Peripheral Nerves

Roots: C5, C6.

• Origin: Terminal branch of the posterior cord of the brachial plexus.
• Course: Passes posteriorly through the quadrangular space (with the posterior circumflex humeral vessels), winds around the surgical neck of the humerus, deep to deltoid.
• Important relations: Surgical neck of humerus; posterior circumflex humeral artery/vein (accompany it through the quadrangular space); inferior glenohumeral joint capsule (lies directly against it, at risk in anterior dislocation).
• Branches: Anterior branch (winds around the surgical neck, supplies deltoid, gives a few terminal cutaneous twigs); posterior branch (supplies teres minor and posterior deltoid, continues as the upper lateral cutaneous nerve of arm).
• Motor supply: Deltoid, teres minor.
• Sensory supply: Skin over the lower lateral shoulder (regimental badge area), via the upper lateral cutaneous nerve of the arm.
• Important anatomical landmarks: Quadrangular space, surgical neck of humerus.
• Common lesion sites: Surgical neck fractures; anterior shoulder dislocation; intramuscular injection into deltoid placed too low/medial; improper crutch use.
• Resulting motor deficit: Weak/absent abduction beyond the first 15° (supraspinatus alone can still initiate); flattened deltoid contour over time (muscle wasting).
• Sensory deficit: Regimental badge area (lower lateral shoulder).
• Important clinical signs: Loss of deltoid contour, inability to actively hold the arm abducted against resistance, sensory testing over the deltoid insertion area.

• Roots: C5, C6, C7.
• Origin: Terminal branch of the lateral cord of the brachial plexus.
• Course: Pierces coracobrachialis, descends between biceps brachii and brachialis, pierces deep fascia lateral to the biceps tendon near the elbow to become the lateral cutaneous nerve of the forearm.
• Important relations: Coracobrachialis (pierces it, key identifying feature); lies between biceps and brachialis for most of its course.
• Branches: Muscular branches to coracobrachialis, biceps brachii, brachialis; terminal cutaneous continuation as the lateral cutaneous nerve of forearm.
• Motor supply: Coracobrachialis, biceps brachii, brachialis (all anterior arm compartment muscles).
• Sensory supply: Lateral forearm (as lateral cutaneous nerve of forearm).
• Important anatomical landmarks: Pierces coracobrachialis.
• Common lesion sites: Rare in isolation; upper trunk/lateral cord injuries (traction, Erb palsy); occasionally injured in shoulder surgery.
• Resulting motor deficit: Weak elbow flexion (brachialis is the primary flexor lost, though brachioradialis can partially compensate) and weak supination (biceps lost).
• Sensory deficit: Lateral forearm numbness.
• Important clinical signs: Weak resisted elbow flexion with the forearm supinated (biceps-dominant position), reduced biceps reflex.

