Step 1 · Anatomy & Embryology

Brachial Plexus & Upper Limb

Name: Brachial Plexus & Upper Limb
Creator: Veli Bakalov, MD

From roots to claw hands: nerve lesions you can localize

Anatomy & Embryology·Limbs & Back·5 min read·9 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Brachial plexus (C5–T1): Roots → Trunks (upper C5–C6, middle C7, lower C8–T1) → Divisions → Cords (lateral, posterior, medial around the axillary artery) → Branches.
Erb palsy = upper trunk C5–C6 from lateral neck traction ("waiter's tip"); Klumpke = lower trunk C8–T1 from upward arm traction (total claw ± Horner).
Axillary nerve (C5–C6): surgical neck fracture or anterior dislocation → weak deltoid and teres minor, numb lateral shoulder.
Radial nerve: midshaft humerus (spiral groove) fracture → wrist drop with triceps spared; axillary compression also weakens triceps.
Median nerve: supracondylar fracture → hand of benediction and ape hand; carpal tunnel → thenar atrophy and numb palmar lateral 3.5 digits (thenar skin spared).
Ulnar nerve: medial epicondyle or hook of hamate → ulnar claw, positive Froment sign (weak adductor pollicis), numb medial 1.5 digits.
Ulnar paradox: distal ulnar lesions claw more because the medial FDP is spared.
Rotator cuff = SITS; supraspinatus (suprascapular nerve) initiates abduction 0–15° and is most often torn; deltoid takes 15–90°.
Winged scapula = long thoracic nerve (C5–C7) → serratus anterior, classically injured in axillary node dissection or mastectomy.

Plexus organization

Five levels. The brachial plexus is built from the ventral rami of C5–T1 and reorganizes itself five times on the way to the arm ("Randy Travis Drinks Cold Beer"):

Roots (C5–T1)Ventral rami; the long thoracic nerve (C5–C7) leaves here.
TrunksUpper C5–C6 (gives the suprascapular nerve), middle C7, lower C8–T1.
DivisionsEach trunk splits into anterior and posterior divisions.
CordsLateral, posterior, and medial, named for their position around the axillary artery.
BranchesThe five terminal nerves.

Terminal nerves. The lateral cord gives the musculocutaneous nerve (C5–C7; biceps, brachialis, sensation of the lateral forearm). The posterior cord gives the axillary (C5–C6) and radial (C5–T1) nerves. The median nerve (C5–T1) is formed by contributions from both the lateral and medial cords, and the medial cord continues as the ulnar nerve (C8–T1). Because the posterior cord carries the extensor supply, it is the one to think of when both deltoid and extensors fail.

Trunk injuries: Erb and Klumpke

Erb vs Klumpke palsy

Feature	Erb-Duchenne	Klumpke
Level	Upper trunk, C5–C6	Lower trunk, C8–T1
Mechanism	Lateral neck traction that widens the neck–shoulder angle: shoulder dystocia, a motorcycle fall onto the shoulder	Upward traction on the arm: grabbing a branch during a fall, breech delivery
Muscles lost	Deltoid, supraspinatus, infraspinatus, biceps, brachialis	Intrinsic hand muscles, including all lumbricals
Posture	"Waiter's tip": arm adducted, internally rotated, extended at the elbow, pronated	Total claw hand
Extra sign	—	Horner syndrome if T1 (sympathetic outflow) is involved

Reading the waiter's tip. Each part of the posture is a lost muscle: the arm hangs adducted without deltoid and supraspinatus, rotates internally without infraspinatus, and stays extended and pronated without biceps (a flexor and the main supinator) and brachialis.

Klumpke look-alikes. A Pancoast (apical lung) tumor compresses C8–T1 and produces Klumpke-type hand findings plus Horner syndrome (ptosis, miosis, anhidrosis). A cervical rib causing thoracic outlet syndrome also targets the lower trunk, with ulnar-sided symptoms and intrinsic wasting.

