Median Nerve Comprehensive Study Notes

Roots & Formation of the Median Nerve

  • Arises from the brachial plexus by two roots
    • Lateral root: C5,C6,C7C5, C6, C7 (from lateral cord)
    • Medial root: C8,T1C8, T1 (from medial cord)
  • Medial root crosses in front of the 3rd part of the axillary artery to unite with the lateral root, forming a characteristic Y-shape
  • Overall root value: C5–T1C5–T1

Course of the Nerve

In the Axilla

  • Lies lateral to the 3rd part of the axillary artery
  • Enters the arm at the lower border of teres major

In the Arm

  • Initially lateral to the brachial artery
  • Crosses anterior to the artery at mid-humerus (level of coracobrachialis insertion) to become medial
  • Continues downward toward the cubital fossa

In the Cubital Fossa

  • Position: medial to brachial artery & biceps brachii tendon
  • Covered by the bicipital aponeurosis, isolating it from the median cubital vein (clinical safety layer during venepuncture)
  • Gives off numerous muscular branches (pronator teres, FCR, palmaris longus, FDS except its ulnar head)
  • Leaves fossa by passing between the two heads of pronator teres, where it also gives origin to the anterior interosseous nerve (AIN)

In the Forearm

  • AIN (purely motor) supplies 2½ deep muscles:
    1. Flexor pollicis longus (FPL)
    2. Lateral half of flexor digitorum profundus (FDP)
    3. Pronator quadratus (PQ)
  • Main trunk passes behind tendinous arch of FDS, then deep to entire FDS mass
  • Approximately 5 cm5\,\text{cm} proximal to the flexor retinaculum, emerges laterally to lie superficial—between FDS tendons & posterior to palmaris longus tendon
  • Special mid-forearm muscular twig → radial head of FDS (tendon for index finger)

At the Wrist

  • Just before carpal tunnel, gives palmar cutaneous branch → crosses superficial to flexor retinaculum to innervate skin of thenar eminence & lateral palm (spared in carpal-tunnel syndrome)
  • Enters palm through the carpal tunnel (deep to flexor retinaculum, superficial to FDS/FDP/FPL tendons & their bursae)

In the Palm

  • Flattens at distal border of retinaculum, dividing into:
    • Lateral division → recurrent branch (motor to thenar muscles except deep head of FPB) + three palmar digital branches
    • Medial division → two palmar digital nerves
  • Total of 5 palmar digital nerves → sensory to palmar surface & nail beds of lateral 3123\tfrac12 digits; motor to 1st & 2nd lumbricals

Branches & Distribution

  • Muscular (direct): pronator teres, FCR, palmaris longus, FDS (except ulnar head), radial head of FDS (index)
  • Anterior interosseous nerve: FPL, lateral 12\tfrac12 FDP, PQ
  • Palmar cutaneous branch: skin over thenar eminence & lateral palm
  • Recurrent (thenar) branch: abductor pollicis brevis, opponens pollicis, superficial head of FPB
  • Palmar digital nerves: lateral 3123\tfrac12 digits (sensory) + lumbricals 1 & 2

Motor Territory (Mnemonic: "LOAF + 5")

  • L umbricals (1 & 2)
  • O pponens pollicis
  • A bductor pollicis brevis
  • F lexor pollicis brevis (superficial head)
  • PLUS: all superficial forearm flexors except FCU & medial 12\tfrac12 FDP, and specific deep muscles via AIN

Sensory Territory

  • Skin: lateral 2/32/3 of palm, palmar surface and nail beds of thumb, index, middle, and lateral half of ring finger; dorsal distal phalanges of same digits

Functional Epithets

  • Labourer’s nerve: powers the long flexors executing coarse forceful actions (digging, lifting)
  • Eye of the hand / Peripheral eye: fine tactile input from pulp of thumb & index finger for precision (e.g.1 evaluating fabric thinness, buttoning a coat)

Clinical Correlations

1. Injury at the Elbow

Possible causes

  • Supracondylar\text{Supracondylar} fracture of humerus
  • Tight tourniquet during venepuncture
  • Entrapment between heads of pronator teres or beneath fibrous arch of FDS

