NeurosciencePain III

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Neuroscience-Pain III

• Ascending Processing and Descending Pain
  Modulation
• Vahn Lewis, UTDB

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The Trigeminal Projections
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Ascending Pain Transmission

• The Spinothalamic Tract
• Neospinothalamic tract (bright pain,
  sensory-discriminative coding)
• Paleospinothalamic tract (multisynaptic,
  affect-motivational coding )

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Projections of the Spinothalamic Tract
Only 20 of fibers from spinothalamic tract
reach the thalamus.
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Thalamic projections

• Localization of head region in the thalamus
• Lateral inhibition and acupuncture?

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Lateral
Medial
lt Sensory-Discriminative -------------Affective-Mo
tivational gt
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Medial
Lateral
lt Caudal -------------------------------- Facial gt
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The Cortical projections for pain

• Sensory Homunculus in the Cortex

Lateral
Medial
ltFacial--------------------------------Caudalgt
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Painful Stimuli Can Activate Regions of the
Limbic Brain

• Pain can elicit emotional responses, memories,
  and learning.
• These aspects of the pain experience are due to
  involvement of the limbic regions of the brain.

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Thalamus
Spinal Cord
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Pain Response in Somatorsensory Cortex and
Cingualte Cortex
Multiple cortical areas relate to pain
processing. PET studies identify SI, SII,
frontal, cingulate, and insular cortical regions
responding to painful stimuli.
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Descending Pain Modulation Pathways
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Stimulation of Periaqueductal Gray (PAG)

• Activation can be profoundly analgesic
• Analgesia mediated by endogenous opiates(?)
• Moderate levels of endogenous opiates in this
  area
• Electrical stimulation is analgesic
• Morphine injected into PAG is analgesic
• Excitatory Amino Acid link to Nucleus Raphe
  Magnus (NRM)

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Electrical Stimulation in N. Raphe Magnus (NRM)

• Analgesia mediated by serotonergic mechanisms
• Many 5HT containing cells in this area
• Analgesia can be produced by 5HT injected into
  spinal cord
• on cells and off cells (Feilds, 1988)

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Nucleus Reticularis ParaGigantoCellularis (RPGC)

• Activation produces analgesia
• Analgesia mediated by noradrenergic neurons

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Spinal (Trigeminal)

• Endogenous opiate in interneurons
• Enkephalin and dynorphin plentiful
• Intrathecal opiates are analgesic
• Dynorphin acts at both kappa and glutamate (NMDA)
  receptors
• Terminals from 5HT and NE neurons converge here

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Possible Functions of Endogenous Analgesic
Systems

• Set thresholds for hyperalgesia
• Dynamic modulation of pain sense by attention
• PAG-NRM / N. RPGC-Dorsal horn system
• Stress induced analgesia (Opiate nonopiate)
• Pain reaction variability
• Gate Control Analgesia
• Stimulation induced analgesia (TENS)
• Counter irritant analgesia

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Pain Induced Analgesia

• Defuse Noxious Inhibitory Control (Le Bars)
• Noxious (painful) stimulation can induce
  analgesia
• Heterosynaptic character
• Brainstem mediated
• Medullary Subnucleus Reticularis Dorsalis (SRD)
  candidate region

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Summary

• Ascending pain pathways in the brain activate
  reticular formation, ,the thalamus, cortex,
  limbic regions.
• Descending -Modulating activity from the brain
  can modify the pain message passing through the
  dorsal horn.
• There are several pain modifying systems in the
  brain