Mechanisms of Pain Processing

Mechanisms of Pain Processing

Mechanisms of Pain Processing 

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Pain Perception

  • The sensation of pain is increased by:
    • Discomfort, fatigue, fear, safness, boredom, insomnia, anxiety, anger, depression, loneliness
  • The sensation of pain decreased by:
    • Sleep, sympathy, companionship, rest, understanding, diversional activities, antidepressants (if appropriate)
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Definitions

  • Pain threshold (pain perception threshold)
    • the point at which pain begins to be felt
  • Pain tolerance level
    • the greatest level of pain that a subject is able to tolerate
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Pain Sensation

  • Pain sensation originates from specific nociceptors
  • Pain can follow injury or occur spontaneously in the CNS (Central Nervous System)
  • Chemicals released when there is tissue injury activate nociceptors in the skin, e.g. bradykinin, histamine, 5-HT (serotonin), and inflammatory mediators
  • These project into the CNS
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Fast and Slow Pain

  • Fast/first pain - small myelinated Ad fibres
    • Usually transient, a sensation of sharp, well-localised pain
    • Triggers reflex to move away from source of pain
  • Slow/second pain - non-myelinated C fibres
    • Less easily localised low conduction velocities cause a dull burning pain
    • Evokes autonomic responses such as elevated heart rate/blood pressure/sweating
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Pain Levels

Pain is generated at 3 different levels:

  • Nociception - peripheral activation and release of pain mediators
    • Primary nociceptors, C and Ad fibres
  • Pain gating - dorsal horn of the spinal cord
    • Ascending and descending secondary fibres
  • Pain perception - the brain (thalamus, limbic and cortical systems)
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Influences on Pain

  • Pain pathways can be influenced by other events to change the sensation
  • Pain receptors are located near touch receptors
  • Electrical activation of these receptors can also help (Transcutaneous Electrical Nerve Stimulation)
  • Stress or adrenaline can reduce pain sensation
  • Descending pathways are activated by these processes that send inhibitory  signals
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Pain Sensation

  • Pain is sensed by chemical messengers activating peripheral nerves
  • Pain is distinguished from other sorts of mechanical and thermal receptors by their higher threshold, since they are normally activated only by stimuli of noxious intensity
  • Noxious stimulus: a noxious stimulus is one that is potentially or actually damaging to body tissue
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Types of Pain

Acute

  • Usually rapid onset
  • Usually identifiable cause
  • Limited duration
  • Tends to get better
  • Biological significance

Chronic

  • Not necessarily an identifiable cause
  • Large emotional component
  • Unpredictable duration
  • Does not get better
  • None/negative meaning
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Origin

  • Nociceptive - stimulation of receptors that respond to heat, vibration, stretch and chemical stimuli released from damaged cells
    • Somatic (e.g. rheumatoid arthritis) or visceral (NSAIDs, weak/strong opioids)
  • Non-nociceptive/neruopathic - from within peripheral and central nervous system. No specific receptors, but nerve cell dysfunction
    • Central or peripheral (e.g. diabetic neuropathy, trigeminal neuralgia)
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Somatic Pain

  • Source - nerves that detect somatic pain are located in the skin and deep tissues
  • Receptors - these specialised nerves, called nociceptors, pick up sensations related to temperature, vibration and swelling in the skin, joints and muscles
  • Characteristics - sharp, well-localised
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Visceral Pain

  • Source - internal organs of main body cavities, e.g. heart, lungs, liver, kidney, bladder, ovaries
  • Receptors activated - receptors for stretch, inflammation and oxygen starvation
  • Characteristics - poorly localised, vague deep ache
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Neuropathic Pain

  • Source - from within the nervous system itself - peripheral or central nervous system
  • Receptors activated - no specific receptors, nerve becomes injured and becomes electrically unstable, firing off signals at random
  • Characteristics - signals interpreted as pain, hypersensitivity, tingling, numbness and weakness
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Examples of Pain

  • Somatic - tension headache, osteoarthritis, toothache, sprained ankle, sore throat, burn
  • Visceral - colic, angina, appendicitis, peritonitis, pleuritis, irritable bowel sydrome
  • Neuropathic - phantom limb pain, trigeminal neuralgia, sciatica, shingles
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