Pain sensation II Hyperalgesia, analgesia and referred pain

🟠 Pain Modulation (Overview)

• 🖇 Modulation = regulation of pain after perception (increase or decrease).
• 🖇 Two outcomes:
• • 🚨 Hyperalgesia → increased pain sensitivity.
  • 📌 Analgesia Systems → suppression of pain.

🟣 Hyperalgesia (Exaggerated Pain Sensitivity)

A. Primary Hyperalgesia

• 📌 Location: at injury site (exact area + surrounding erythema/flair).
• 📌 Threshold: Low → very sensitive, abnormal.
• 📌 Mechanism: Local reflex.
• • A-delta fiber branches back before reaching spinal cord.
  • Releases Substance P + Prostaglandins → vasodilation + redness → ↑ sensitivity.

B. Secondary Hyperalgesia

• 📌 Location: area around the primary site (further away).
• 📌 Threshold: Normal or High (variable).
• 📌 Mechanism: Convergence-Fasciculation.
• • Neurons from injured + adjacent areas converge on same 2nd order neuron in dorsal horn.
  • Cortex interprets injury pain as if entire converged area is painful.

🟣 Pain Control Systems (Analgesia)

A. Stress Analgesia

• 📌 Severe stress (e.g., soldier in battle) → pain suppressed.
• 📌 Due to activation of natural opioid system.

B. Gate Control Theory (GCT)

• 📌 “Gates” in Substantia Gelatinosa (between 1st & 2nd ON).
• 📌 Touch/pressure fibers (large fibers) inhibit pain fibers.
• 📌 Example: rubbing an injured area reduces pain → closes the gate.
• 📌 Cortex can also modulate gates (corticofugal influence).

C. Pain Control Analgesia System (PCAS)

• 🚨 Most important system.
• 📌 Uses endogenous opioids (Endorphins, Enkephalins).
• 📌 Four key complexes:
• 1. Hypothalamus (PVN)
  2. Midbrain (PAG)
  3. Medulla
  4. Spinal Cord inhibitory complex (P.A.I.C)
• 📌 Mechanism:
• • PVN → Endorphins/Enkephalins → PAG.
  • PAG → activates Medulla.
  • Medulla → sends Serotonin to spinal cord.
  • Spinal cord inhibitory neurons block Glutamate + Substance P release.
• 📌 Cortex supervises system (e.g., during stress analgesia).
• 📌 Clinical use: Acupuncture, TENS stimulate this system.

🟠 Key Questions to Remember

1. 🖇 Mechanism of Primary Hyperalgesia → Local reflex (Substance P/Prostaglandins).
2. 🖇 Mechanism of Secondary Hyperalgesia → Convergence-Fasciculation.
3. 🖇 Threshold in Primary Hyperalgesia → Low.
4. 🖇 Threshold in Secondary Hyperalgesia → Normal or High.
5. 🚨 Components + Mechanism of PCAS (most important).
6. 📌 Principle of Gate Control Theory → touch fibers close the gate.
7. 📌 What is Stress Analgesia? → pain suppression under stress.