🔹 Brachial plexus – roots, trunks, cords
Formation: ventral rami of C5–T1. These roots merge into upper (C5–C6), middle (C7), and lower (C8–T1) trunks. Each trunk splits into anterior and posterior divisions, which form cords (lateral, posterior, medial) named by their relation to the axillary artery.
🔸 Major terminal nerves
- Musculocutaneous (C5–C6) — anterior arm
- Median (C5–T1) — anterior forearm & hand
- Ulnar (C8–T1) — medial hand & forearm
- Axillary (C5–C6) — shoulder
- Radial (C5–T1) — posterior arm & forearm
🔸 Functional division
- Anterior divisions → flexor muscles (median, ulnar, musculocutaneous)
- Posterior divisions → extensor muscles (axillary, radial)
- Proximal muscles: C5–C6; distal hand: C8–T1 (proximal–distal gradient).
💡 High yield: the brachial plexus gives off collateral nerves (dorsal scapular, long thoracic, suprascapular, subscapular, thoracodorsal, medial brachial/antebrachial cutaneous) that innervate shoulder girdle muscles and skin.
💪 Motor innervation – terminal & collateral
| Nerve | Key muscles | Primary action |
|---|---|---|
| Musculocutaneous | Biceps brachii, brachialis | Elbow flexion, supination |
| Median | Forearm flexors (except FCU & ulnar half FDP), thenar, lumbricals 1–2 | Wrist/digit flexion, thumb opposition, pronation |
| Ulnar | FCU, ulnar half FDP, hypothenar, interossei, lumbricals 3–4, adductor pollicis | Digit abduction/adduction, MP flexion, IP extension (4–5) |
| Axillary | Deltoid, teres minor | Shoulder abduction (15°–110°), lateral rotation |
| Radial | Triceps, brachioradialis, wrist/finger extensors, supinator | Elbow/wrist/MP extension, supination |
Collateral nerves
- Dorsal scapular → rhomboids
- Long thoracic → serratus anterior (protraction, upward rotation)
- Suprascapular → supraspinatus (0–15° abduction) & infraspinatus (lateral rotation)
- Subscapular (upper/lower) → subscapularis, teres major
- Thoracodorsal → latissimus dorsi
- Medial pectoral → pectoralis major & minor
Segmental innervation
- C5–C6: shoulder muscles (deltoid, rotator cuff, biceps)
- C6–C7: distal arm & proximal forearm
- C7–C8: distal forearm muscles
- C8–T1: intrinsic hand muscles
🖐️ Sensory innervation (hand & forearm)
- Median nerve: palmar surface of lateral 3½ digits, thenar eminence, lateral palm (palmar cutaneous branch superficial to carpal tunnel → spared in carpal tunnel syndrome).
- Ulnar nerve: medial 1½ digits, hypothenar eminence, medial palm.
- Radial nerve: dorsum of hand over first dorsal web space, anatomic snuffbox, dorsal thumb.
🔬 Clinical pearl: The palmar cutaneous branch of the median nerve does NOT pass through the carpal tunnel, so palmar skin over the thenar eminence is often spared in carpal tunnel syndrome.
⚠️ Brachial plexus lesions
Erb-Duchenne (C5–C6) · waiter's tip
- Mechanism: head & shoulder forced apart (birth, trauma, disk herniation).
- Affected: axillary, suprascapular, musculocutaneous nerves.
- Posture: arm adducted & medially rotated (loss of abductors/lateral rotators), forearm extended & pronated (loss of biceps/brachialis).
- Sensory: lateral forearm to thumb (musculocutaneous).
Klumpke (C8–T1) · claw/ape hand
- Mechanism: forceful arm abduction (fall, thoracic outlet, birth).
- Affected: ulnar nerve ± median hand muscles.
- Signs: claw hand (ulnar) + ape hand (median) — intrinsic hand weakness, Horner syndrome possible.
- Sensory: medial forearm & medial 1½ digits.
📌 Exam key: C5–C6 lesions affect proximal muscles (shoulder, elbow); C8–T1 affect distal hand muscles. Reflex: biceps (C5–C6) lost in upper lesion.
