# Lecture 9: Upper Limb - Neurovasculature ## Unit 1.3: Human Gross Anatomy I - Musculoskeletal System --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the formation and organization of the brachial plexus 2. Identify the major nerves of the upper limb and their terminal distributions 3. Trace the arterial supply from the subclavian artery to the hand 4. Describe the venous and lymphatic drainage of the upper limb 5. Correlate nerve injury patterns with clinical presentations 6. Identify important neurovascular relationships and compression sites --- ## The Brachial Plexus The brachial plexus is the network of nerves that provides motor and sensory innervation to the entire upper limb. Understanding its organization is essential for diagnosing and localizing upper limb neurological deficits. ### Formation and Location The brachial plexus is formed by the ventral rami of spinal nerves C5 through T1. Some individuals have contributions from C4 (a "prefixed" plexus) or T2 (a "postfixed" plexus). The plexus begins in the posterior triangle of the neck, extends behind the clavicle, and reaches its terminal branches in the axilla. ### Organization: The Five Components The plexus is organized into five successive components, proceeding from proximal to distal: roots, trunks, divisions, cords, and branches. The mnemonic "Robert Taylor Drinks Cold Beer" helps remember this sequence. The roots are the five ventral rami themselves (C5, C6, C7, C8, and T1). These emerge from the intervertebral foramina and pass between the anterior and middle scalene muscles. The roots combine to form three trunks. The superior trunk is formed by the union of C5 and C6 roots. The middle trunk is simply the continuation of the C7 root. The inferior trunk is formed by C8 and T1 roots. The trunks lie in the posterior triangle of the neck, above the clavicle. Each trunk divides into an anterior and posterior division—six divisions total. The anterior divisions generally supply the flexor (anterior) compartments of the limb, while the posterior divisions supply the extensor (posterior) compartments. The divisions pass behind the clavicle. The divisions recombine to form three cords, named by their relationship to the axillary artery. The lateral cord is formed by the anterior divisions of the superior and middle trunks (C5, C6, C7). The posterior cord is formed by all three posterior divisions (C5-T1). The medial cord is simply the continuation of the anterior division of the inferior trunk (C8, T1). The cords and their terminal branches are located in the axilla. The terminal branches arise from the cords and include the major nerves supplying the upper limb: the musculocutaneous nerve (from the lateral cord), the median nerve (from lateral and medial cords), the ulnar nerve (from the medial cord), the radial nerve (from the posterior cord), and the axillary nerve (from the posterior cord). Panel A: Cervical spine with spinal nerves C5-T1 emerging as roots (red) between scalene muscles, forming three trunks (orange): superior (C5+C6), middle (C7), inferior (C8+T1) in the posterior triangle of neck. Panel B: Each trunk dividing into anterior (blue) and posterior (green) divisions passing behind clavicle, recombining into three cords (purple) around the axillary artery: lateral, posterior, and medial cords. Panel C: Terminal branches color-coded emerging from cords: musculocutaneous (yellow), median (orange), ulnar (purple), radial (green), and axillary (teal) with root values labeled. Panel D: Anatomical landmark scale showing scalenes, clavicle, and axillary artery relationships with mnemonic "Robert Taylor Drinks Cold Beer" for roots, trunks, divisions, cords, and branches. --- ## Branches of the Brachial Plexus The branches of the brachial plexus can be divided into supraclavicular branches (arising from the roots and trunks, proximal to the clavicle) and infraclavicular branches (arising from the cords, distal to the clavicle). ### Supraclavicular Branches These nerves arise early in the plexus and supply structures of the shoulder girdle. The dorsal scapular nerve arises directly from the C5 root and pierces the middle scalene muscle. It courses deep to the levator scapulae along the medial border of the scapula, innervating both the rhomboid major and minor and part of the levator scapulae. Injury causes lateral winging of the scapula and difficulty retracting the scapula. The long thoracic nerve arises from the posterior aspects of C5, C6, and C7 roots. It descends on the superficial surface of the serratus anterior muscle, which it innervates. The long course of this nerve along the chest wall makes it vulnerable to injury during axillary surgery or trauma. Damage causes "winging" of the scapula—the medial border lifts away from the chest wall when the patient pushes against a wall. The nerve to subclavius arises from the superior trunk (C5, C6) and innervates the subclavius muscle. It may give off a contribution to the phrenic nerve (the accessory phrenic nerve). The suprascapular nerve arises from the superior trunk (C5, C6) and passes through the suprascapular notch (beneath the superior transverse scapular ligament) to