Upper Limb Anatomy and Brachial Plexus

Major Upper Limb Anatomy

Organizational Principles and Surface Landmarks

The upper limb is specialized for mobility and precise manipulation rather than weight bearing. Anatomically, it is divided into the pectoral girdle (clavicle and scapula), arm (humerus), forearm (radius and ulna), and hand (carpals, metacarpals, phalanges). A key Step 1 principle is that muscle compartments are organized by shared action, innervation, and blood supply. The upper limb also contains clinically important fibro-osseous tunnels and spaces where nerves, vessels, or tendons can be compressed.

Bones, Joints, and High-Yield Injuries

The clavicle is the most commonly fractured bone in children; fractures classically occur at the junction of the middle and lateral thirds. The medial fragment is pulled superiorly by the sternocleidomastoid, while the lateral fragment is pulled inferiorly by the weight of the upper limb. The scapula provides the glenoid cavity for the humeral head and attachment sites for rotator cuff muscles. The humerus has several nerve-related fracture sites: surgical neck, radial groove, and medial epicondyle.

Site	Associated Structure	Classic Deficit
Surgical neck of humerus	Axillary nerve, posterior circumflex humeral artery	Deltoid weakness, impaired abduction from 15–90°, sensory loss over lateral shoulder
Midshaft humerus/radial groove	Radial nerve, deep brachial artery	Wrist drop, loss of wrist/finger extension, dorsal hand sensory loss
Medial epicondyle	Ulnar nerve	Medial hand sensory loss, intrinsic hand weakness, clawing of digits 4–5
Scaphoid fracture	Retrograde blood supply from radial artery branches	Anatomic snuffbox tenderness; risk of avascular necrosis of proximal fragment

The glenohumeral joint is a ball-and-socket synovial joint with high mobility and low intrinsic stability. It is stabilized by the glenoid labrum, capsule, glenohumeral ligaments, and rotator cuff. The shoulder most commonly dislocates anteriorly and inferiorly, risking axillary nerve injury. The elbow joint includes humeroulnar and humeroradial articulations for flexion-extension and the proximal radioulnar joint for pronation-supination. The wrist primarily involves the distal radius and proximal carpal row; the ulna is separated from carpals by the triangular fibrocartilage complex.

Muscle Compartments and Actions

The arm has two major compartments. The anterior compartment flexes the elbow and is supplied by the musculocutaneous nerve: biceps brachii, brachialis, and coracobrachialis. The posterior compartment extends the elbow and is supplied by the radial nerve: triceps brachii and anconeus. In the forearm, the anterior compartment generally flexes the wrist and digits and pronates the forearm, while the posterior compartment extends the wrist and digits and supinates the forearm.

Region	Major Muscles	Primary Actions	Dominant Innervation
Rotator cuff	Supraspinatus, infraspinatus, teres minor, subscapularis	Stabilize humeral head; initiate abduction, lateral rotation, medial rotation	Suprascapular, axillary, upper/lower subscapular nerves
Anterior arm	Biceps brachii, brachialis	Elbow flexion; biceps also supinates	Musculocutaneous nerve
Posterior arm	Triceps brachii	Elbow extension	Radial nerve
Anterior forearm	Pronator teres, FCR, palmaris longus, FCU, FDS, FDP, FPL, pronator quadratus	Flexion of wrist/digits; pronation	Median nerve except FCU and medial FDP by ulnar nerve
Posterior forearm	ECRL/B, ED, EDM, ECU, supinator, APL, EPL, EPB, EI	Extension of wrist/digits; supination	Radial nerve/deep radial/posterior interosseous nerve

Hand Anatomy and Functional Compartments

The hand contains extrinsic tendons from forearm muscles and intrinsic muscles that refine grip and dexterity. The thenar eminence controls thumb opposition, abduction, and flexion; most thenar muscles are innervated by the recurrent branch of the median nerve. The hypothenar eminence moves the little finger and is ulnar-innervated. The lumbricals flex metacarpophalangeal joints and extend interphalangeal joints; lateral two are median-innervated, medial two are ulnar-innervated. The interossei are ulnar-innervated: palmar interossei adduct digits (PAD), dorsal interossei abduct digits (DAB).

Arterial, Venous, and Lymphatic Anatomy

The subclavian artery becomes the axillary artery at the lateral border of the first rib and the brachial artery at the inferior border of teres major. The brachial artery bifurcates in the cubital fossa into the radial and ulnar arteries. In the hand, the ulnar artery primarily forms the superficial palmar arch, while the radial artery primarily forms the deep palmar arch. Collateral circulation around the scapula and elbow helps maintain perfusion after gradual arterial occlusion but may be inadequate after acute trauma.

Superficial veins are clinically important: the cephalic vein runs laterally and drains into the axillary vein; the basilic vein runs medially and joins brachial veins to form the axillary vein. The median cubital vein connects them in the cubital fossa and is commonly used for venipuncture. Lymph from the upper limb drains mainly to axillary lymph nodes; most drainage passes to humeral nodes, then central and apical nodes.

Clinically Important Spaces and Tunnels

• Axilla: pyramidal space transmitting major neurovascular structures from neck to upper limb; bounded by pectoral muscles anteriorly, scapular/subscapular region posteriorly, thoracic wall medially, and humerus laterally.
• Cubital fossa: contains, from lateral to medial, biceps tendon, brachial artery, and median nerve; remembered as TAN when arranged medial-to-lateral as tendon, artery, nerve in some clinical descriptions.
• Carpal tunnel: bounded by carpal bones and flexor retinaculum; contains median nerve and flexor tendons. Ulnar nerve and artery pass superficial to the flexor retinaculum through Guyon canal.
• Anatomic snuffbox: bordered by EPL medially and APL/EPB laterally; contains radial artery and overlies the scaphoid.

