arm through the radial and axillary nerves, and why a posterior cord lesion never weakens flexion.
Key points
Nerve lesion → deficit map
| Nerve | Motor loss | Sensory loss | Classic cause |
|---|---|---|---|
| Axillary | Abduction 15–90° | Regimental badge patch | Surgical neck fracture |
| Radial | Wrist and finger extension (wrist drop) | First dorsal web space | Midshaft humerus / 'Saturday night' |
| Median | Thumb opposition, LOAF muscles | Lateral 3½ digits, palmar | Carpal tunnel, supracondylar fracture |
| Ulnar | Interossei, adductor pollicis (claw) | Medial 1½ digits | Medial epicondyle, Guyon's canal |
| Musculocutaneous | Elbow flexion and supination | Lateral forearm | Direct axillary trauma |
Common traps
- Ulnar claw is *worse* with a distal (wrist) lesion — the intact flexor digitorum profundus still flexes the fingers. This is the ulnar paradox.
- A supracondylar fracture threatens the median nerve and brachial artery; a midshaft fracture threatens the radial nerve. Don't swap them.
Mnemonic
Randy Travis Drinks Cold Beer — Roots, Trunks, Divisions, Cords, Branches.
Clinical case
A 34-year-old falls from a motorbike and lands on his shoulder. His right arm hangs adducted and internally rotated with the forearm pronated. Sensation is reduced over the lateral arm; hand grip is normal.
High-yield
Winged scapula with medial border lift = long thoracic nerve. Add Horner's and the lesion is lower trunk / T1.