reach the supraspinous fossa. It innervates the supraspinatus, then winds around the lateral border of the scapular spine to innervate the infraspinatus. Entrapment at the suprascapular notch causes weakness of shoulder abduction and external rotation. ### Infraclavicular Branches These branches arise from the three cords and include both smaller branches to local muscles and the major terminal nerves of the limb. From the lateral cord arise the lateral pectoral nerve (innervating the pectoralis major, particularly its clavicular head), the musculocutaneous nerve, and the lateral contribution to the median nerve. From the medial cord arise the medial pectoral nerve (innervating both pectoralis major and minor), the medial cutaneous nerve of the arm, the medial cutaneous nerve of the forearm, the ulnar nerve, and the medial contribution to the median nerve. From the posterior cord arise the upper subscapular nerve (to subscapularis), the thoracodorsal nerve (to latissimus dorsi), the lower subscapular nerve (to subscapularis and teres major), the axillary nerve, and the radial nerve. Panel A: Supraclavicular branches showing dorsal scapular nerve (C5) piercing middle scalene to rhomboids, long thoracic nerve (C5-C7) descending on serratus anterior, nerve to subclavius, and suprascapular nerve passing through scapular notch. Panel B: Lateral cord (C5-C7) infraclavicular branches including lateral pectoral nerve to pectoralis major, musculocutaneous nerve, and lateral root of median nerve. Panel C: Medial cord (C8-T1) branches including medial pectoral nerve, medial cutaneous nerves of arm and forearm, ulnar nerve, and medial root of median nerve with formation from two roots shown. Panel D: Posterior cord (C5-T1) branches including upper and lower subscapular nerves, thoracodorsal nerve to latissimus dorsi, axillary nerve, and radial nerve with target muscles listed. --- ## Terminal Nerves: Detailed Anatomy ### The Musculocutaneous Nerve The musculocutaneous nerve (C5-C7) is the motor nerve to the anterior compartment of the arm. It arises from the lateral cord and has a distinctive course—it pierces the coracobrachialis muscle (which it innervates) before continuing distally between the biceps brachii and brachialis muscles. The nerve innervates all three muscles of the anterior arm: coracobrachialis, biceps brachii, and brachialis. These muscles flex the shoulder (coracobrachialis), flex the elbow (biceps and brachialis), and supinate the forearm (biceps). After emerging lateral to the biceps tendon at the elbow, the nerve continues as the lateral cutaneous nerve of the forearm, providing sensory innervation to the lateral forearm from elbow to wrist. Injury to the musculocutaneous nerve is uncommon because of its protected course through the arm. When it occurs, patients have weakness of elbow flexion (though not complete loss, as brachioradialis is preserved) and supination, along with sensory loss over the lateral forearm. ### The Median Nerve The median nerve (C5-T1) is formed by the union of contributions from both the lateral and medial cords, typically joining anterior to the axillary artery. It is the primary motor nerve to the anterior forearm and provides crucial sensory innervation to the lateral hand. In the arm, the median nerve accompanies the brachial artery, characteristically crossing from lateral to medial about halfway down the arm. It gives no branches in the arm. At the elbow, the median nerve passes anterior to the brachial artery and then between the two heads of the pronator teres to enter the forearm. Within the forearm, it travels between the flexor digitorum superficialis and flexor digitorum profundus muscles. The motor branches in the forearm supply the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis. The anterior interosseous nerve, a pure motor branch, arises in the proximal forearm and supplies the flexor pollicis longus, the lateral half of the flexor digitorum profundus (to digits 2 and 3), and the pronator quadratus. At the wrist, the median nerve enters the hand through the carpal tunnel. It gives off the recurrent (thenar) branch to the thenar muscles (except the adductor pollicis) and palmar digital nerves that provide sensory innervation to the palmar surfaces of the lateral 3.5 digits (including the nail beds dorsally) and motor innervation to the first and second lumbricals. High median nerve injury (at or above the elbow) causes loss of most forearm flexors and produces the characteristic "hand of benediction" when the patient attempts to make a fist—the index and middle fingers remain extended because their flexors are paralyzed. Low median nerve injury (at the wrist, as in carpal tunnel syndrome) causes thenar weakness and sensory loss in the lateral digits without affecting forearm function. Panel A: Median nerve formation from lateral cord (C5-C7) and medial cord (C8-T1) joining anterior to axillary artery, with arm course alongside brachial artery crossing from lateral to medial. Panel B: Forearm entry between two heads of pronator teres, anterior interosseous branch to FPL, lateral FDP, and pronator quadratus, main trunk traveling between FDS and