Upper Limb — Brachial Plexus, Axillary Artery, Shoulder Joint & Muscles of Arm and Forearm

The Brachial Plexus — Construction and Clinical Corridors

The brachial plexus is the most clinically significant nerve network in the upper limb, formed by the ventral rami of C5, C6, C7, C8, and T1 spinal nerves. It constructs itself in a recognizable sequence: roots first, then trunks, divisions, cords, and finally terminal branches. Understanding this ladder-like assembly makes its anatomical relationships intuitive rather than memorized.

The five roots travel between the anterior and middle scalene muscles in the posterior triangle of the neck. At the lateral border of the scalene muscles, they converge into three trunks — the upper trunk (C5–C6), the middle trunk (C7), and the lower trunk (C8–T1). These trunks pass posterior to the clavicle at the supraclavicular region, where they divide into anterior and posterior divisions. The six divisions reorganize into three cords at the lateral border of the first rib: the lateral cord (anterior divisions of upper and middle trunks), medial cord (anterior division of lower trunk), and posterior cord (posterior divisions of all three trunks). The cords finally give rise to the terminal branches in the axilla.

The terminal branches include the musculocutaneous nerve (C5–C7), which pierces the coracobrachialis and descends in the anterior arm compartment, providing motor innervation to the biceps brachii, brachialis, and coracobrachialis, with cutaneous supply to the lateral forearm via the lateral antebrachial cutaneous nerve. The median nerve (C5–T1) descends medial to the brachial artery and gives no branches in the arm; in the forearm it innervates most anterior compartment muscles except the flexor carpi ulnaris and the ulnar half of the flexor digitorum profundus. The ulnar nerve (C8–T1) passes medially to the axillary artery, pierces the medial intermuscular septum, and runs posterior to the medial epicondyle at the elbow where it is famously superficial and vulnerable to trauma. The radial nerve (C5–T1) spirals around the posterior humerus in the radial groove, then passes anterior to the lateral epicondyle where it divides into deep and superficial branches. The axillary nerve (C5–C6) exits through the quadrangular space and wraps around the surgical neck of humerus to supply the deltoid and teres minor. The long thoracic nerve (C5–C7) runs down the lateral chest wall on the serratus anterior — injury causes winging of the scapula.

Clinical correlations: Erb’s palsy from C5–C6 root injury (common during shoulder dystocia or compression against the clavicle) produces the classic “waiter’s tip” posture: arm hangs adducted and internally rotated, elbow extended and forearm pronated, wrist flexed. Klumpke’s palsy from C8–T1 injury causes intrinsic hand muscle paralysis resulting in a claw hand deformity. The radial nerve is at particular risk during humeral shaft fractures — mid-shaft fractures injure the nerve in the radial groove, causing wrist drop and sensory loss over the posterior arm, posterior forearm, and dorsum of the hand (first web space spared with distal injury). Lacerations to the cubital fossa risk musculocutaneous nerve injury.

Axillary Artery — Segments and Branching Pattern

The axillary artery begins at the lateral border of the first rib as a continuation of the subclavian artery and ends at the inferior border of the teres major muscle, where it becomes the brachial artery. It is divided into three parts by the pectoralis minor muscle.

Part 1 (lateral to pectoralis minor) gives off the highest thoracic artery, a small branch to the upper chest wall. Part 2 (deep to pectoralis minor) gives off the thoracoacromial artery (supplying the clavicular head of deltoid, chest wall, and acromion) and the lateral thoracic artery (supplying serratus anterior and the breast — enlarged in breast carcinoma). Part 3 (lateral to pectoralis minor) gives off the subscapular artery (largest branch, running on the subscapularis muscle and dividing into the thoracodorsal and circumflex scapular arteries) and the anterior and posterior circumflex humeral arteries (which loop around the surgical neck of the humerus, with the posterior circumflex passing through the quadrangular space alongside the axillary nerve).

