The nerves of the arm come from the brachial plexus, formed by the C5 to T1 roots in the neck and armpit. Five main nerves leave it, and each has a classic injury pattern.
Brachial plexus
Roots join into trunks, divisions and cords that pass under the clavicle into the armpit. Traction on the arm at birth or in a fall can stretch the plexus and cause an upper (Erb) or lower (Klumpke) palsy.
Radial nerve
The radial nerve winds around the back of the humerus and extends the elbow, wrist and fingers. A midshaft humerus fracture or pressure in the armpit can cause wrist drop.
Median nerve
The median nerve runs through the carpal tunnel and controls thumb opposition and much of the grip. Carpal tunnel syndrome causes numbness in the thumb, index and middle fingers, often worse at night.
Ulnar nerve
The ulnar nerve passes behind the medial epicondyle, the funny bone, and runs the small muscles of the hand. Damage weakens finger spreading and causes numbness in the little finger and half of the ring finger.
Axillary and musculocutaneous nerves
The axillary nerve supplies the deltoid and can be injured by a shoulder dislocation. The musculocutaneous nerve bends the elbow through the biceps.
| Nerve | Motor test | Sensory test |
|---|---|---|
| Radial | Extend the wrist and fingers | Back of the hand at the first web space |
| Median | Touch thumb to little finger, OK sign | Tip of the index finger |
| Ulnar | Spread the fingers apart | Tip of the little finger |
| Axillary | Lift the arm out to the side | Skin over the lateral deltoid |
| Musculocutaneous | Bend the elbow | Lateral forearm |
At the bedside
Check and document these nerves before and after any splint, reduction or cast on the arm. A new deficit after treatment needs prompt review.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).