FDP. Panel C: Carpal tunnel entry cross-section showing nerve position, hand branches with recurrent thenar branch to APB, FPB, and OP, and palmar digital branches to lateral 3.5 digits with sensory territory shaded. Panel D: Clinical insets showing "hand of benediction" deformity from high lesion and thenar atrophy from low lesion at the wrist. --- ### The Ulnar Nerve The ulnar nerve (C8-T1) arises from the medial cord as its direct continuation. It is the primary motor nerve to the intrinsic muscles of the hand. In the arm, the ulnar nerve initially travels with the brachial artery but at mid-arm it pierces the medial intermuscular septum to enter the posterior compartment. Importantly, it gives no motor branches in the arm. At the elbow, the ulnar nerve passes posterior to the medial epicondyle in the cubital tunnel—this is the location of the "funny bone." The nerve is superficial and palpable here, making it vulnerable to compression and trauma. The nerve enters the forearm by passing between the two heads of the flexor carpi ulnaris. In the forearm, it innervates only two muscles: the flexor carpi ulnaris and the medial half of the flexor digitorum profundus (to digits 4 and 5). It travels with the ulnar artery in the distal forearm. The dorsal cutaneous branch arises proximal to the wrist and provides sensory innervation to the dorsomedial hand. This branch is clinically important: it is affected in cubital tunnel syndrome but spared in Guyon's canal compression, helping distinguish between these two lesion sites. At the wrist, the ulnar nerve enters the hand through Guyon's canal (ulnar canal), lying between the pisiform and the hook of the hamate. It divides into a superficial branch (sensory to the medial 1.5 digits) and a deep branch (motor to most intrinsic hand muscles). The deep branch curves around the hook of the hamate and supplies the hypothenar muscles, all interossei (dorsal and palmar), the third and fourth lumbricals, and the adductor pollicis. The ulnar nerve is thus the primary motor nerve of the hand. Ulnar nerve injury produces a "claw hand" deformity in digits 4 and 5, with hyperextension at the MCP joints and flexion at the IP joints. Froment's sign is positive—when asked to hold a piece of paper between the thumb and index finger, the patient compensates for adductor pollicis weakness by flexing the thumb IP joint using the median nerve-innervated flexor pollicis longus. Panel A: Ulnar nerve arising from medial cord (C8-T1) in axilla, traveling with brachial artery then piercing medial intermuscular septum to enter posterior compartment, passing posterior to medial epicondyle through cubital tunnel. Panel B: Forearm course entering between FCU heads with branches to FCU and medial FDP, dorsal cutaneous branch departing proximal to wrist with sensory territory on dorsum shown. Panel C: Wrist entry through Guyon's canal (pisiform medial, hook of hamate lateral), dividing into superficial branch (sensory to medial 1.5 digits) and deep branch (motor, curving around hamate hook to hypothenar, interossei, lumbricals 3-4, adductor pollicis). Panel D: Clinical insets showing claw hand deformity in digits 4-5 and positive Froment's sign demonstration. --- ### The Radial Nerve The radial nerve (C5-T1) is the largest branch of the brachial plexus and arises from the posterior cord. It is the motor nerve to all extensors of the upper limb. From its origin, the radial nerve passes posteriorly through the triangular interval (bounded by the long head of triceps, the humerus, and teres major) to reach the posterior arm. It then winds around the humerus in the spiral (radial) groove, accompanied by the profunda brachii artery. This intimate relationship with the humeral shaft makes the radial nerve vulnerable in humeral fractures. In the posterior arm, the nerve supplies all three heads of the triceps and the anconeus. After emerging from the spiral groove on the lateral arm, it pierces the lateral intermuscular septum to enter the anterior compartment, where it gives branches to brachioradialis and extensor carpi radialis longus. At the cubital fossa, the radial nerve divides into its two terminal branches. The superficial branch is purely sensory, continuing distally under the brachioradialis before emerging on the dorsum of the hand to supply the lateral dorsal hand and proximal lateral 2.5 digits. The deep branch (posterior interosseous nerve) is purely motor. It winds around the lateral aspect of the proximal radius, passing through the supinator muscle (through a fibrous arch called the arcade of Frohse), before entering the posterior forearm. It innervates the extensor carpi radialis brevis, the supinator, and all other posterior forearm muscles: extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis. Radial nerve injury in the spiral groove produces "wrist drop"—inability to extend the wrist and fingers against resistance. The triceps is spared because its nerve branches arise proximal to the groove. Isolated posterior interosseous nerve injury (as from compression at the arcade of Frohse) causes finger and thumb extension weakness without wrist drop (ECRL is spared) and without