HUF 2-51 Perception of pain

Question 1

Nociception

Answer 1

• Reception of noxious signals
  => Activation of nociceptors (pain receptors)
• Nociception w/o pain

Question 2

Pain

Answer 2

• Subjective perception of unpleasant feeling as a result of nociception
• Complex interplay between signalling systems, modulation from higher centres and unique perception of individual
• Pain w/o nociception

Question 3

Classification of pain

Answer 3

Duration:

• Acute
• Chronic

Nature
- Nociceptive
> Somatic
> Visceral
- Non-nociceptive
> Neuropathic
> Psychogenic

Question 4

Properties of nociceptors

Answer 4

1. High threshold
   - Thermonociceptors: >45 °C
   - Mechanonociceptors: >60 g/cm2
2. Polymodal (thermo, mechano, chemical…)
3. TRPV1
   - Transient Receptor Potential Vanilloid Type 1
   - Transduction of noxious signals in terminals of pain fibres
4. Free n. endings in skin, vessel walls, CT…
5. Sensitized by chemicals as a result of tissue damage
   e. g. BK, 5-HT, PG, K+, Histamine (from mast cells, CGRP, SP),
   * CGRP, SP dilates BV

Question 5

Nociceptive primary aff. fibres

Answer 5

1. Aδ
   - Thin-myelinated (fast)
   - Sharp, fast (first) pain
2. C
   - Unmyelinated (slow)
   - Slow, delayed (second) pain; chronic pain

Question 6

Nociceptive aff. and gating in spinal cord

Answer 6

Aδ => Lamina I, V
C => Lamina II

Aβ => Lamina IV (non-noxious fibres; gating of pain)

Gating theory of pain: Aβ activates inhibitory interneuron of projection neuron
∴ Aβ blocked => pain

2° neurons:

1. Nociceptive specific
2. Wide dynamic range (noxious and non-noxious)

Question 7

Spinothalamic tract

Answer 7

Decussate in midline of spinal cord
=> Join with those from trigeminal nu. (CN5; face)
=> CL nu., VP nu. of thalamus (process sensory info)
=>
1. Somatosensory cortex (location of pain)
2. Association cortical areas (insular cortex, cingulate cortex, prefrontal cortes)
=> unpleasant feeling and emotional aspects of pain

Question 8

Spinoreticular tract

Answer 8

Decussate in midline of spinal cord
=> Reticular formation of medulla and pons (additional relay / integration centres)
=> Somatosensory and association cortex

Question 9

Spino-mesencephalic tract

Answer 9

Terminate in periaqueductal gray matter in midbrain

=> Descending control of pain perception

Question 10

Projected pain

Answer 10

• Pain which is not originated from nociceptors
• Site that the noxious agent act is not where the pain is sensed
  e.g.
  1. Direct mechanical stimulation at elbow
  => Discharge from ulnar n.
  => Projected into hand

2. Displaced intervertebral disk
   => Compressed fibre impulses
   => Pain projected onto area innervated by spinal n.

Question 11

Referred pain

Answer 11

• Nociceptive stimulation of viscera produces sensation of pain not in affected organ
• Pain in distinct, superficial part of body
• N. supplying referred region (somatic) and affected organ (viscera) converge onto same spinal neuron
  => Brain learns from experience that signal is more likely from somatic aff.

e.g. Esophagus, heart, urinary/bladder, left ureter, Right prostate

Question 12

Central control of pain

Answer 12

• Descending system suppressing transmission of pain

# Periaqueductal gray (midbrain)
# Locus ceruleus (pons; NA)
# Nucleus raphe-magnus (medulla; 5-HT)
=> Dorsolateral funiculus

• Endogenous opioid peptides (e.g. enkephalin, endorphin, dynorphin)
• Endogenous opioid receptors can be activated by morphine

Question 13

Pain suppression by opioid peptides

Answer 13

1. Presynaptic action
   - Block Ca2+ influx into n. terminals
2. Postsynaptic action
   - Open K+ channels
   => Hyperpolarisation (less excitable)

Mimicked by local injection of morphine into spinal cord
e.g. Caesarean section

Question 14

Management of pain

Answer 14

1. Pharmacological
   - Non-narcotic analgesic: NSAID, Paracetamol
   - Narcotic analgesic: Morphine (+ analogues), which bind opiate receptors of endogenous pain control system
   - Psychological drugs: Barbiturates
   - Local anaesthetics
2. Physical
   - Heat: activate thermoceptors, ↑ circulation, ↓ metabolic waste
   - Cold: ↓ inflammation
   - Massage: Aβ
   - Electrical stimulation: ↑ descending inhibitory pathway
   - Neurosurgery: interruption of ascending pain pathway
3. Psychological methods
   - Cognitive behavioural therapy
   - Biofeedback
   - Relaxation
   - Imagery
   - Hypnosis