Physiology of pain

Question 1

List and describe the three dimensions of pain

Answer 1

Sensory: discriminative
• Relates to the location, intensity, duration and quality of pain

Motivation: affective:
• Conscious or unconscious drive for a person to initiate, sustain or direct behaviour
• Related to the unpleasant feeling associated with pain, e.g., feel unwell, depressed, etc
• You want to escape from the pain

Cognitive: evaluative:
• Pain is evaluated in terms of current and past experience
• Interpreted in relation to the actual situation, e.g. fearful dental patient – everything is painful!

Question 2

What are the three entities that may cause pain in the head and neck area?

Answer 2

• Intraoral pain conditions: e.g. tooth, periodontal tissues, tongue, salivary glands, may refer to other teeth and distant areas in head, neck and mandible – diagnostic dilemma
• Musculoskeletal pain disorders: Includes cervical spine disorders and TMDs – the most common non-dental orofacial pains

• Medical conditions masquerading as orofacial pain disorders: Pain source may not be the same as the pain site; differential diagnosis very difficult:
○ Intracranial disorders (e.g. neoplasia, haematoma)
○ Neurovascular disorders (e.g. primary headache, migraine)
○ Neuropathic pain disorders (e.g. trigeminal neuralgia, burning mouth syndrome)
○ Extracranial disorders (e.g. sinuses, eyes, ears, neck)

Question 3

What are the peripheral mechanisms of nociception and pain?

Answer 3

• Nociceptionis the neural processes of encoding and processing noxious stimuli
• Activation of nociceptors does not necessarily lead to pain
• Nociceptors are present as free-nerve endings. They are widely distributed in the skin, mucosa, and deep tissues
• Aβ fibres: touch and pressure fibres
• Aδ and C fibres: convey nociceptive information
• They convert a noxious stimulus into action potentials
• Nociceptors provide information about the location, intensity and duration of a noxious stimulus (sensory- discriminative dimension of pain)

Question 4

How do nociceptors work in physiological pain?

Answer 4

• Acute mildly tissue-damaging stimuli (heat, cold)

* Acts on ion channels, and the action potentials travel to the brain

Question 5

How do nociceptors work in inflammatory pain?

Answer 5

• Involves immune cells (mast cells, macrophages)
• These cells release chemicals like H+, interleukins and pro-inflammatory cytokines
• These chemicals bind to receptors which leads to the generation of action potentials, and 2nd messenger cascades
• These 2nd messenger cascades activate nociceptors (peripheral sensitisation)
• The nociceptors are now very active (more likely to fire off action potentials)
• Once activated, they respond to light touches where pain is generated from a non-painful stimulus (allodynia)
• They’ll also experience increased pain to a painful stimulus (hyperalgesia)

Question 6

What are the action of NSAID analgesics in pain

Answer 6

• NSAID analgesics are said to block the peripheral transmission of pain
• Aspirin and other NSAIDs have good analgesic effects in inflammatory pain
• Decrease peripheral sensitization process – reduce allodynia and hyperalgesia

Question 7

Describe the process of nociception and pain

Answer 7

• There are two types of afferent nociceptive nerve fibres
  
  • Aδ fibres: myelinated thus sharp, pricking pain that’s well localised
  • C fibres: unmyelinated, thus dull pain that is poorly localised. It is a secondary or slow pain
• The first order neurons travel from the trigeminal nerves to the brain stem to a place called “subnucleus caudalis”
• Here, these trigeminal neurones synapse with second order neurons which ascend to the thalamus
• Second order neurons synapse with third order neurons called Ventroposteromedial (VPM). * It is in the nucleus of the thalamus, which will discriminate where the pain is coming from in the body

Question 8

What is central sensitisation?

Answer 8

• Refers to changes in the excitability of 2nd order (and higher) neurones in the Trigeminal Brainstem Sensory Nuclear Complex
• This can occur because an intense barrage of action potentials come from the first order neurons
• This intensely depolarises the second order neurones, and so they may continuously fire action potential after actional potential
• May explain allodynia and hyperalgesia
• Continued central sensitisation may partly contribute to chronic pain

Question 9

What is referred pain?

Answer 9

• Referred pain is pain felt at a site distant from the site of injury
• Nociceptive afferents from different orofacial regions converge onto the same 2nd order neurone
• The brain gets confused as to which nociceptive afferent is the source of the problem
• Thus, pain source may not be the pain site

Question 10

Describe the modulation of pain

Answer 10

• Pain can be made to become more or less intense; modulated
• The nociceptive pathway can be turned off partly/ completely or opened up further
• Descending inhibitory pathways: PAG/ GABA will dampen the transmission of pain from the brain stem complex. OR, this can be amped up
• Also, when you get hurt, you rub that area. This activates the Aβ fibers, which will inhibit the pathways of the Aδ and C fibers, alleviating the pain a little
• Gate control theory