sensory loss. Panel A: Radial nerve arising from posterior cord (C5-T1) as largest branch, passing through triangular interval with profunda brachii artery, winding in spiral groove around posterior humerus with vulnerability to humeral fracture noted. Panel B: Branches to all three heads of triceps and anconeus, nerve piercing lateral intermuscular septum with branches to brachioradialis and ECRL in lateral arm. Panel C: Cubital fossa division into superficial branch (sensory, coursing under brachioradialis to dorsal hand) and deep branch/PIN (motor, through supinator at arcade of Frohse to all posterior forearm extensors). Panel D: Clinical insets showing wrist drop from spiral groove injury with triceps spared, and PIN syndrome showing finger drop without wrist drop or sensory loss. --- ### The Axillary Nerve The axillary nerve (C5-C6) arises from the posterior cord and provides motor and sensory innervation to the shoulder region. From its origin, the nerve passes posteriorly through the quadrangular space—bounded by the subscapularis and teres minor above, teres major below, the long head of triceps medially, and the surgical neck of the humerus laterally. The posterior circumflex humeral artery accompanies it through this space. After passing through the quadrangular space, the nerve winds around the surgical neck of the humerus, deep to the deltoid muscle. It innervates the teres minor (via a branch given off within the quadrangular space) and all portions of the deltoid. The sensory branch, the superior lateral cutaneous nerve of the arm, supplies the skin over the lateral shoulder—the "regimental badge" area (named for the location where military insignia are worn). The axillary nerve is vulnerable in anterior shoulder dislocations and fractures of the surgical neck of the humerus. Injury causes weakness of shoulder abduction (particularly from 15 to 90 degrees, where deltoid is the primary abductor) and numbness over the deltoid. Panel A: Axillary nerve arising from posterior cord (C5-C6), exiting posteriorly through quadrangular space with posterior circumflex humeral artery, boundaries marked (subscapularis and teres minor superiorly, teres major inferiorly, long head of triceps medially, surgical neck of humerus laterally). Panel B: Branch to teres minor within quadrangular space, nerve winding under deltoid muscle with branches to all three parts of deltoid (anterior, middle, posterior). Panel C: Sensory branch (superior lateral cutaneous nerve of arm) to "regimental badge" area with territory shaded on shoulder. Panel D: Clinical inset showing anterior shoulder dislocation with nerve stretched and patient demonstrating weakness of shoulder abduction compared to normal side. --- ## Arterial Supply of the Upper Limb The arterial supply to the upper limb begins with the subclavian artery and continues as a chain of vessels to the hand. ### The Subclavian Artery The right subclavian artery arises from the brachiocephalic trunk, while the left subclavian artery arises directly from the aortic arch. The anterior scalene muscle divides the subclavian artery into three parts, each with characteristic branches. The first part (medial to the anterior scalene) gives rise to the vertebral artery (ascending to supply the brain), the internal thoracic artery (descending to supply the anterior thoracic wall), and the thyrocervical trunk (branching to supply the thyroid, neck, and shoulder). The second part (posterior to the anterior scalene) gives rise to the costocervical trunk, which supplies the posterior neck and upper intercostal spaces. The third part (lateral to the anterior scalene) may give rise to the dorsal scapular artery (though this often arises from the transverse cervical artery of the thyrocervical trunk). ### The Axillary Artery The axillary artery is the continuation of the subclavian artery, beginning at the lateral border of the first rib and ending at the lower border of teres major. It is surrounded by the cords of the brachial plexus, which are named by their relationship to it. Pectoralis minor divides the axillary artery into three parts. The first part (proximal to pectoralis minor) gives off the superior thoracic artery. The second part (posterior to pectoralis minor) gives off the thoracoacromial trunk (with its pectoral, acromial, clavicular, and deltoid branches) and the lateral thoracic artery (supplying the breast and serratus anterior). The third part (distal to pectoralis minor) gives off the subscapular artery (the largest branch, dividing into circumflex scapular and thoracodorsal arteries) and the anterior and posterior circumflex humeral arteries (wrapping around the surgical neck of the humerus). ### The Brachial Artery The brachial artery is the continuation of the axillary artery, beginning at the lower border of teres major. It courses along the medial aspect of the arm, medial to the biceps brachii, before dividing into the radial and ulnar arteries in the cubital fossa. The major branch of the brachial artery is the profunda brachii (deep brachial artery), which accompanies the radial nerve through the spiral groove to supply the posterior arm. Superior and