• Roots: C5, C6, C7, C8, T1.
• Origin: Terminal branch (and largest branch) of the posterior cord of the brachial plexus.
• Course: Enters the arm posterior to the brachial artery, passes through the triangular interval, runs in the radial (spiral) groove of the humerus with profunda brachii vessels. It pierces the lateral intermuscular septum, descends between brachialis and brachioradialis at the elbow (within the cubital fossa laterally). It then divides into superficial and deep terminal branches near the lateral epicondyle.
• Important relations: Radial groove of humerus (mid-shaft fractures); triangular interval; cubital fossa (lateral border); arcade of Frohse (deep branch entrapment site within supinator).
• Branches:
• In the axilla/arm: posterior cutaneous nerve of arm, lower lateral cutaneous nerve of arm, posterior cutaneous nerve of forearm; muscular branches to triceps, anconeus.
• At the elbow: divides into superficial branch (sensory) and deep branch (motor, becomes posterior interosseous nerve after piercing supinator).
• Motor supply: Triceps, anconeus, brachioradialis, ECRL (radial nerve trunk); ECRB, supinator, and all posterior forearm muscles (via the deep branch/posterior interosseous nerve).
• Sensory supply: Posterior arm and forearm skin; dorsum of the lateral hand and the dorsal aspect of the lateral 3½ digits (excluding the nail beds, which receive median/ulnar sensation) via the superficial branch.
• Important anatomical landmarks: Radial groove, triangular interval, arcade of Frohse.
• Common lesion sites:
• Axilla: Crutch pressure, improperly fitted slings, affects triceps and all distal radial territory.
• Radial groove: Mid-shaft humeral fractures, "Saturday night palsy" (compression against a hard edge, e.g., sleeping with arm over a chair), triceps typically spared or partially spared (branches often arise proximal to the compression point), wrist and finger extensors lost.
• Deep branch/posterior interosseous nerve: Entrapment at the arcade of Frohse, radial head fractures/dislocations, elbow surgery, wrist extension may be preserved (ECRL/brachioradialis spared, being supplied proximal to this point) but finger extension and radial deviation on extension are lost.
• Resulting motor deficit: Wrist drop (loss of wrist and finger extension) in proximal lesions; with posterior interosseous nerve lesions specifically, wrist extension is preserved but deviates radially (ECU lost while ECRL/ECRB partially spared), and finger extension at the MCP joints is lost.
• Sensory deficit: Dorsum of the hand, first dorsal web space (classic testing area), sensory loss absent in isolated posterior interosseous nerve lesions (purely motor distal to its origin).
• Important clinical signs: Wrist drop, inability to extend fingers at the MCP joints, weak grip strength (finger flexors are mechanically disadvantaged without wrist stabilisation from extensors).

• Roots: C5, C6, C7, C8, T1.
• Origin: Union of the lateral root (from lateral cord) and medial root (from medial cord), anterior/lateral to the axillary artery.
• Course: Descends through the arm alongside the brachial artery (lateral initially, crossing to medial around mid-arm). It enters the forearm between the two heads of pronator teres. It passes deep to the fibrous arch of FDS, running between FDS and FDP.
• It enters the carpal tunnel deep to the flexor retinaculum, most superficial structure within it. It terminates by dividing into digital branches in the palm.
• Important relations: Brachial artery (crosses it in the arm); pronator teres (passes between its two heads); FDS arch (passes deep to it); carpal tunnel (most superficial content).
• Branches:
• Muscular branches (arm, none; forearm, to pronator teres, FCR, palmaris longus, FDS).
• Anterior interosseous nerve: arises just distal to pronator teres, supplies FPL, lateral half FDP, pronator quadratus.
• Palmar cutaneous branch: arises proximal to the flexor retinaculum, supplies skin over the lateral palm and thenar eminence (spared in carpal tunnel syndrome).
• Recurrent (motor) branch: arises just distal to the flexor retinaculum, supplies thenar muscles (ABP, FPB superficial head, opponens pollicis).
• Common and proper palmar digital nerves: sensory to the lateral 3½ digits; motor to the 1st and 2nd lumbricals.
• Motor supply: Most anterior forearm muscles (except FCU, medial FDP), thenar muscles, 1st and 2nd lumbricals.
• Sensory supply: Lateral palm, palmar aspect of the lateral 3½ digits, and the nail beds/dorsal tips of the same digits.

Lesion at elbow (proximal median nerve injury):

• Loss of pronation (pronator teres, pronator quadratus), weak wrist flexion (radial deviation lost as FCR is affected, FCU intact preserves some ulnar-deviated flexion).
• Loss of FPL and lateral FDP (index/middle finger DIP flexion lost: "Pointing/Benediction sign" on attempted fist making, index and middle fingers remain extended).
• Thenar wasting and weakness (ape hand deformity component: loss of thumb opposition/abduction).
• Sensory loss over the lateral palm and lateral 3½ digits.

Lesion at wrist (distal median nerve injury, e.g., laceration, carpal tunnel syndrome):

• Thenar muscle weakness/wasting (recurrent branch), weak opposition and abduction of the thumb.
• Sensory loss over the lateral 3½ digits, palm typically spared if the lesion is within the carpal tunnel (palmar cutaneous branch arises proximally). However, the palm IS affected in a laceration at the wrist proximal to the retinaculum, since the palmar cutaneous branch would also be cut.
• Long flexors (FDS, FDP, FPL) are spared since they are innervated more proximally.

