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Definition of Pain (IASP)

Nociception and Pain. Definition of Pain (IASP) “An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.”. Nociception vs Pain. Tranduction* Conduction Spinal Processing Perception.

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Definition of Pain (IASP)

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  1. Nociception and Pain Definition of Pain (IASP) “An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.”

  2. Nociception vs Pain • Tranduction* • Conduction • Spinal Processing • Perception *Nociceptors are a specific subset of peripheral sensory organs which respond to noxious stimuli.

  3. Categories of Pain Physiological Clinical Persistent

  4. Physiological Pain • “Fast” pain – carried by Ad fibers • Sharp • Well-localized • “Slow” pain – carried by C fibers • Aching • Poorly localized

  5. Nociceptor Activation Heat  VR1 nucleus Ca+2 Glutamate ATP  P2X2 H+ ASIC Na+/Ca+2 ? PKA/PKC  Bradykinin  Mechanical VR1= vanilloid receptor ASIC=acid sensing ion channels P2X2=purinergic receptor

  6. Nociceptors in Teeth • 1. A fibers • thermoreceptors • mechanoreceptor • 2. C fibers • polymodal (chemoreceptors)

  7. Hydrodynamic Mechanism of Dental Pain • Odontoblast • Predentin • Dentin • Odontoblastic Process • Subodontoblastic Nerve Plexus • A Fiber • Axon Terminal in Tubule

  8. Spinal Processing • Nociceptive nerve endings synapse in the spinal cord (substantia gelatinosa) or medulla (nucleus of the spinal tract of CN V). • Information passed to thalamus through the activation of secondary (projection) neurons. Spinal Nucleus (C.N. V) Substantia Gelatinosa

  9. Referred Pain Visceral Nociceptors • Fibers usually run with autonomic fibers • Large receptive fields • Converge on neurons that receive somatic input

  10. AMPA Receptor (a-Amino-3-hydroxy-5-methylisoxazole-4-proprionic acid) • Natural agonist is glutamate • Permeable to Na+ and K+, but not Ca2+ • Channel opening short (10 ms) SP C Glu PN Na+ Glu K+ Ad

  11. Ascending CNS Pathways • Spino- and trigemino-thalamic tracts • Thalamus (sensory-discrimination) • Reticular/limbic systems (motivational-affective) • Cortex (cognitive-evaluative)

  12. Gate Control Theory of Pain (Melzack and Wall, 1965) Ad & C Ab

  13. Descending CNS Pathways Inhibition Facilitation

  14. Inhibition Facilitation 5-HT, NA(a2) GABA, GLY ENK, DNY bEN, ACh 5-HT, NA(a1) GLUT, SP ACh (nic) PAIN + EXIN ININ + + ACh(nic) CCK DNY (N) bEN, GABA GLY, ENK DNY (k) DRG DRG SP GLUT SP GLUT - - + + Nx + Nx + PN PN

  15. Endogenous Opioids & Stress-induced Analgesia • Enkephalins – dorsal horn • Dynorphins – hypothalamus, PAG, dorsal horn • -endorphins (involved in stress-induced analgesia) – hypothalamus

  16. Clinical Pain (Inflammatory or Post-surgical Pain) Pain Response “Pain Threshold” Stimulus Intensity An increased response to a normally painful stimulus An painful response to a normally innocuous stimulus Hyperalgesia Allodynia

  17. Primary Hyperalgesia “Peripheral Sensitization” Na+ Channels TTx-S/TTx-R B: Transcription B VR1, Na+ channels. Heat  VR1 Glutamate  SP H+ ASIC A ATP  P2X2  PG’s  Bradykinin  NGF Cytokines A: Sensitization

  18. Secondary Hyperalgesia “Central Sensitization” Periphery CNS Injury site DRG/TG • Injury and inflammatory response results in increased nociceptor activation • Afferent barrage leads to: • Increased excitability • Decreased inhibition

  19. Glu SP Mg++ AMPA NMDA Ca++ IP3 PN NMDA Receptor (N-methyl-D-aspartate) • Natural agonist is glutamate; usually blocked by Mg2+ • Depolarization opens channel by removing block • Channel opening has a long duration (100 ms) permitting summation of inputs (“wind-up”)

  20. Analgesics

  21. Na+ Channels TTx-S/TTx-R VR1 Glutamate SP ASIC P2X2  PG’s  Opioids  Cytokines The Pharmacological Approach

  22. Inhibition Facilitation 5-HT, NA(a2) GABA, GLY ENK, DNY bEN, ACh 5-HT, NA(a1) GLUT, SP ACh (nic) PAIN + EXIN ININ + + ACh(nic) CCK DNY (N) bEN, GABA GLY, ENK DNY (k) DRG DRG SP GLUT SP GLUT - - + + Nx + Nx + PN PN

  23. The Problem of Persistent Pain • Peripheral Processes • Spontaneous activity • Sympathetic activity • Nociceptor sensitization • Central Processes • Central sensitization • Spinal reorganization • Cortical reorganization • Loss of inhibitory pathways

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