inferior ulnar collateral arteries contribute to the anastomotic network around the elbow. Panel A: Subclavian artery with three parts relative to anterior scalene showing first part branches (vertebral, internal thoracic, thyrocervical trunk), second part (costocervical trunk), and third part (dorsal scapular). Panel B: Axillary artery with three parts relative to pectoralis minor showing first part (superior thoracic), second part (thoracoacromial trunk, lateral thoracic), and third part (subscapular, circumflex humeral arteries around surgical neck). Panel C: Brachial artery coursing medial to biceps with profunda brachii departing alongside radial nerve, superior and inferior ulnar collateral arteries to elbow anastomosis. Panel D: Division in cubital fossa into radial (lateral) and ulnar (medial) arteries with continuation to superficial and deep palmar arches, all vessels color-coded red with major landmarks labeled. --- ### The Radial and Ulnar Arteries The radial artery is the lateral terminal branch of the brachial artery. Though smaller than the ulnar artery, it is more superficial. It courses distally under cover of the brachioradialis muscle in the upper forearm, then becomes palpable in the lower forearm—the common site for pulse assessment. At the wrist, the radial artery passes through the anatomical snuffbox and then between the two heads of the first dorsal interosseous muscle to enter the deep palm, where it forms the deep palmar arch. The radial artery gives off the radial recurrent artery (contributing to the elbow anastomosis), the superficial palmar branch (completing the superficial palmar arch), and in the hand, the princeps pollicis artery (to the thumb) and the radialis indicis artery (to the lateral index finger). The ulnar artery is the larger, medial terminal branch. It passes deep to the pronator teres and then deep to the flexor digitorum superficialis before emerging in the distal forearm to travel with the ulnar nerve. At the wrist, it passes through Guyon's canal to enter the palm, where it forms the superficial palmar arch. The ulnar artery gives off the anterior and posterior ulnar recurrent arteries (for elbow anastomosis) and the common interosseous artery, which divides into anterior and posterior interosseous arteries supplying the deep forearm compartments. The deep palmar branch completes the deep palmar arch. ### The Palmar Arches The superficial palmar arch is formed predominantly by the ulnar artery and is completed by the superficial palmar branch of the radial artery. It lies at the level of the flexed fingers (across the mid-palm) and gives rise to common palmar digital arteries, which divide into proper palmar digital arteries supplying adjacent sides of the fingers. The deep palmar arch is formed predominantly by the radial artery and is completed by the deep branch of the ulnar artery. It lies more proximal than the superficial arch, on the bases of the metacarpals, and gives rise to palmar metacarpal arteries that join the common digital arteries from the superficial arch. Panel A: Brachial artery dividing in cubital fossa into radial (lateral, superficial under brachioradialis) and ulnar (medial, deep under pronator teres) arteries with recurrent branches contributing to periarticular anastomosis. Panel B: Common interosseous artery from ulnar dividing into anterior and posterior interosseous arteries, radial artery pulse point at wrist, and radial artery entering anatomical snuffbox in lateral view. Panel C: Palmar view showing superficial arch (ulnar-dominant, mid-palm) giving common and proper digital arteries, and deep arch (radial-dominant, more proximal) giving palmar metacarpal arteries with princeps pollicis and radialis indicis. Panel D: Cross-section at mid-palm showing relative positions of superficial and deep arches with anastomoses indicated, color intensity distinguishing artery depth. --- ## Venous Drainage The venous drainage of the upper limb consists of deep veins accompanying the arteries and a prominent superficial venous system visible beneath the skin. ### Deep Veins The deep veins are venae comitantes (paired veins) that accompany the arteries of the same name. They follow the arterial pattern: digital veins drain into palmar veins, which form radial and ulnar veins that unite to form brachial veins. The brachial veins join the basilic vein to form the axillary vein, which becomes the subclavian vein at the lateral border of the first rib. ### Superficial Veins The superficial veins are of great clinical importance for venipuncture and intravenous access. The cephalic vein begins at the anatomical snuffbox from the lateral end of the dorsal venous network. It ascends along the lateral (radial) aspect of the forearm and arm, passes in the deltopectoral groove between the deltoid and pectoralis major muscles, and drains into the axillary vein. The basilic vein begins from the medial end of the dorsal venous network. It ascends along the medial (ulnar) aspect of the forearm, crosses anterior to the medial epicondyle, and continues up the medial arm. At approximately mid-arm level, it pierces the deep fascia to join the brachial