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Y2 s1 pain physiology



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  • 1. Physiology of pain Prof. Vajira Weerasinghe Professor in Physiology, Faculty of Medicine University of Peradeniya & Consultant Neurophysiologist, Teaching Hospital, Peradeniya
  • 2. Topics covered in the lecture
    • What is pain (International definition of pain)
    • Dual nature of pain: fast pain and slow pain
    • What causes pain : pain stimuli
    • Nerve pathways carrying pain signals to the brain
    • Brain areas involved in pain perception
    • Pain modulatory pathways
    • Neurochemicals involved in pain pathways
    • Gate control theory of pain
  • 3. What is pain?
    • Pain is a difficult word to define
    • Patients use different words to describe pain
    • eg.
    • Aching, Pins and needles, Annoying, Pricking, Biting, Hurting, Radiating, Blunt, Intermittent, Burning, Sore, Miserable, Splitting, Cutting, Nagging, Stabbing, Crawling, Stinging, Crushing, Tender, Dragging, Numbness, Throbbing, Dull, Overwhelming, Tingling, Electric-shock like, Penetrating, Tiring, Excruciating, Piercing, Unbearable
    • Different words in Sinhala or in Tamil
  • 4. What is pain?
    • There is an International definition of pain formulated by the IASP (International Association for the study of pain)
    • Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage IASP – International Association for the Study of Pain 2009
  • 5. What is pain?
    • Pain is
      • subjective
      • protective
      • and it is modified by developmental, behavioural, personality and cultural factors
    • It is a symptom
    • Associated signs are crying, sweating, increased heart rate, blood pressure, behavioural changes etc
  • 6. Dual nature of pain
    • Fast pain
      • acute
      • pricking type
      • well localised
      • short duration
      • Thin myelinated nerve fibres are involved (A delta)
    • Slow pain
      • chronic
      • throbbing type
      • poorly localised
      • long duration
      • Unmyelinated nerve fibres are involved (c fibres)
  • 7.
    • No stimuli, but pain is felt
      • phantom limb pain
        • eg. in amputated limb
    • Stimuli present, but no pain felt
        • eg. soldier in battle field, sportsman in arena
    • Pain due to a stimulus which does not normally provoke pain
        • Allodynia
    • Pain initiated or caused by a primary lesion or dysfunction in the nervous system
        • Neuropathic pain
    Different situations
  • 8. Pain
    • Pain as a sensation
      • physiologically (nociception)
    • Pain as an emotional experience
      • psychologically
  • 9. Stimuli
    • Physical
      • pressure etc
    • Electrical
    • Thermal
      • cold, hot
    • Chemical
      • H+, lactic acid, K+, histamine, bradykinin, acetylcholine, proteolytic enzymes
      • Prostaglandins
        • these increase the sensitivity (decrease the threshold) for other nociceptive stimuli
  • 10. Receptors
    • There are no specialised receptors
    • Free nerve endings are sensitive to pain stimuli
    • Free nerve endings are distributed everywhere (both somatic and visceral tissues)
    • Slow adapting type of receptors
  • 11. Nerve pathways carrying pain signals to the brain
    • Pain signals enter the spinal cord
    • First synapse is present in the dorsal horn of the spinal cord
    • Then the second order neuron travels through the lateral spinothalamic tracts
  • 12. afferent fibres
    • two types
      • A  (thin myelinated)
      • C (unmyelinated)
  • 13. central connections
    • afferent fibre enters the spinal cord
    • synapses in laminae ii,iii
      • substantia gelatinosa
    substantia gelatinosa Neurotransmitter at the first synapse of the pain pathway is substance P
  • 14.
    • crosses the midline
    • ascends up as the lateral spinothalamic tract
    ascending pathway Pain lateral spinothalamic tract C fibre substantia gelatinosa
  • 15. lateral spinothalamic tract thalamus sensory cortex C fibre thalamocortical tracts
  • 16. Pain perception
    • This occurs at different levels
      • thalamus is an important centre of pain perception
        • lesions of thalamus produces severe type of pain known as ‘thalamic pain’
      • Sensory cortex is necessary for the localisation of pain
      • Other areas are also important
        • reticular formation, limbic areas, hypothalamus and other subcortical areas
  • 17. Descending pain modulatory system
    • several lines of experimental evidence show the presence of descending pain modulatory system
      • discovery of morphine receptors
          • they were known to be present in the brain stem areas
      • discovery of endogenous opioid peptides
        • eg. Endorphines, enkephalins, dynorphin
  • 18. midbrain pons medulla spinal cord periaqueductal grey nucleus nucleus raphe magnus substantia gelatinosa
  • 19. opioid peptides
    • short peptides originally known to be secreted in CNS and later found to be present in GIT etc
  • 20. opioid peptides
    •  endorphin
        • earliest to discover, large peptide, present in the pituitary
    • encephalins - met & leu
        • widely distributed
    • dynorphin
    • Endomorphine 1 & 2
    • Pronociceptins
  • 21. receptors
    • delta
    • mu
    • kappa
    • More recently discovered: ORL1 receptor
  • 22.
    • Final pain perception depends on activity of the
      • Ascending pain impulse transmitting tracts
      • Descending pain modulatory (inhibitory) tracts
    sensory cortex C fibre
  • 23. Gate control theory
    • This explains how pain can be relieved very quickly by a neural mechanism
    • First described by P.D. Wall & Melzack (1965)
    • “ There is an interaction between pain fibres and touch fibre input at the spinal cord level in the form of a ‘ gating mechanism ’
  • 24. Gate control theory When pain fibre is stimulated, gate will be opened & pain is felt pain pain is felt + gate is opened
  • 25. Gate control theory When pain and touch fibres are stimulated together, gate will be closed & pain is not felt pain is not felt touch pain + - gate is closed
  • 26.  
  • 27. Gate control theory
    • This theory provided basis for various methods of pain relief
      • Massaging a painful area
      • Applying irritable substances to a painful area (counter-irritation)
      • Transcutaneous Electrical Nerve Stimulation (TENS)
      • Acupuncture ?
  • 28. Summary
    • Pain is not just a sensation but is a more complex phenomenon
    • Pain can be blocked at many places
    • Chemicals play an important role in causing pain as well as in reducing pain
    • Neural mechanisms also play a role in pain interaction
    • This complex nature of pain perception makes it a very difficult entity to control
  • 29.
    • “ Pain is a more terrible lord of mankind than even death itself”
    • Dr. Albert Schweitzer (1875-1965)