Terminal nerve lesions: from injury to deficit

Classic upper-limb nerve injuries

Nerve	Classic injury	Motor finding	Sensory loss
Axillary (C5–C6)	Surgical neck fracture, anterior shoulder dislocation	Deltoid (abduction 15–90°) and teres minor weak	Lateral shoulder ("regimental badge" or sergeant's patch)
Radial (C5–T1)	Midshaft humerus fracture (spiral groove), Saturday night palsy, crutches	Wrist drop; weak BEST muscles	Posterior arm and forearm, dorsal hand
Median (C5–T1)	Supracondylar fracture (proximal), carpal tunnel (distal)	Hand of benediction, ape hand, thenar atrophy	Palmar lateral 3.5 digits
Ulnar (C8–T1)	Medial epicondyle (proximal), hook of hamate (distal)	Ulnar claw, Froment sign	Medial 1.5 digits
Musculocutaneous (C5–C7)	Upper trunk injury	Weak elbow flexion and supination	Lateral forearm
Long thoracic (C5–C7)	Axillary node dissection, mastectomy, stab wound	Winged scapula	None

Radial nerve, the great extensor nerve. Its BEST muscles are Brachioradialis, Extensors (wrist and fingers), Supinator, and Triceps. Sleeping with the arm draped over a chair or bench ("Saturday night palsy") presses the nerve against the humerus; review sources sometimes list it with axillary compression, as from crutches. Location decides the triceps: branches to the triceps leave proximal to the spiral groove, so a midshaft fracture causes wrist drop with normal elbow extension, whereas an axillary lesion weakens the triceps too.

The hand: median, ulnar, and the tunnels

Median nerve. A proximal lesion (supracondylar humerus fracture) loses the long flexors of the lateral digits, so attempting a fist leaves the index and middle fingers extended ("hand of benediction"), plus a thenar "ape hand" from loss of opposition. A distal lesion (carpal tunnel) spares the forearm flexors and gives thenar atrophy with numbness of the palmar first 3.5 digits. The median-supplied hand muscles are LOAF: Lateral 2 lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis.

Ulnar nerve. Injured proximally at the medial epicondyle ("funny bone") or distally at the hook of hamate (cyclists, falls onto the hypothenar eminence). Weak interossei and medial 2 lumbricals leave the 4th and 5th digits hyperextended at the MCP and flexed at the IP joints (ulnar claw). A positive Froment sign reflects a weak adductor pollicis: the patient flexes the thumb IP joint with the median-supplied flexor pollicis longus to pinch paper. Proximal lesions also weaken flexor carpi ulnaris, so the wrist deviates radially on flexion.

Ulnar paradox: a distal (wrist) lesion claws more obviously than a proximal one. A proximal lesion also paralyzes the medial flexor digitorum profundus, so the 4th and 5th fingers cannot flex at the DIP joints and the claw is milder even though the injury is worse.

Compression tunnels.

Carpal tunnel (median): worsened by pregnancy, hypothyroidism, acromegaly, dialysis, and repetitive use; Phalen and Tinel signs are positive. Sensation over the thenar eminence is spared because the palmar cutaneous branch runs superficial to the tunnel.
Guyon canal (ulnar at the wrist): cyclists.
Cubital tunnel (ulnar at the medial epicondyle): the elbow site.

Shoulder: rotator cuff, abduction, and winging

Rotator cuff (SITS)

Muscle	Action	Nerve
Supraspinatus	Initiates abduction (0–15°); empty-can test; most commonly torn or impinged	Suprascapular
Infraspinatus	External rotation	Suprascapular
Teres minor	External rotation	Axillary
Subscapularis	Internal rotation	Upper and lower subscapular

Abduction sequence. Supraspinatus initiates abduction from 0–15°, the deltoid (axillary nerve) carries it from 15–90°, and above 90° the trapezius and serratus anterior rotate the scapula upward to reach 180°.

Winged scapula. The long thoracic nerve (C5–C7) supplies serratus anterior, which holds the scapula against the chest wall. Injury during axillary lymph node dissection or mastectomy, or from a stab wound, makes the medial scapula wing out and prevents abduction above the horizontal.