Segmental innervation patterns

Dermatomes & Myotomes - The Body's Wiring

• Dermatome: Area of skin supplied by a single spinal nerve root.
• Myotome: Group of muscles supplied by a single spinal nerve root.

Key Landmarks:

• C4: Shoulder cape
• T4: Nipple line (📌 T-4 = Teat-four)
• T10: Umbilicus (📌 T-10 = Bellybu-ten)
• L4: Knee cap
• S2-S4: Perineum

Key Myotome Actions:

• C5: Shoulder abduction
• C7: Elbow extension
• L5: Big toe extension
• S1: Plantarflexion

⭐ Herpes zoster (shingles) reactivates in a dorsal root ganglion, causing a painful, unilateral vesicular rash restricted to a single dermatome.

Upper Limb Innervation - Arming the Nerves

• Key Myotomes (Motor) & Reflexes:

  • C5: Deltoid (abduction), Biceps (flexion) → Biceps reflex
  • C6: Wrist extension → Brachioradialis reflex
  • C7: Triceps (extension), Wrist flexion → Triceps reflex
  • C8: Finger flexion (grip)
  • T1: Finger abduction (interossei)

• Key Dermatomes (Sensory):

  • C5: Lateral shoulder (deltoid badge area)
  • C6: Lateral forearm, thumb, index finger
  • C7: Middle finger
  • C8: Medial forearm, ring, little finger
  • T1: Medial arm/axilla

⭐ Erb-Duchenne Palsy: Injury to C5-C6 roots (upper trunk) results in a "waiter's tip" hand: adducted, internally rotated arm with an extended elbow.

Lower Limb Innervation - Leg Day Nerves

• Femoral Nerve (L2-L4):
  • Motor: Hip flexion (iliopsoas), knee extension (quadriceps).
  • Sensory: Anterior thigh, medial leg.
  • Reflex: Patellar (L4).
• Obturator Nerve (L2-L4):
  • Motor: Thigh adduction.
  • Sensory: Medial thigh.
• Sciatic Nerve (L4-S3): Splits into tibial and common peroneal.
  • Tibial (L4-S3): Plantarflexion, inversion. Sensation to sole.
  • Common Peroneal (L4-S2): Dorsiflexion, eversion. Sensation to anterolateral leg & dorsum of foot.

📌 Mnemonic: "L4, kick the door" (knee extension); "S1, point to the sun" (plantarflexion).

⭐ High-Yield: Injury to the common peroneal nerve (e.g., fibular neck fracture) causes foot drop: inability to dorsiflex (L5) and evert the foot, leading to a high-stepping gait.

Clinical Correlations - Pinched & Pointless

• Radiculopathy ("Pinched Nerve"): Nerve root compression, typically from a herniated disc or spondylosis, causing segmental deficits.
  • Dermatomal Signs: Radiating pain (sciatica), paresthesia ("pins & needles"), or sensory loss in a specific skin distribution.
  • Myotomal Signs: Weakness in specific muscle groups (e.g., foot drop with L5).
  • Reflexes: Diminished or absent deep tendon reflexes (DTRs) corresponding to the affected nerve root (e.g., ↓ Achilles reflex with S1 compression).

⭐ A positive Straight Leg Raise (Lasègue's sign) is highly suggestive of lumbar radiculopathy, typically from L4-S1 disc herniation.

High‑Yield Points - ⚡ Biggest Takeaways

• C3-C5 innervates the diaphragm (phrenic nerve), essential for breathing.
• C5 controls shoulder abduction (deltoid); C6 manages wrist extension and the biceps reflex.
• C7 handles wrist flexion and finger extension, tested by the triceps reflex.
• L4 is key for knee extension via the patellar reflex and sensation over the medial malleolus.
• S1 mediates plantarflexion (Achilles reflex) and sensation on the lateral foot.