Brachial Plexus

The brachial plexus is the somatic nerve network that supplies motor and sensory innervation to the upper limb. It is formed by the ventral rami of spinal nerves C5–T1, with variable contributions from C4 or T2. It passes between the anterior and middle scalene muscles, traverses the posterior triangle of the neck, enters the axilla deep to the clavicle, and becomes organized around the axillary artery. Step 1 questions often test the ability to localize a lesion by matching a motor deficit, sensory loss, reflex change, and mechanism of injury to a specific plexus level or terminal branch.

Organization: Roots, Trunks, Divisions, Cords, Branches

The plexus is classically organized as roots → trunks → divisions → cords → terminal branches. A useful structural principle is that ventral divisions generally supply anterior/flexor compartments, whereas dorsal divisions generally supply posterior/extensor compartments.

Level	Components	High-yield anatomy
Roots	C5, C6, C7, C8, T1 ventral rami	Give off dorsal scapular, long thoracic, and contributions to phrenic nerve regionally; pass between anterior and middle scalene muscles.
Trunks	Upper: C5–C6; Middle: C7; Lower: C8–T1	Upper trunk injury causes Erb-Duchenne palsy; lower trunk injury causes Klumpke palsy.
Divisions	Anterior and posterior divisions of each trunk	Anterior divisions supply flexor compartments; posterior divisions supply extensor compartments.
Cords	Lateral, posterior, medial	Named by position relative to the second part of the axillary artery.
Terminal branches	Musculocutaneous, axillary, radial, median, ulnar	Major named nerves tested with characteristic motor and sensory deficits.

Branches and Core Functions

The posterior cord is formed from all posterior divisions and gives rise to the axillary nerve and radial nerve, both serving extensor-related functions. The lateral cord derives from anterior divisions of the upper and middle trunks and contributes to the musculocutaneous nerve and lateral root of the median nerve. The medial cord derives from the anterior division of the lower trunk and gives the ulnar nerve and medial root of the median nerve.

Nerve	Roots	Major motor function	Key sensory territory	Classic lesion
Musculocutaneous	C5–C7	Arm flexion: biceps brachii, brachialis, coracobrachialis	Lateral forearm via lateral antebrachial cutaneous nerve	Weak elbow flexion and supination
Axillary	C5–C6	Deltoid and teres minor; arm abduction from 15° to 90°	“Regimental badge” area over lateral shoulder	Surgical neck fracture of humerus or anterior shoulder dislocation
Radial	C5–T1	Wrist, finger, and elbow extension	Posterior arm/forearm and dorsal hand, especially dorsal first web space	Wrist drop after midshaft humeral fracture or compression in radial groove
Median	C6–T1	Forearm pronation, wrist flexion, thenar muscles, lateral lumbricals	Palmar lateral 3.5 digits and fingertips dorsally	Carpal tunnel syndrome; ape hand; benediction sign with proximal lesion
Ulnar	C8–T1	Interossei, medial lumbricals, adductor pollicis, hypothenar muscles, FCU, medial FDP	Medial 1.5 digits	Claw hand; cubital tunnel or medial epicondyle injury

Important Nonterminal Branches

Several proximal branches are frequently tested because their injuries produce distinctive scapular or shoulder findings. The long thoracic nerve arises from roots C5–C7 and innervates the serratus anterior, which protracts and upwardly rotates the scapula. Injury, classically during axillary lymph node dissection, trauma, or repetitive overhead activity, causes winged scapula and difficulty abducting the arm above 90° because the glenoid cannot rotate superiorly. The dorsal scapular nerve arises from C5 and innervates the rhomboids and levator scapulae, contributing to scapular retraction and elevation. The suprascapular nerve from the upper trunk C5–C6 innervates supraspinatus and infraspinatus, affecting initiation of abduction and external rotation.

Classic Brachial Plexus Lesions

Erb-Duchenne palsy results from injury to the upper trunk C5–C6, often due to excessive lateral traction on the neck, such as shoulder dystocia during delivery or a fall causing increased angle between neck and shoulder. The affected limb assumes the “waiter’s tip” posture: arm adducted and medially rotated, elbow extended, forearm pronated. This reflects weakness of deltoid, supraspinatus, infraspinatus, biceps, and brachialis.

Klumpke palsy results from injury to the lower trunk C8–T1, typically from sudden upward traction on the arm, such as grabbing an object during a fall. It causes intrinsic hand muscle weakness and a “claw hand” appearance. If T1 sympathetic fibers are involved, it may be associated with Horner syndrome: ptosis, miosis, and anhidrosis due to disruption of sympathetic outflow.

Lesion	Plexus level	Mechanism	Deficit
Erb-Duchenne palsy	Upper trunk C5–C6	Excess separation of neck and shoulder	Waiter’s tip: adduction, medial rotation, pronation
Klumpke palsy	Lower trunk C8–T1	Upward traction on abducted arm	Claw hand; possible Horner syndrome
Thoracic outlet syndrome	Often lower plexus or subclavian vessels	Compression by cervical rib, scalene hypertrophy, or first rib region	Paresthesias, weakness, vascular symptoms worsened by arm elevation

Step 1 Localization Strategy

To localize a lesion, first identify whether the pattern is a single peripheral nerve, cord, trunk, or root. A single terminal nerve lesion produces a restricted motor and sensory deficit, such as isolated wrist drop with radial nerve injury. A plexus-level lesion affects multiple terminal nerve distributions sharing common roots or trunks. Reflexes help: biceps reflex C5–C6, brachioradialis C5–C6, and triceps C7–C8. Root lesions often include neck pain or dermatomal findings, whereas plexus lesions more often follow traction, compression, or penetrating trauma and do not necessarily respect a single dermatome.