🩺 Branch lesions (radial, median, ulnar, others)
| Nerve | Common injury site | Motor loss | Sensory loss | Classic sign |
|---|---|---|---|---|
| Radial | Axilla (crutch), mid‑shaft humerus | Elbow/wrist/MP extension, supination | Posterior arm/forearm, dorsal thumb | Wrist drop |
| Median | Elbow (supracondylar fracture) | Wrist flexion (ulnar deviation), pronation, thumb opposition, lateral digit flexion | Palmar lateral 3½ digits | Hand of benediction, ape hand |
| Median | Wrist (carpal tunnel) | Thenar atrophy, thumb opposition, lumbricals 1–2 | Palmar 3½ digits (lateral palm spared) | Ape hand (no benediction) |
| Ulnar | Elbow, wrist, hook of hamate | Hypothenar, interossei, lumbricals 3–4, adductor pollicis | Medial 1½ digits | Claw hand (more clawing with wrist lesion) |
| Axillary | Surgical neck humerus | Shoulder abduction (to horizon) | Lateral arm (deltoid) | — |
| Musculocutaneous | Rare | Elbow flexion, supination | Lateral forearm | — |
| Long thoracic | Mastectomy, lateral chest wound | Serratus anterior (abduction above horizon) | — | Winged scapula |
| Suprascapular | — | 0–15° abduction (supraspinatus), lateral rotation (infraspinatus) | — | — |
🔹 Remember: Proximal lesions produce more signs. Sensory deficits often precede motor weakness. "Wrist drop" = radial nerve; "claw hand" = ulnar; "ape hand" = median (thenar atrophy).
🩸 Arterial supply & anastomoses
Subclavian
→
Axillary
→
Brachial
→
Radial / Ulnar
→
Palmar arches
- Axillary artery (1st rib to teres major) — branches: lateral thoracic, subscapular, posterior humeral circumflex (with axillary nerve at surgical neck).
- Brachial artery — profunda brachii (with radial nerve in radial groove).
- Radial artery → deep palmar arch; Ulnar artery → superficial palmar arch.
- Collateral circulation: shoulder (suprascapular ↔ subscapular), hand (palmar arches).
🏥 Fracture correlates: surgical neck humerus → axillary nerve + posterior humeral circumflex artery. Mid‑shaft humerus → radial nerve + profunda brachii.
📐 Carpal tunnel & syndrome
Boundaries: flexor retinaculum (anterior), carpal bones (posterior).
Contents: 9 tendons (FDS ×4, FDP ×4, FPL ×1), median nerve. No vessels or ulnar/radial nerve branches.
Carpal tunnel syndrome
- Compression of median nerve due to reduced space.
- Motor: thenar atrophy, weak thumb opposition (ape hand).
- Sensory: numbness in palmar 3½ digits; lateral palm spared (palmar cutaneous branch superficial).
Other carpal injuries
- Scaphoid fracture — most common; risk of avascular necrosis (proximal pole).
- Lunate dislocation — most commonly dislocated; may compress median nerve.
- Hook of hamate fracture — may injure ulnar nerve.
🧪 USMLE pearl: In carpal tunnel syndrome, sensory loss spares the thenar eminence. Thenar atrophy + ape hand = median nerve at wrist.
🔄 Rotator cuff & shoulder
SITS muscles: Supraspinatus, Infraspinatus, Teres minor, Subscapularis. Tendons reinforce glenohumeral joint.
- Supraspinatus — most commonly torn; pain during abduction.
- Rotator cuff tear — chronic use or fall with abducted arm.
- Humeral head dislocation — most often inferior; may injure axillary or radial nerve.
- Surgical neck fracture — axillary nerve & posterior humeral circumflex artery at risk.
📌 Imaging correlate: AP shoulder (external rotation) and AP elbow views are commonly used; note that the scaphoid is the most frequently fractured carpal bone.
⚖️ Quick comparison – root vs branch lesions
| Lesion | Affected roots/nerves | Motor pattern | Sensory |
|---|---|---|---|
| Erb-Duchenne | C5–C6 (upper trunk) | Shoulder adducted/medially rotated, elbow extended/pronated | Lateral arm/forearm |
| Klumpke | C8–T1 (lower trunk) | Intrinsic hand weakness (claw + ape) | Medial arm/forearm, medial 1½ digits |
| Radial nerve (axilla) | C5–T1 | Wrist drop, loss of elbow/wrist/MP extension | Posterior arm, forearm, dorsal thumb |
| Median nerve (wrist) | C6–T1 | Ape hand, thenar atrophy, lumbricals 1–2 | Palmar 3½ digits (thenar spared) |
| Ulnar nerve (elbow) | C8–T1 | Claw hand (4–5), weak grip, loss of adduction/abduction | Medial 1½ digits |
“Wrist drop” = radial · “Benediction” = median at elbow · “Claw” = ulnar · “Ape” = median (thenar).
- Surgical neck humerus → axillary nerve + posterior humeral circumflex artery.
- Mid‑shaft humerus (radial groove) → radial nerve + profunda brachii.
- Supracondylar humerus → median nerve (at elbow).
- Medial epicondyle → ulnar nerve.
- Hook of hamate → ulnar nerve.
- Scaphoid fracture → risk of avascular necrosis (proximal pole).
- Lunate dislocation → may compress median nerve.