Pain

What is pain?

Pain is an unpleasant sensory and emotional experience associated with potential or actual tissue damage. Nociception is the detection of a noxious stimulus by the brain, and nociceptive pain is that caused by stimulation of peripheral pain nerves called nociceptors.


What do nociceptors detect?

Nociceptors may detect or or more of the following stimuli:

  • Chemical mediators
  • Histamine
  • Serotonin
  • H+ ions
  • Potassium
  • Substance P
  • Mechanical stress
  • Thermal damage

How can nociceptive pain be classified?

Nociceptive pain can be classified as either somatic or visceral. Somatic pain is well localised and can be deep or superficial:

  • A fractured bone causes deep somatic pain
  • Skin abrasions cause superficial somatic pain

Visceral pain is generally poorly localised, and often referred to other dermatomes whose somatic pathways share the same spinal roots.

  • Diaphragmatic irritation often presents with shoulder tip pain
  • Appendix pain presents as diffuse, visceral, referred lower abdominal pain around the umbilicus (T10), before localising to the right iliac fossa when the peritoneum is irritated, causing somatic pain

Alternatively, nociceptive pain can be categorised by its cause, such as inflammatory, traumatic, ischaemic.


What is chronic pain?

  • Pain that continues after the cause has been removed, beyond the expected recovery time
  • Some say it should be three months, but it is variable

What is neuropathic pain?

This is pain that is caused by a dysfunction within the nervous system itself, rather than pain caused by an external damaging stimulus. The patient is feeling pain, and it can be horribly debilitating, despite there being no actual tissue damage.

Neuropathic pain may demonstrate the following features:

Hyperalgesia

Disproportionately severe pain response to a stimulus that is normally only slightly or moderately painful.

Allodynia

A sensation of pain to a normally painless stimulus. Trigeminal neuralgia can result in excruciating pain when the face is subjected to even light touch.

Phantom pain

Pain felt in a part of the body that is absent, such as an amputated leg.


What types of pain fibres are there?

  • Aδ fibres
    • Small myelinated fibres diameter up to 5μm
    • Sharp pain
    • 50 m/s conduction velocity
  • C fibres
    • The smallest fibres less than 2μm
    • Dull pain
    • 2 m/s conduction velocity
  • Both enter the dorsal horn in the spinal cord to synapse with higher order neurons before travelling to the substantia gelatinosa

Describe the gate control theory of pain

  • Pain signals are transferred from the peripheries to the brain via ‘gates’ in the substantia gelatinosa of the spinal cord
    • This consists of an inhibitory interneurone
      • The Aβ fibre stimulates the interneurone, reducing transmission of the pain signal to the brain from the C fibre
      • The C fibres inhibit the interneurone, and increase pain signal transmission

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