Carpal tunnel syndrome: Entrapment of the median nerve within the tunnel; sensory symptoms in the lateral 3½ digits sparing the palm; thenar weakness/wasting in advanced cases; positive Tinel and Phalen signs.

Recurrent branch: Motor branch to the thenar muscles, arising just distal to the flexor retinaculum, typically curving around the distal border of the retinaculum before entering the thenar muscles, superficial and vulnerable in palmar lacerations or poorly placed carpal tunnel release incisions.

Palmar cutaneous branch: Arises proximal to the wrist crease, runs superficial to the flexor retinaculum (not through the tunnel) to supply the central palm and thenar eminence skin, explains sparing of palm sensation in carpal tunnel syndrome.

• Roots: C8, T1.
• Origin: Terminal branch (direct continuation) of the medial cord of the brachial plexus.
• Course: Descends medial to the brachial artery in the arm, pierces the medial intermuscular septum around mid-arm to enter the posterior compartment briefly. It passes posterior to the medial epicondyle of the humerus (in the cubital/ulnar groove).
• It enters the forearm between the two heads of flexor carpi ulnaris, runs on FDP. It becomes superficial in the distal forearm alongside the ulnar artery (nerve medial to artery). It enters the hand via Guyon canal.
• Important relations: Medial epicondyle (cubital tunnel); FCU (passes between its two heads); ulnar artery (in the distal forearm and Guyon canal); hook of hamate and pisiform (Guyon canal boundaries).
• Branches:
• Muscular branches (forearm), FCU, medial half of FDP.
• Palmar cutaneous branch: arises in the distal forearm, supplies skin over the medial palm (does not pass through Guyon canal).
• Dorsal cutaneous branch: arises proximal to the wrist, winds around the ulna to supply the dorsomedial hand and dorsal aspect of the medial 1½ digits (explains why more proximal ulnar lesions affect the dorsum of the hand while Guyon canal lesions do not).
• Superficial branch (in Guyon canal), cutaneous to the palmar medial 1½ digits, motor to palmaris brevis.
• Deep branch (in Guyon canal), motor to hypothenar muscles, all interossei, adductor pollicis, medial two lumbricals; curves around the hook of hamate.

Motor supply: FCU, medial half FDP, hypothenar muscles, adductor pollicis, all interossei, medial two lumbricals, supplies the majority of intrinsic hand muscles.

Sensory supply: Medial 1½ digits (palmar and dorsal aspects), medial palm and dorsomedial hand.

Medial epicondyle relationship: The ulnar nerve lies directly posterior to the medial epicondyle, superficial and palpable ("funny bone"), extremely vulnerable to direct trauma, prolonged leaning/compression, and fracture-related injury at this exact site; also the site of surgical transposition for chronic entrapment (cubital tunnel syndrome).

Guyon canal: A distal site of ulnar nerve compression, with a sensory/motor pattern that spares the dorsum of the hand (dorsal cutaneous branch already given off proximally), a key distinguishing feature from a lesion at the elbow.

Ulnar claw: Occurs with a distal (wrist-level) ulnar nerve lesion more prominently than a proximal one, paradoxically. Loss of the medial two lumbricals (which flex MCP/extend IP of the ring and little fingers) combined with intact long flexors produces hyperextension at the MCP and flexion at the IP joints of the ring and little fingers. FDP is still working, since it is affected proximally but not distally. The FDP paralysis actually reduces clawing when the lesion is high.

Froment sign: Test for adductor pollicis weakness (ulnar nerve). Asked to pinch paper between thumb and index finger, the patient compensates for weak thumb adduction by flexing flexor pollicis longus (median nerve, unaffected) at the thumb IP joint, visible thumb IP flexion during the pinch is a positive sign.

Ulnar paradox: A proximal (elbow-level) ulnar nerve lesion produces a less obvious claw than a distal (wrist-level) lesion, because the proximal lesion also paralyses the medial half of FDP (ring/little finger DIP flexors), removing the deforming flexor pull that produces clawing at the IP joints. A distal lesion spares FDP (innervated more proximally), so its unopposed pull, combined with lost lumbrical function, produces a more pronounced claw. "The nerve is more proximally injured, yet the clinical deformity looks less severe", the paradox.

