Pain & Nociception

  • By: Terri
  • Date: November 4, 2010
  • Time to read: 4 min.

What is pain?

  • It is an unpleasant sensation, with a protective function
  • Definition:
    • unpleasant sensory & emotional experience associated with actual/potential tissue damange

What influences the pain you feel?

  • Individual coping strategies
  • Attitudes
  • Previous pain experience
  • Cognitive understanding
  • Cultural factors
  • Current emotional state


  • Sudden
    • Mechanical
    • Vascular
    • Traumatic
    • Toxic
  • Gradual
    • Infective
    • Inflammatory
    • Neoplastic
    • Degenerative
    • Endocrine
    • Nutritional

Aggravating/Relieving factors

  • Movement
  • Respiration
  • Ingestion (food)
  • Type
  • Posture

Character/ Quality of pain

  • Dull, aching
  • Burning
  • Gnawing
  • Colicky
  • Crampy
  • Throbbing
  • Stabbing
  • Sharp, tearing
  • Radiating
  • Gripping
  • Shooting
  • Excruciating
  • Sharp lancing

Types of pain

  • Sharp/Bright VS Burning/Dull
  • Fast VS Slow pain
  • A-delta fibres VS ‘C’ fibres
  • Withdrawal VS Immobilization
  • Cutaneous VS Visceral pain
Cutaneous Visceral
Sharp Dull,aching
Well-localised Poorly localised
Not associated with ANS symp Associated with ANS symp
Not referred to other site Freq referred to other site

Referred pain




  • Cardiac pain
    • pain in the chest, axilla, arm (T1, T2)
  • Gall bladder pain
    • pain in the shoulder (C3, C5)
  • Pain of appendicitis
    • pain in umbilicus
  • Ureteric pain
    • pain in testes (loin)
  • Irritation of nerve endings in meninges and cranial vessels
    • headache


  • Dermatomal rule
    • Pain is referred to a structure, which is developed from the same dermatome from which the pain producing structure is developed.
Convergence theory
  • afferents from skin and visceral structure converge on the projection neuron and impulses generated by them ascend in the same tract – but the brain has ‘learnt’ that impulses usually come from the skin and interpret as such
  • Facilitation theory
    • A collateral from the visceral afferent converges on the projection neuron, together with the cutaneous afferent, partially depolarizing the projection neuron
    • A subthreshold stimulus (eg light touch) from the cutaneous neurone can produce an action potential
    • image


      Postinjury & Neuropathic pain

      • Persists while the injury heals
        • when the nerves are intact
      • Neuropathic pain
        • Persists even after the injury has healed
        • if the nerves are damaged
        • Difficult to manage, resistant to analgesic
      • Hyperalgesia
        • Minor pain can produce an exaggerated response
      • Allodynia
        • Innocuous pain such as touch can produce pain


      • Brain itself has very few pain receptors
        • but structures nearby have pain receptors
          • blood vessels at base of brain
          • meninges
          • muscles of scalp
          • neck & face
          • paranasal sinuses
          • eyes & teeth
      • Mediated by
        • mechanical receptors
          • eg stretching
        • chemical receptors
          • eg histamine, serotonin
      • Impulses carried by
        • 5th & 9th cranial nerves
          • by upper cervical cranial roots
      • Types of headache
        • Chronic/recurrent (benign)
          • muscle tension/ tension headache
        • Subacute (over days/weeks)
          • serious intracranial disease
            • encephalitis
            • viral/chronic meningitis
        • Single, severe episode
          • subarachnoid haemorrhage (SAH)
          • migraine
          • acute meningitis
        • Pressure headaches
          • headaches of raised intracranial pressure
          • intracranial mass lesions displace and stretch the meninges & basal blood vessels
            • pain is provoked when these structures are moved physically either by the mass/by raised ICP
            • made worse by coughing, straining, sneezing

      Pain mechanism/process

      • Stimulus
        • chemical/mechanical/thermal/biological
      • Pain producing substances (PPS)
        • kinins
        • K+
        • histamine
        • substance P
        • ANG II
        • CCK
        • NE
        • Capsaicin
      • Receptor
        • free nerve endings
          • non specific (no adequate stimulus – stimulus for which the receptors are most specific)
          • adaptation – very little/none
            • image
          • Types
            • Mechanoreceptors
              • in skin
              • definite threshold levels
              • respond to strong stimuli (pin prick, hot temp)
              • fast pain
              • warming signal – withdrawal from noxious stimulus
            • Polynodal nociceptors
              • present in almost all tissues
              • no definite threshold level
              • respond to tissue damange
              • slow pain
              • pain causes immobilization of affected part
                • to prevent further damage
      • Sensory nerves (afferents)
        • A-delta fibres
          • via lateral spinothalamic tract
          • together with temperature sense
        • C fibres
          • via spinoreticulothalamic tract
      • Perception
        • via thalamus
      • Postcentral gyrus (Somatic sensory area 1 – SSA1) & SSA2
        • integration centre
          • postcentral gyrus (SSA1)
          • somatic sensory area 2 (primary & secondary sensory homunculi) / cerebral cortex

      Pain pathways
      Read from Dr Joachim’s notes on the pathways

      • Neospinothalamic tract
      • Paleospinothalamic tract

      1) Neospinothalamic tract (lateral pain system)


      • Afferent: A-delta fibres
      • To SSA1 & SSA2 of opposite side
        • via lateral spinothalamic tract





      2) Paleospinothalamic tract (Medial pain system)


      • Afferent: C fibres
      • Goes to limbic cortex (emotions)
        • via spinoreticulothalamic pathway


      Sensory homunculus

      In somatic sensory area 1


      • Postcentral gyrus (Broadmann’s area 1, 2, 3)

      In somatic sensory area 2


      • In Sylvian fissure/lateral sulcus


      Subcortical perception & affect

      • pain can be felt in the absence of the cerebral cortex
        • but it is required for discriminative, meaningful & exact interpretation & some of the emotional components
        • can feel, but can’t understand it
      • the built-in unpleasant affect is mediated by the cingulate gyrus

      Role of thalamus

      • Understand type, cause, site, amount of pain
      • Meaningful interpretation of pain is lost in thalamic syndrome
        • damage to posterior thalamic nuclei
        • attacks of prolonged, severe, extremely unpleasant pain
          • spontaneous/in response to trivial stimuli

      Chronic pain syndrome – Phantom limb

      • evokes a sensation which is projected to the site of receptor
        • eventhough it may no longer exist
        • eg. amputated limb


      • spontaneous burning pain long after seemingly trivial injuries
      • often accompanied by
        • hyperalgesia
          • abnormally increased pain sense
        • allodynia
          • Pain from stimuli which are not normally painful
      • associated with reflex sympathetic dystrophy
        • shiny skin in the affected area
        • increased hair growth
        • overgrowth of NA sympathetic fibres
      • relieved by alpha adrenergic blockade


      Modulation of pain


      Gatecontrol Theory

      • SG cells in dorsal horn
      • They inhibit impulse transmission in the projection neuron
      • A-delta and A-beta fibres stimulate it
        • stimulation of inhibition!
        • Therefore: inhibition of impulse transmission
      • C fibres inhibit it
        • inhibition of inhibition!
        • Therefore: no inhibition of impulse transmission
        • therefore pain lasts longer


      • Endogenous opioids
        • Enkephalins, dynorphins
        • a type of analgesic



      • NMDA receptor blockers
        • esctasy pill


      • Analgesics
        • narcotics
        • NSAIDs
        • Opiates
        • NMDA receptor inhibitors


      Pain surgery




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