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The perception of pain

The perception of pain . Ching -Liang Lu. What is 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. What is pain?. Pain is

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The perception of pain

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  1. The perception of pain Ching-Liang Lu

  2. What is pain? Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damageIASP – International Association for the Study of Pain 2009

  3. What is pain? • Pain is • subjective • not a stimulus, but an experience • protective (acute) • modified by developmental, behavioural, personality and cultural factors • Associated signs are crying, sweating, increased heart rate, blood pressure, behavioural changes etc

  4. Pain • Dimensions of the ‘PAIN” • Sensory-discriminative • Affective-motivate • Cognitive

  5. Nociceptive vs Neuropathic Pain Nociceptive Pain Caused by activity in response to potentially tissue-damaging stimuli Neuropathic Pain Initiated or caused by primary lesion or dysfunction in the nervous system Phantom pain Postherpeticneuralgia Pos postoperative pain toperativepain Trigeminalneuralgia Arthritis Sickle cellcrisis Neuropathic low back pain Mechanicallow arthritis,back pain Central post-stroke pain Distalpolyneuropathy(eg, diabetic, HIV) Sports/e sports or exercise injuries xerciseinjuries *Complex regional pain syndrome

  6. Nociceptive pain: Tissue injury/inflammation

  7. Postherpetic neuralgia

  8. Acute vs Chronic Pain

  9. What is Acute Pain? • Physiologic response to tissue damage • Warning signals damage/danger • Helps locate problem source • Has biologic value as a symptom • Responds to traditional medical model • Life temporarily disrupted (self limiting) Acute pain is not that bad.

  10. What is Chronic Pain? • Chronic pain is persistent or recurrent pain, lasting beyond the usual course of acute illness or injury, or more than 3 - 6 months, and adversely affecting the patient’s well-being • Pain that continues when it should not

  11. Social Consequences Marital/family relations Intimacy/sexual activity Social isolation Socioeconomic Consequences Healthcare costs Disability Lost workdays Domains of Chronic Pain Quality of Life Physical functioning Ability to perform activities of daily living Work Recreation Psychological Morbidity Depression Anxiety, anger Sleep disturbances Loss of self-esteem

  12. Pain measurement

  13. Nociceptors • 3 classes of nociceptors • Mechanical: pinch, punctate, squeeze • Thermal: above 45C or below 5C • Polymodal: mechanical, thermal, chemical

  14. Somatic receptor types

  15. Thermosensation • Heat • DRG • 45% small- to medium-diameter neurons: threshold of ∼42°C  heat-sensitive C and type II AδfibresTRPV-1 receptor • 5-10% medium- to large-diameter cells: threshold of ∼51°C type I AδTRPV-2 receptor • TRPV-3 TRPV -4: warm range 31∼39°C

  16. Thermosensation • Cold • TPRVM8 receptor: ∼26°C ; (cool to cold) express in 10-20 % of small-diameter neuron • TRPA1: ∼17°C painful cold ?

  17. TRP channel family

  18. Mechanosensation • Mechanoreceptor: • ? Yet to be identified in mammalian • non-mammalian • Bacteria: MscL, MscS, DEG/EnaC ion channel, • Yeast, flies, worms: TRP channel • Fly, Zebra fish: NOMPC (TRP channel)

  19. AFFERENT (SENSORY) NEURON

  20. Nociceptive afferents First pain: Sharp, faster A-delta fibers Second pain: Dull, slower C-fibers Blocking each nerve blocks the sensation Compound Action Potential

  21. SPINAL CORD LAMINAE Dorsal horn

  22. Lamina of spinal cord

  23. Referred pain • Signals from muscles and viscera can be felt as pain elsewhere • Example: myocardial infarction and angina can be felt in chest and left arm • Mechanism: convergence of afferents muscle/ viscera afferents and somatic afferents. • Convergence on the same projection neurons in the dorsal horn • The brain cannot tell the difference

  24. Neurotransmitters • Fast synaptic potentials • Glutamate (amino acid, excitatory) • Efficient reuptake of amino acids • Range: postsynaptic neurons in vicinity • Slow synaptic potentials • Neuropeptides e.g. Substance P(excitatory), GABA (inhibitory) • No reuptake mechanisms • Range: diffusion, many neurons, unlocalized nature of pain • Other Neuropeptides • Released and increased in persistent pain conditions • Enhances and prolong the actions of glutamate

  25. Substance P in dorsal horn

  26. Pain modulation below spinal level • PERIPHERAL • CENTRAL • Peripheral (Sensitization) TISSUE INJURY RELEASE OF SUBBSTANCE-P CHEMICAL MEDIATORS OF AND GLUTAMATE INFLAMMATION STIMULATE NOCICEPTORS IN THE PERIPHERY

  27. Peripheral Sensitization NSAID

  28. Peripheral sensitization 53C 30 sec at site A&D

  29. “Wind-up” Response to second stimulus is stronger than the response to the first one in C fiber Change in temporal integration Central sensitization for chronic pain:wind-up

  30. "Wind-up" is NMDA mediated. Response with and without an NMDA antagonist.

  31. Spinal microglia activation in the chronic visceral pain (chronic pancreatitis) Liu PY, Lu CL. Gastroenterology 2012; 142: 165-73.

  32. Spinal microglia in visceral pain I.T. tetracycline (microglia inhibitor) TNBS Liu PY, Lu CL. Gastroenterology 2012; 142: 165-73.

  33. Ascending pathwaysSpinal cord ThalamusCortex

  34. Ascending pathwaysSpinal cord ThalamusCortex • Thalamic nuclei • Lateral nuclear group: spinothalamic tract, NS and WDR, laminae I and V, small receptive fields, encoding location of injury •  Infarction  Thalamic syndrome (spontaneous burning pain, allodynia) • Medial nuclear group:spinoreticulothalamic tract, laminae VII and VIII

  35. New pathway for visceral pain:  Dorsal column pathway

  36. Lateral system: S1/2 Discrimitive component Thalamus Medial system: Affective component

  37. PFC PFC PFC ACC ACC PCC PPC PPC Insula PCC PCC Thalamus Insula Insula Vermis Amygdala Vermis Neuroimaging of acute pain Muscle pain Cutaneous pain Visceral pain Chen et al. Tooth pain Lu et al., 2004 Lin et al. (preliminary) Niddam et al., 2002

  38. Somatosensory area

  39. Cortical neuronal correlates upon balloon distension in GI tract (volunteers) Esophagus Rectum Antrum Fundus Mertz H. Gastroenerology 2000; 118:8342 Ladabaum U. Gastroenerology 2001; 120:369 Aziz Q. Gastroenerology 1997; 113:50 Lu CL Neurogastroenterol 2004;16:575 Gastroenerology 2005; 128:1529

  40. Brain  Pain perception No Brain, No Pain Rene Descartes (1596-1650)

  41. Pain modulation(Central) • Gate control theory • Descending inhibition

  42. Gate control theory • P.D. Wall & Melzack (1965) • “There is an interaction between pain fibresand touch fibre input at the spinal cord level in the form of a ‘gating mechanism’

  43. pain is felt + pain gate is opened Gate control theory When pain fibre is stimulated, gate will be opened & pain is felt

  44. Gate control theory touch - + pain gate is closed pain is not felt When pain and touch fibres are stimulated together, gate will be closed & pain is not felt

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