Clinical correlation: The axillary artery is the most commonly injured vessel in anterior shoulder dislocations due to its proximity to the glenohumeral joint. Iatrogenic injury during procedures such as axillary lymph node dissection or shoulder arthroplasty is also a recognized complication. Pseudoaneurysm formation can occur after axillary artery catheterization procedures.

Shoulder Joint — Glenohumeral Architecture

The glenohumeral joint is the most mobile — and therefore the most unstable — joint in the body. It is a ball-and-socket synovial joint formed by the large globular head of the humerus (approximately one-third of a sphere) articulating with the relatively shallow glenoid cavity of the scapula. The glenoid cavity is deepened and stabilized by the fibrocartilaginous glenoid labrum.

The articular surfaces are covered with hyaline cartilage. The glenoid cavity is smaller than the humeral head, creating an inherent mismatch that is partially compensated by the labrum. The fibrous capsule attaches proximally to the glenoid rim and distally to the anatomical neck of the humerus, becoming loose inferiorly to permit abduction. The capsule is reinforced by the coracohumeral ligament (strengthening the superior capsule), the glenohumeral ligaments (three thickened bands — superior, middle, and inferior — on the anterior capsule that resist excessive rotation and translation), and the transverse humeral ligament (bridging the bicipital groove to retain the long head of biceps tendon).

Four rotator cuff muscles — the SITS muscles — dynamically stabilize the joint. Supraspinatus originates from the supraspinous fossa and inserts on the middle facet of the greater tubercle; it initiates the first 15 degrees of shoulder abduction and is the most frequently torn rotator cuff muscle. Infraspinatus originates from the infraspinous fossa and inserts on the posterior facet of the greater tubercle; it externally rotates the shoulder. Teres minor originates from the lateral border of the scapula and inserts on the inferior facet of the greater tubercle; it also externally rotates. Subscapularis originates from the subscapular fossa and inserts on the lesser tubercle; it internally rotates the shoulder and forms the anterior wall of the rotator interval. All four are innervated by suprascapular (supraspinatus and infraspinatus), axillary (teres minor), and subscapular (subscapularis) nerves respectively.

The coracoacromial arch — formed by the coracoid process, acromion, and coracoacromial ligament spanning between them — forms a protective bony-ligamentous roof over the humeral head. The subacromial bursa lies deep to the arch and superficial to the supraspinatus tendon.

Clinical correlations: Anterior glenohumeral dislocation is the most common type (approximately 95 percent of dislocations), typically occurring when the arm is abducted and externally rotated. The humeral head displaces anteroinferiorly and often causes a Hill-Sachs lesion (compression fracture of the posterolateral humeral head) and Bankart lesion (avulsion of the inferior glenohumeral ligament from the anterior glenoid rim). Axillary nerve damage during dislocation produces deltoid paralysis and sensory loss over the lateral shoulder — the “regimental badge” area. Recurrent dislocations may require surgical stabilization (Bankart repair or Latarjet procedure). Impingement syndrome results from friction between the undersurface of the acromion and the supraspinatus tendon, classified by Neer as Stage I (edema and hemorrhage, reversible), Stage II (fibrosis and tendinitis, reversible with rest), or Stage III (bone spurs and tendon tears, requiring surgical decompression). A rotator cuff tear presents with pain, weakness in active abduction, and a positive drop arm test.

Muscles of the Arm — Anterior and Posterior Compartments

The anterior compartment of the arm contains muscles that flex the elbow and/or supinate the forearm. All are innervated by the musculocutaneous nerve (C5–C7). Biceps brachii has two heads: the long head originates from the supraglenoid tubercle (intra-articular but exits via the bicipital groove), and the short head originates from the coracoid process. Both insert on the radial tuberosity via the bicipital aponeurosis. It is a powerful supinator and elbow flexor. Rupture of the long head causes the “Popeye deformity” — a visible bulge of the muscle belly in the mid-arm. Brachialis originates from the anterior humerus and inserts on the ulnar tuberosity; it is the primary elbow flexor regardless of forearm position. Coracobrachialis originates from the coracoid process and inserts on the medial humerus; it flexes and adducts the shoulder.

