Pain Mechanisms

Pain Mechanisms

  • Nociception: Physiologic process that communicates tissue damage to the CNS
    • Involves 4 processes:
      • Transduction – occurs when there is release of chemical mediators
        • Noxious stimuli cause cell damage with the release of sensitizing chemicals
          • Prostaglandins
          • Bradykinin
          • Serotonin
          • Substance P
          • Histamine
        • These substances activate nociceptors and lead to generation of action potential
      • Transmission – involves the conduct of the action potential from the periphery (injury site) to the spinal cord and then to the brainstem, thalamus, and cerebral cortex
        • Action potential continues from
          • Site of injury to spinal cord
          • Spinal cord to brainstem and thalamus
          • Thalamus to cortex for processing
      • Perception – conscious awareness of pain
        • Conscious experience of pain
      • Modulation – involves signals from the brain going back down the spinal cord to modify incoming impulses
        • Neurons originating in the brainstem descend to the spinal cord and release substances (e.g., endogenous opioids) that inhibit nociceptive impulses

 

Classification of Pain

  • By underlying pathology
    • Nociceptive: Somatic or Visceral
    • Neuropathic: CNS or PNS Damage
  • By duration 
    • Acute
    • Chronic

Acute Pain

  • Sudden onset
  • Less than 3-month for normal healing to occur
  • Mild to severe 
  • Generally a precipitating event or illness can be identified
  • Manifestations reflect sympathetic nervous system activation:
    • Increased heart rate
    • Increased respiratory rate
    • Increased blood pressure

Chronic pain

  • Persistent pain and causes may be unknown
  • Gradual or sudden onset
  • More than 3-month duration; may start acute but continues past normal recovery time
  • Does not go away; characterized by periods of waxing and waning
  • Behavioral manifestations
    • Decreased physical movement/activity
    • Fatigue
    • Withdrawal from others and social interaction
  • Can be disabling and accompanied by anxiety and depression
  • Treatment goals
    • Control to the extent possible
    • Focus on enhancing function and quality of life