veins and form the axillary vein. The median cubital vein is a large communicating vein connecting the cephalic and basilic veins across the anterior elbow. It is the preferred site for venipuncture because of its superficial, relatively fixed position. The bicipital aponeurosis lies between the median cubital vein and the underlying brachial artery and median nerve, providing some protection during venipuncture. Panel A: Dorsal hand view showing dorsal venous network as blue meshwork with lateral origin of cephalic vein and medial origin of basilic vein marked. Panel B: Anterior arm view with cephalic vein (light blue) ascending laterally through deltopectoral groove to axillary vein, and basilic vein (dark blue) ascending medially, piercing deep fascia at mid-arm to join brachial veins. Panel C: Median cubital vein connecting cephalic and basilic at cubital fossa with venipuncture site marked, cross-section showing bicipital aponeurosis protecting underlying brachial artery and median nerve. Panel D: Deep veins shown as paired venae comitantes alongside arteries for comparison, with complete drainage pattern from hand to axillary vein illustrated. --- ## Lymphatic Drainage The lymphatic drainage of the upper limb ultimately passes through the axillary lymph nodes, which also receive lymph from the breast and thoracic wall. ### Superficial and Deep Lymphatics Superficial lymphatic vessels follow the superficial veins. Most drain toward the axilla, though some vessels accompanying the cephalic vein may drain directly into the apical axillary nodes or infraclavicular nodes. Deep lymphatic vessels follow the deep blood vessels and drain to the humeral (lateral) group of axillary lymph nodes, which lie along the axillary vessels. ### Axillary Lymph Nodes The axillary lymph nodes are organized into five groups, with lymph flowing toward the apex of the axilla. The pectoral (anterior) nodes lie along the lateral thoracic vessels on the medial wall of the axilla. They receive lymph from the anterior thorax and lateral breast—an important pathway for breast cancer metastasis. The subscapular (posterior) nodes lie along the subscapular vessels on the posterior wall of the axilla. They drain the posterior thorax and scapular region. The humeral (lateral) nodes lie along the axillary vessels on the lateral wall of the axilla. They are the primary drainage site for the upper limb. The central nodes lie within the fat of the axilla and receive lymph from the pectoral, subscapular, and humeral groups. The apical nodes lie at the apex of the axilla, medial to the axillary vessels. They receive lymph from all other axillary node groups (and sometimes directly from the upper limb and breast). They drain to the subclavian lymphatic trunk, which on the right joins the right lymphatic duct and on the left joins the thoracic duct. Panel A: Arm view showing superficial lymphatics (thin green lines) following superficial veins toward axilla and deep lymphatics (darker green) following deep vessels. Panel B: Axillary lymph node groups on cross-section including pectoral/anterior nodes on medial wall with breast drainage pathway, subscapular/posterior nodes on posterior wall, and humeral/lateral nodes on lateral wall receiving upper limb lymph. Panel C: Central nodes in axillary fat receiving from pectoral, subscapular, and humeral groups with converging arrows, and apical nodes at apex receiving from all groups draining to subclavian trunk. Panel D: Infraclavicular/deltopectoral nodes along cephalic vein shown in inset with drainage pattern arrows showing progression toward apical nodes. --- ## Important Neurovascular Relationships and Compression Sites Understanding sites where nerves and vessels are vulnerable to compression or injury is clinically essential. ### Nerve Compression Syndromes Multiple locations along the course of upper limb nerves are prone to compression. Thoracic outlet syndrome involves compression of the brachial plexus and/or subclavian vessels in the scalene triangle or costoclavicular space. Patients may experience arm pain, paresthesias (often in ulnar distribution), fatigue, and color changes in the hand. Suprascapular neuropathy results from compression of the suprascapular nerve at the suprascapular notch. Patients have weakness of shoulder abduction (supraspinatus) and external rotation (infraspinatus). Quadrangular space syndrome is compression of the axillary nerve within the quadrangular space, causing deltoid weakness and shoulder numbness. Saturday night palsy describes radial nerve compression in the spiral groove, typically from draping the arm over a chair while intoxicated. Patients have wrist drop but preserved triceps function. Pronator syndrome is median nerve compression between the heads of the pronator teres. Symptoms resemble carpal tunnel syndrome but include forearm flexor weakness. Carpal tunnel syndrome is the most common compression neuropathy, with median nerve compression under the flexor retinaculum causing thenar weakness and sensory loss in the lateral 3.5 digits. Cubital tunnel syndrome is ulnar nerve compression posterior to the medial epicondyle, causing intrinsic hand