The posterior compartment contains the triceps brachii, innervated by the radial nerve. It has three heads: the long head (origin: infraglenoid tubercle), lateral head (origin: posterior humerus above the radial groove), and medial head (origin: posterior humerus below the radial groove). All three converge to insert on the olecranon process of the ulna via the triceps tendon. The long head also assists in shoulder extension. The triceps is the primary extensor of the elbow.

Clinical correlation: Radial nerve injury at the spiral groove causes inability to extend the wrist (wrist drop) and fingers, with sensory loss over the posterior arm, posterior forearm, and dorsal hand. The posterior interosseous nerve — the deep branch of the radial nerve — can be compressed in the proximal forearm (supinator syndrome), causing pure motor deficits without sensory loss.

Muscles of the Forearm — Compartments and Functions

The forearm contains 20 muscles divided into anterior (flexor-pronator) and posterior (extensor-supinator) compartments, each further subdivided.

The anterior compartment (median and ulnar nerves, and musculocutaneous) is organized into three layers. The superficial layer (from lateral to medial: pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris) all originate from the medial epicondyle via the common flexor origin. The intermediate layer contains only flexor digitorum superficialis (FDS), which splits into four tendons that insert on the middle phalanges of digits 2–5 — it flexes the PIP joint and, secondarily, the MCP and elbow. The deep layer contains flexor digitorum profundus (FDP — flexes DIP joints of digits 2–5, supplied by the median nerve for the index and middle fingers via the anterior interosseous nerve, and ulnar nerve for the ring and little fingers), flexor pollicis longus (flexes the thumb IP joint, via the anterior interosseous nerve), and pronator quadratus (pronates the forearm, via the anterior interosseous nerve).

The posterior compartment (radial nerve) is organized into two layers. The superficial layer (from lateral to medial: brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris) all originate from the lateral epicondyle via the common extensor origin. The deep layer (from lateral to medial: supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis) — the “mobile wad” of Henry — crosses the posterior radius.

Clinical correlations: Lateral epicondylitis (“tennis elbow”) involves the extensor carpi radialis brevis origin and causes pain over the lateral elbow. Medial epicondylitis (“golfer’s elbow”) involves the flexor-pronator origin. Pronator syndrome involves median nerve compression in the pronator teres (arch between the two heads), causing forearm pain and median nerve sensory symptoms in the palm. The anterior interosseous nerve branch of the median nerve can be compressed by the pronator quadratus or an aberrant artery, causing weakness of FPL, FDP to index, and pronator quadratus — the “OK sign” is lost (cannot form a circle between thumb and index finger).

Cubital Fossa and Adjacent Spaces

The cubital fossa is a triangular space anterior to the elbow whose boundaries are the brachioradialis laterally, the pronator teres medially, and the interepicondylar line superiorly. The floor is formed by the brachialis (anterior surface) and the supinator muscle (posterior surface). The roof is the deep fascia of the forearm reinforced by the bicipital aponeurosis.

The contents from lateral to medial are: the biceps tendon, the brachial artery (which bifurcates into radial and ulnar arteries), and the median nerve. The brachial artery here is the site for measuring blood pressure and for arterial access during cardiac catheterization.

Clinical correlations: The median cubital vein — a communicating vein between the cephalic and basilic veins — lies in the subcutaneous tissue over the cubital fossa and is the most common site for venepuncture. Compression of the brachial artery during supracondylar humeral fractures in children can cause Volkmann’s ischemic contracture — a devastating compartment syndrome of the forearm. The radial nerve is superficial at the lateral arm and is at risk during surgical approaches to the elbow.