Key deficits & signs

  • Loss of pronation → forearm held supine (paralysis of pronator teres & PQ)
  • Weak wrist flexion; wrist drifts ulnarward (paralysis of FCR, intact FCU/FDP-medial)
  • Inability to flex PIP & DIP of index & middle fingers → Benediction attitude when attempting a fist
  • Loss of FPL function → no thumb IP flexion
  • Ape-thumb deformity: flattened thenar eminence, thumb adducted (paresis of thenar group, intact adductor pollicis via ulnar nerve)
  • Sensory loss over lateral palm & 3123\tfrac12 digits (palmar + dorsal tips)

2. Injury at the Mid-Forearm

  • Classically produces pointing index finger: radial head of FDS paralysed → index finger remains extended during fist formation
  • Other findings mimic distal forearm/wrist lesion

3. Injury at the Wrist (Distal Forearm) – Laceration/Suicide Cuts

  • Damages median nerve & its palmar cutaneous branch proximal to retinaculum
  • Ape-thumb deformity (thenar paralysis)
  • Sensory loss over thenar eminence + lateral palm + 3123\tfrac12 digits (both aspects)

4. Carpal Tunnel Syndrome (CTS)

Pathophysiology

  • Compression of median nerve beneath flexor retinaculum inside a narrow osteofibrous tunnel crowded with 9 flexor tendons & synovial sheaths
  • Causes narrowing/pressure rise:
    • Idiopathic tenosynovitis
    • Myxoedema\text{Myxoedema} (thyroxine deficiency)
    • Fluid retention of pregnancy
    • Lunate fracture-dislocation
    • Osteoarthritis of wrist

Clinical picture

  • Nocturnal burning pain / "pins & needles" in lateral 3123\tfrac12 digits
  • Thenar weakness; untreated cases → ape-thumb
  • Sensory sparing of thenar eminence (palmar cutaneous branch runs superficial to retinaculum)

Diagnostic signs

  • Tinel’s sign: percussion over flexor retinaculum reproduces symptoms (Fig. 13.11)
  • Phalen’s test: sustained wrist flexion for ≈1 min\approx 1\,\text{min} recreates paresthesia (Fig. 13.12)
  • Nerve conduction velocity < 30 m/s30\,\text{m/s} across tunnel is confirmatory

Management

  • Conservative: splinting, anti-inflammatory therapy, treat underlying endocrinopathy or pregnancy fluid retention
  • Surgical: longitudinal division of flexor retinaculum to decompress tunnel

Illustrative Deformities (Clinical Images)

  • Benediction hand (often popularly misattributed; classically ulnar claw, but here due to median motor loss in index & middle fingers)
    • Historical aside: St. Peter’s chronic ulnar claw famously resembled papal blessing gesture → "benediction hand"
  • Ape-thumb: thenar wasting & lateral rotation of thumb due to loss of opposition/abduction; thumb lies in plane of palm
  • Pointing index finger: selective inability to flex index finger due to radial FDS tendon paralysis

Key Numeric Facts

  • Emergence from beneath FDS: ∼5 cm\sim 5\,\text{cm} proximal to retinaculum
  • Root values: C5–T1C5–T1
  • Conduction velocity criterion for CTS: <30\,\text{m/s}

Integrated Summary (Motor & Sensory)

  • Motor: all anterior forearm muscles except FCU & medial 12\tfrac12 FDP; thenar group (except adductor pollicis & deep FPB head); 1st & 2nd lumbricals
  • Sensory: lateral palm, palmar digits 1,2,31,2,3 & radial half of 44, dorsal distal phalanges of same digits; spares dorsum proximal segments & medial palm

Real-World Relevance & Ethics

  • Ergonomics: prolonged wrist flexion/typing → CTS; employer responsibility for proper work-station design, periodic breaks, early symptom reporting
  • Surgical stewardship: avoid iatrogenic nerve compression from tight tourniquet during phlebotomy
  • Clinical empathy: nocturnal pain of CTS frequently disrupts sleep; patient education and timely referral prevent permanent thenar atrophy