Upper Limb Anatomy: Bones, Muscles, Nerves, and Vessels

The shoulder (glenohumeral) joint is a ball-and-socket synovial joint that sacrifices stability for exceptional range of motion. The humeral head articulates with the shallow glenoid fossa of the scapula — the labrum (a fibrocartilaginous rim) deepens the socket but the joint relies heavily on muscular support for stability. The four rotator cuff muscles (SITS: supraspinatus, infraspinatus, teres minor, subscapularis) are the primary stabilizers. The suprascapular nerve (from the upper trunk of the brachial plexus) innervates supraspinatus and infraspinatus, passing through the suprascapular notch under the transverse scapular ligament. The axillary nerve (from the posterior cord) innervates the deltoid and teres minor, passing through the quadrangular space. Both nerves are vulnerable to injury — the axillary nerve during shoulder dislocation, and the suprascapular nerve from compression at the notch.

The arm (brachium) has an anterior (flexor) compartment and a posterior (extensor) compartment. The anterior compartment contains biceps brachii, brachialis, and coracobrachialis — all innervated by the musculocutaneous nerve (C5-C7). The posterior compartment contains triceps brachii (all three heads) and anconeus — innervated by the radial nerve (C5-T1). The brachial artery runs in the medial bicipital groove and is the primary arterial supply to the arm, continuing as the main vessel of the upper limb until it bifurcates into the radial and ulnar arteries at the cubital fossa. The radial nerve is clinically significant in the arm because it courses through the spiral (radial) groove on the posterior humerus — humeral shaft fractures can damage the nerve here, causing wrist drop (inability to extend the wrist and fingers).

The cubital fossa is a triangular depression on the anterior elbow, bounded by the pronator teres (medially), brachioradialis (laterally), and an imaginary line between the epicondyles (superiorly). Its contents from lateral to medial are: the radial nerve (deep, between brachioradialis and brachialis), the biceps tendon, the brachial artery, and the median nerve. The brachial artery bifurcates here into the radial and ulnar arteries. The cubital fossa is clinically important: the brachial artery is palpated here for blood pressure measurement, and the median cubital vein (superficial to the bicipital aponeurosis) is the most common site for venipuncture (blood draws). The bicipital aponeurosis provides a protective layer between the superficial median cubital vein and the deeper brachial artery.

The forearm has an anterior (flexor) compartment with three muscle layers. Superficial layer (5 muscles, lateral to medial): pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, flexor carpi ulnaris. Common flexor origin: medial epicondyle of the humerus. Most are innervated by the median nerve, except flexor carpi ulnaris and the medial half of flexor digitorum profundus (ulnar nerve). The posterior (extensor) compartment also has superficial and deep layers. Common extensor origin: lateral epicondyle. All are innervated by the radial nerve (posterior interosseous branch). Key clinical correlations: medial epicondylitis ("golfer's elbow") involves the common flexor origin, while lateral epicondylitis ("tennis elbow") involves the common extensor origin. Carpal tunnel syndrome compresses the median nerve at the wrist, affecting the thenar muscles and sensation to the lateral 3.5 fingers.

The hand contains intrinsic muscles organized into three groups: the thenar eminence (opponens pollicis, abductor pollicis brevis, flexor pollicis brevis — mostly median nerve), the hypothenar eminence (opponens digiti minimi, abductor digiti minimi, flexor digiti minimi brevis — all ulnar nerve), and the central compartment (lumbricals, interossei, adductor pollicis — mostly ulnar nerve). The critical nerve to remember: the ulnar nerve innervates most intrinsic hand muscles. The "ulnar paradox" is high-yield: a distal ulnar nerve lesion at the wrist causes more obvious clawing of the ring and little fingers than a proximal lesion at the elbow, because with a proximal lesion the FDP to those fingers is also paralyzed (so the fingers can't flex at the DIP, reducing the visible claw deformity). Sensation: the median nerve supplies the palmar surface of the lateral 3.5 fingers, the ulnar nerve supplies the medial 1.5 fingers, and the radial nerve supplies the dorsal hand (anatomical snuffbox region).

The arterial supply of the upper limb follows a continuous pathway: subclavian → axillary (at lateral border of rib 1) → brachial (at lower border of teres major) → radial and ulnar arteries (at cubital fossa). The radial artery is palpable at the wrist (radial pulse) in the anatomical snuffbox and just lateral to the flexor carpi radialis tendon — this is where the radial pulse is taken clinically. The ulnar artery enters the hand through Guyon's canal and forms the superficial palmar arch. The radial artery enters via the anatomical snuffbox and forms the deep palmar arch. Allen's test evaluates the integrity of both arches before procedures like radial artery cannulation. Understanding this arterial pathway from subclavian to fingertip is essential for clinical reasoning about ischemia, trauma, and vascular access.