weakness and numbness in the medial 1.5 digits. Ulnar tunnel (Guyon's canal) syndrome is ulnar nerve compression at the wrist. Because the dorsal cutaneous branch arises proximal to the wrist, dorsal sensation is preserved (distinguishing this from cubital tunnel syndrome). Posterior interosseous nerve (PIN) syndrome involves compression at the arcade of Frohse within the supinator muscle. Patients have finger and thumb extension weakness without wrist drop or sensory loss. ### Neurovascular Bundles Throughout the upper limb, nerves and vessels travel together in predictable relationships. In the arm, the brachial artery, median nerve, and basilic vein form a neurovascular bundle. In the distal forearm, the ulnar nerve and artery travel together. In the hand, digital nerves and arteries course together along the sides of the fingers. Panel A: Proximal compression sites including thoracic outlet (scalene triangle and costoclavicular space with brachial plexus and subclavian vessels), suprascapular notch (nerve under superior transverse scapular ligament), and quadrangular space (axillary nerve compressed by boundaries). Panel B: Mid-limb compression sites including spiral groove (radial nerve wrapped around humerus with fracture risk), pronator teres (median nerve between two heads), and arcade of Frohse (PIN entering supinator). Panel C: Distal compression sites including carpal tunnel (cross-section showing median nerve compression), cubital tunnel (ulnar nerve posterior to medial epicondyle), and Guyon's canal (ulnar nerve at wrist with dorsal branch arising proximally). Panel D: Full arm schematic showing all compression sites with nerves in yellow, arteries in red, and compression points highlighted in orange, each syndrome name labeled. --- ## Scapular Anastomosis The scapular anastomosis is a network of arterial connections around the scapula that provides collateral circulation should the subclavian or proximal axillary artery become occluded. ### Contributing Arteries From the subclavian artery, the suprascapular artery (from the thyrocervical trunk) and the dorsal scapular artery (from the subclavian or transverse cervical) contribute to the anastomosis. From the axillary artery, the subscapular artery and its circumflex scapular branch form the major distal contribution. These vessels anastomose on the posterior surface of the scapula, creating a bypass route that allows blood to reach the axillary artery distal to an occlusion. This is clinically relevant in subclavian artery stenosis or after trauma. Panel A: Subclavian artery giving rise to thyrocervical trunk with suprascapular artery passing over superior transverse scapular ligament into supraspinous fossa on posterior view of scapula. Panel B: Dorsal scapular artery from subclavian or transverse cervical descending along medial scapular border, and axillary artery with subscapular artery giving circumflex scapular branch through triangular space. Panel C: Anastomotic connections shown as network on posterior scapular surface with arrows indicating direction of collateral blood flow if subclavian is occluded proximally. Panel D: Overview showing occlusion point marked on subclavian with collateral flow pathway highlighted in brighter red, demonstrating bypass route from subclavian to axillary territory. --- ## Dermatomes and Myotomes Understanding the segmental innervation of the upper limb is essential for neurological examination and lesion localization. ### Upper Limb Dermatomes The dermatomes of the upper limb are arranged in a sequential pattern from C5 to T1. C5 supplies the lateral arm, over the deltoid region—tested by pinprick over the lateral shoulder. C6 supplies the lateral forearm extending to the thumb and index finger—tested over the lateral forearm or thumb. C7 supplies the middle finger—tested over the middle finger. C8 supplies the medial forearm, ring finger, and little finger—tested over the little finger. T1 supplies the medial arm—tested over the medial upper arm. ### Upper Limb Myotomes Each spinal nerve root is associated with key movements for clinical testing. C5 is tested by shoulder abduction (deltoid muscle). C6 is tested by elbow flexion (biceps) and wrist extension. C7 is tested by elbow extension (triceps) and wrist flexion. C8 is tested by finger flexion (grip strength). T1 is tested by finger abduction (interossei). The reflexes also follow segmental patterns: the biceps reflex tests C5-C6, the brachioradialis reflex tests C5-C6, and the triceps reflex tests C7. Panel A: Anterior and posterior views of upper limb with dermatome territories shaded: C5 (blue) over lateral deltoid, C6 (green) over lateral forearm to thumb and index finger, C7 (yellow) over middle finger, C8 (orange) over medial forearm and ring/little fingers, T1 (red) over medial arm. Panel B: Myotome testing positions for C5 (shoulder abduction), C6 (elbow flexion, wrist extension), C7 (elbow extension, wrist flexion), C8 (finger flexion), and T1 (finger abduction against resistance). Panel C: Reflex testing positions showing biceps (C5-C6), brachioradialis (C5-C6), and triceps (C7) with root values labeled. Panel D: Summary diagram