• Roots: C5, C6, C7.
• Origin: Direct branches from the roots of the brachial plexus, uniting before descending.
• Course: Passes through/behind the middle scalene, descends on the superficial surface of serratus anterior, along the mid-axillary line.
• Important relations: Extremely superficial course on serratus anterior, vulnerable during axillary surgery, mastectomy, and blunt trauma to the lateral chest wall.
• Branches: None; terminates as muscular branches into serratus anterior digitations.
• Motor supply: Serratus anterior.
• Sensory supply: None.
• Important anatomical landmarks: Runs along the mid-axillary line, superficial to serratus anterior.
• Common lesion sites: Axillary lymph node dissection/mastectomy, penetrating trauma, blunt trauma (backpack palsy from prolonged strap pressure), stretch injury during overhead activity.
• Resulting motor deficit: Winged scapula: loss of scapular protraction and stabilisation against the thoracic wall; medial border and inferior angle protrude posteriorly, most evident pushing against resistance (e.g., wall push-up test).
• Sensory deficit: None.
• Important clinical signs: Winging of the scapula on forward pushing/wall press test; difficulty with overhead arm elevation past 90° (serratus anterior contributes to scapular rotation).

• Roots: C5, C6.
• Origin: Branch of the upper trunk of the brachial plexus.
• Course: Passes through the suprascapular notch (deep to the superior transverse scapular ligament) into the supraspinous fossa, then curves around the lateral border of the scapular spine through the spinoglenoid notch into the infraspinous fossa.
• Important relations: Suprascapular notch (artery passes over the ligament, nerve passes under, "Army over, Navy under"); spinoglenoid notch (second potential entrapment site).
• Branches: Muscular branches to supraspinatus and infraspinatus; articular branches to the glenohumeral and acromioclavicular joints.
• Motor supply: Supraspinatus, infraspinatus.
• Sensory supply: None cutaneous; deep sensory/proprioceptive fibres to the shoulder and AC joints (roughly 70% of shoulder joint sensation).
• Important anatomical landmarks: Suprascapular notch, spinoglenoid notch.
• Common lesion sites: Entrapment at the suprascapular notch (ligament thickening, ganglion cysts) or spinoglenoid notch (often from a paralabral cyst associated with labral tears); traction injury (overhead throwing athletes, heavy backpack use).
• Resulting motor deficit: Weak abduction initiation and lateral rotation; notch-level lesions affect both supraspinatus and infraspinatus, spinoglenoid-level lesions affect infraspinatus alone.
• Sensory deficit: None cutaneous; deep shoulder pain is common due to joint afferent involvement.
• Important clinical signs: Weakness of the first 15° of abduction and of resisted external rotation; infraspinatus/supraspinatus wasting visible on inspection in chronic cases.

• Roots: C6, C7, C8.
• Origin: Branch of the posterior cord of the brachial plexus, between the upper and lower subscapular nerves.
• Course: Descends along the posterior axillary wall with the thoracodorsal vessels, on the surface of subscapularis, to reach latissimus dorsi.
• Important relations: Runs with the subscapular/thoracodorsal artery; posterior axillary wall, close to axillary lymph nodes (subscapular group), vulnerable during axillary dissection alongside the long thoracic nerve.
• Branches: Terminates as muscular branches into latissimus dorsi.
• Motor supply: Latissimus dorsi.
• Sensory supply: None.
• Important anatomical landmarks: Posterior axillary wall, with the thoracodorsal vessels.
• Common lesion sites: Axillary lymph node dissection, latissimus dorsi flap harvest (deliberately preserved or divided depending on flap type).
• Resulting motor deficit: Weak arm adduction, extension, and medial rotation (especially noticeable in movements like climbing or pushing up from a chair using the arms).
• Sensory deficit: None.
• Important clinical signs: Weakness of resisted shoulder extension/adduction (e.g., asking the patient to cough while palpating the posterior axillary fold, or resisted adduction testing); latissimus dorsi wasting in chronic injury.