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Module 202 Theme 2 > Physiology of pain 1 > Flashcards

Physiology of pain 1 Flashcards

1
Q

Why do we feel pain?

A

Warning sign - survival
Avoid harmful situations
Prevents further injury or death
Tells us to rest following an injury

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2
Q

What are the sensations of pain?

A 
Burning
Stabbing 
Sharp
Radiating
Deep ache
Freezing
Itch
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3
Q

What are the 3 classifications of pain?

A

Nociceptive - normal functioning of nociceptors

Inflammatory - pain in response to inflammation

Neuropathic - Pain in response to injury of nervous system

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4
Q

What are nociceptors?

A

Primary sensory neuron that detects pain
Pseudounipolar - cell body in dorsal root ganglion
From skin, muscle, joints, viscera, meninges to the dorsal horn

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5
Q

How are sensory nerve fibres classified?

A

By diameter and myelin content
Aalpha and Abetea fibres
Adelta fibres
C fibres

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6
Q

Describe Aalpha and Abeta nerve fibres

A

Myelinated
large diameter
light touch and proprioception

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7
Q

Describe Adelta fibres

A

Thinly myelinated
Medium diameter
Light touch, temperature and nociception

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8
Q

Describe C fibres

A

Unmyelinated
Small diameter
Temperature
Nociceptive

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9
Q

What do meissner’s detect?

A

Stroking/fluttering

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10
Q

What do pacinicans detect?

A

Vibration

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11
Q

What do merkel’s discs detect?

A

Pressure

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12
Q

What do ruffini detect?

A

Stretch

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13
Q

What does it feel like when Adelta fibre nociceptors respond?

A

Sharp pricking pain

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14
Q

What does it feel like when C fibre nociceptors respond?

A

Slow dull ache

Burning pain

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15
Q

Name the peaks found on an electrophysiology recording of the whole sensory neuron

A

Aalpha Abeta
Adelta
C fibres

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16
Q

Describe the two pain responses

A

Fast sharp pricking pain - well localised, activation of reflex arcs (Adelta)

Slow dull ache - poorly localised (fibres)

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17
Q

What activates nociceptors?

A

Pressure

Heat

Cold

Chemical

Tissue damage/inflammation

18
Q

Which type of pain has no first response?

A

Visceral pain

19
Q

What is meant by polymodal nociceptors?

A

Most C fibres

Respond to pressure, temperature and chemical

20
Q

Describe the transduction mechanism of pressure

A
  • Mechanically sensitive ion channels
  • Not yet identified in eukaryotic cells
    Potential channels - acid sensing ion channels, Transient receptor potential family of channels
21
Q

Describe the transduction mechanism of temperature

A
  • Transient receptor potential family of channels

- Detect different temperatures

22
Q

Describe the central pain pathway

A

Pain information ascends the spinothalamic pathway

23
Q

What is TRPV1?

A

Vanilloid subtype responds to heat (eg. chilli - capsaicin is and agonist)

24
Q

Describe the spinothalamic tract

A

First order neuron - Enter dorsal horn, form tract of lissauer, synapse in substantia gelatinosa (lamina 1 and 2)
Glutamate and substance P excites second order neurons

Second order neurons - Cross in the dorsal horn in each level, ascend in anterolateral column to the thalamus

Third order neurons - Ascend to primary somatosensory cortex - encode the sensory components (tells you where it hurts and modality) Lower body to medial cortex and upper body to lateral cortex. Projections to insula and cingulate cortex. Encodes the emotional components of pain (unpleasantness, negative effect)

25
Q

What is referred pain?

A

Convergence of visceral and cutaneous nociceptors on same second order neurons in spinal cord
Brain perceives the pain as cutaneous

26
Q

Describe the descending regulation of pain

A

Stress induced analgesia - battle victims with no pain

Higher cortical regions can activate descending modulatory pathways

27
Q

Name the two important regions in downregulation of pain

A

Periaqueductal gray matter (PAG)
Rostral ventromedial medulla (RVM)

Cortical regions project to PAG
PAG projects to RVM
RVM projects to dorsal horn

Modulates activity of spinothalamic tract

28
Q

How is pain inhibited?

A

PAG neurons excite RVM neurons which inhibit (or excite) the spinothalamic tract
Serotonergic projections act on dorsal horn inhibitory interneurons
Also parallel noradrenalin pathway

29
Q

Describe the endogenous opioid system

A

Opioids play an important role in the descending inhibition of pain
(E.g. Endorphins, enkephalins)

Especially stressed-induced analgesia

Opioids are inhibitory
- Act on inhibitory metabotropic receptors

Released from interneurons at multiple sites:

  • Midbrain (Periaqueductal gray matter)
  • Medulla (Rostral ventromedial medulla)
  • Dorsal horn
30
Q

Name some chemicals released as part of tissue damage in activating or modulating nociceptors

A 
ATP
H+
Serotonin/5-HT
(from platelets)
Histamine 
(from mast cells)
Bradykinin
Prostaglandin 
(conversion of arachidonic acid (lipid) by cyclooxygenase (COX) enzymes)
Nerve growth factor
31
Q

What is the effect of ATP on nociceptors

A

Binds to the purinergic receptors P2X
Switch on
Cause pain

32
Q

What is the effect of H+ on nociceptors

A

Binds to acid sensing ion channels
Switch on
Cause pain

33
Q

What is the effect of serotonin on nociceptors

A

Binds to 5HT3 receptors
Switch on
Cause pain

34
Q

Describe neurogenic inflammation

A

Activation of one branch of a nociceptor axon, triggers release of substance P and CGRP from another
Causes - Vasodilation, increased permeability, activation of mast cells release of histamine
More inflammation

35
Q

What can inflammation do to pain?

A

Cause pain hypersensitivity - pain is more painful
“Pain hypersensitivity after an injury helps healing by ensuring that contact with the injured tissue is minimized until repair is complete”

36
Q

What is Allodynia?

A

Non-noxious stimuli produce a painful response

37
Q

What is Hyperalgesia

A

Noxious stimuli produce an exaggerated pain response

38
Q

What are the mechanisms of pain hypersensitivity?

A 
Peripheral sensitisation ( hyperalgesia)
Central sensitisation (hyperalgesia and allodynia) - mechanism in neuropathic pain
39
Q

What is peripheral sensitisation?

A

Increase in responsiveness of the peripheral ends of nociceptors
Driven by tissue injury or inflammation -

40
Q

What chemicals are involved in peripheral sensitisation?

A

bradykinin and NF reduce the threshold of heat activated channels
Prostaglandin reduces threshold of sodium channels

41
Q

Give a common example of peripheral sensitization

A

Sunburn

42
Q

Describe the action of bradykinin

A
  • Binds to receptor (metabotropic – G protein-coupled)
  • Activation of protein kinase phosphorylates TRPV1
    Phosphorylation of channel reduces its threshold (i.e. it fires more easily)

Module 202 Theme 2 (12 decks)

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