correlating dermatome territories with myotome movements and reflex arcs for clinical neurological examination. --- ## Clinical Correlations ### Erb-Duchenne Palsy (Upper Brachial Plexus Injury) Erb-Duchenne palsy results from injury to the C5 and C6 roots or the upper trunk of the brachial plexus. It is classically seen as a birth injury from excessive lateral neck traction during delivery, but can also occur from trauma in adults (motorcycle accidents, falls onto the shoulder). The affected muscles include those innervated by C5-C6: the deltoid (shoulder abduction), supraspinatus and infraspinatus (shoulder external rotation), biceps (elbow flexion, supination), and brachioradialis (elbow flexion). The characteristic posture is called the "waiter's tip" position: the arm hangs at the side, adducted and internally rotated (unopposed subscapularis and pectoralis major), the elbow is extended (weak biceps), and the forearm is pronated with the wrist flexed. The position resembles a waiter expecting a tip placed into the palm. ### Klumpke's Palsy (Lower Brachial Plexus Injury) Klumpke's palsy affects the C8 and T1 roots or the lower trunk. It may occur from birth injury (traction on an abducted arm) or in adults from traction injuries or infiltrating tumors (Pancoast tumor of the lung apex). The intrinsic hand muscles are primarily affected, producing a claw hand deformity with hyperextension at the MCP joints and flexion at the IP joints. Sensory loss occurs in the medial arm, forearm, and hand. If the T1 sympathetic fibers are involved, Horner syndrome may be present: ptosis, miosis, and anhidrosis on the affected side. ### Saturday Night Palsy Saturday night palsy describes radial nerve injury from compression in the spiral groove. The name derives from the scenario of falling asleep with the arm draped over a chair after alcohol intoxication, compressing the nerve against the humerus. Patients present with wrist drop (inability to extend the wrist) and finger drop (inability to extend the MCP joints). Importantly, the triceps is spared because its nerve branches arise proximal to the spiral groove. Recovery depends on the severity of the injury. Most cases involve neurapraxia (conduction block without axonal damage) and recover spontaneously over weeks to months. Panel A: Erb-Duchenne palsy (C5-C6) with plexus diagram highlighting upper trunk in red, patient in "waiter's tip" position (arm adducted, internally rotated, elbow extended, forearm pronated), affected muscles listed, and mechanism of lateral neck traction during delivery. Panel B: Klumpke's palsy (C8-T1) with plexus diagram highlighting lower trunk, claw hand deformity (MCP hyperextension, IP flexion), sensory loss on medial arm/forearm/hand, and Horner syndrome features (ptosis, miosis) from T1 sympathetic involvement. Panel C: Saturday night palsy showing radial nerve in spiral groove with compression mechanism (arm over chair), wrist drop and finger drop demonstrated with patient unable to extend against gravity. Panel D: Comparative note showing triceps spared in Saturday night palsy with preserved elbow extension, distinguishing from higher radial nerve lesions. --- ## Summary The brachial plexus is formed by the ventral rami of C5-T1 and is organized into roots, trunks, divisions, cords, and branches. The cords are named by their relationship to the axillary artery. The major terminal nerves—musculocutaneous, median, ulnar, radial, and axillary—each have characteristic courses, motor territories, and sensory distributions. The arterial supply proceeds from subclavian to axillary to brachial to radial and ulnar arteries, ultimately forming the superficial and deep palmar arches. The superficial palmar arch is ulnar-dominant; the deep arch is radial-dominant. Superficial venous drainage is via the cephalic vein (lateral) and basilic vein (medial), connected at the elbow by the median cubital vein—the preferred venipuncture site. Lymphatic drainage is to the axillary nodes, which are organized into pectoral, subscapular, humeral, central, and apical groups. Understanding common compression sites allows recognition of clinical syndromes: thoracic outlet syndrome, carpal tunnel syndrome, cubital tunnel syndrome, and Saturday night palsy, among others. Brachial plexus injuries produce characteristic patterns: Erb-Duchenne palsy (C5-C6, waiter's tip) and Klumpke's palsy (C8-T1, claw hand with possible Horner syndrome). --- ## Key Terms | Term | Definition | |------|------------| | Brachial plexus | Network of nerves formed by C5-T1 ventral rami that innervates the upper limb | | Musculocutaneous nerve | C5-C7; supplies anterior arm muscles; sensory to lateral forearm | | Median nerve | C5-T1; supplies forearm flexors and thenar muscles; sensory to lateral palm | | Ulnar nerve | C8-T1; supplies FCU, medial FDP, and most intrinsic hand muscles; sensory to medial hand | | Radial nerve | C5-T1; supplies all upper limb extensors; sensory to posterior arm, forearm, and dorsal hand | | Carpal tunnel syndrome | Median nerve compression at the wrist causing thenar weakness and lateral digit numbness | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*