Physiology of pain 1 Flashcards
1
Q
Definition of pain
A
- Unpleasant sensory experience associated with tissue damage
2
Q
Classification of pain
A
Nociceptive - normal functioning of nociceptors
Inflammatory - Pain in response to inflammation
Neuropathic - Pain in response to injury to the nervous system
3
Q
What are nociceptors
A
- Are primary sensory neurons that detect pain
4
Q
Classification of sensory nerve fibres
A
- alpha and beta fibres
- A delta fibre
- C fibre
5
Q
Features of alpha and beta fibres
A
- Myelinated
- Large diameter
- Light touch, proprioception
6
Q
Features of A delta fibres
A
- Thinly myelinated
- Medium diameter
- Light touch, temp, nociception
7
Q
Features of C fibres
A
- Unmyelinated
- Small diameter
- Temperature, nociception
8
Q
What does it feel like when alpha delta fibres respond
A
- Sharp pricking pain
9
Q
What does it feel like when C fibres respond?
A
- Slow dull ache
- Burning pain
10
Q
Features of fast sharp pricking pain
A
- Well localised
- Activation of reflex arcs
11
Q
Localisation of slow dull aches
A
- Poorly localised
12
Q
Visceral pain and first response
A
- Visceral pain has no response
13
Q
What activates nociceptors
A
- Pressure
- Heat
- Cold
- Chemical
- Tissue damage/inflammation
14
Q
What is polymodality
A
- Polymodality is the feature of a single receptor of responding to multiple modalities, such as free nerve endings which can respond to temperature, mechanical stimuli (touch, pressure, stretch) or pain (nociception)
- Most C-fibre nociceptors are polymodal
15
Q
How is pressure detected
A
- Mechanically sensitive ion channels
- Not yet identified in eukaryotic cells
- Acid sensing ion channels
- Transient receptor potential(TRP) family of channels
16
Q
How is temperature detected
A
- Transient receptor potential family of channels
- Detect different temperatures
17
Q
Which tract does pain information ascend via
A
- Spinothalamic tract
18
Q
Where do first-order neurons enter and synapse in spinothalamic tract
A
- Enter dorsal horn
- Form tract of lissauer
- Synapse in substantia gelatinosa
19
Q
Path of second-order neurons in spinothalamic tract
A
- Cross in dorsal horn at each level
- Ascend in anterolateral column to thalamus
20
Q
How do we feel referred pain
A
- Convergence of visceral and cutaneous nociceptors on same second order neurons in spinal cord
- Brain perceives pain as cutaneous
21
Q
Referred pain for angina
A
- Angina perceived as pain in upper chest wall and left arm
22
Q
Path of third-order neurons in spinothalamic tract
A
- Ascend to primary somatosensory cortex
- Projections to insula and cingulate cortex
- Encode the emotional components of pain(unpleasantness, negative-affect)
23
Q
What regions of the brain are activated in the pain network
A
- Many cortical regions activated(including limbic system, prefrontal cortex)
24
Q
What are the two important regions of the descending regulation pathway of pain
A
- Periaqueductal gray matter(PAG)
- Rostral ventromedial medulla(RVM)
25
Purpose of the descending regulation of pain pathway
- Modulates activity of spinothalamic tract
26
How is pain inhibited by the action of the descending regulation of pain pathway
- Cortical regions project to PAG
- PAG projects to RVM
- RVM projects to dorsal horn
- PAG neurons excite RVM neurons, which inhibit(or excite) the spinothalamic tract
27
Where do serotonergic projections act in the descending pain regulation pathway
- Act on dorsal horn inhibitory interneurons
28
Role of opioids in the descending inhibition of pain
- Opiods are inhibitory
| - Act on inhibitory metabotropic receptors
29
Where are opioids released
Released from interneurons at multiple sites
- Midbrain(Periaqueductal gray matter)
- Medulla(Rostral ventromedial medulla)
- Dorsal horn
30
Effect of ATP on nociceptors
- ATP binds to purinergic receptors(P2X)
31
Effect of H+ on nociceptors
- Activates acid-sensing ion channels by binding to them
32
Effect of serotonin on nociceptors
- Binds to 5-HT3 receptors(activation)
33
What does switching nociceptors cause and give an example
- Switch on = PAIN
| - In runners, lactic acid build up leads to tissue acidosis(increase in protons) = activation of nociceptors = PAIN
34
What is neurogenic inflammation
- Inflammation arising from the local release by afferent neurons of inflammatory mediators
35
Process of neurogenic inflammation
- Activation of one branch of a nociceptor axon, triggers release of substance P and CGRP from another(leads to cascade)
- Release of histamine causes more inflammation
36
Causes of histamine release in neurogenic inflammation
- Vasodilation
- Increased permeability
- Activation of mast cells
37
What is an effect of inflammation that helps with repair
- Inflammation can cause pain hypersensitivity
| - Helps after an injury as it ensures that contact with the injured tissue is minimized until repair is complete
38
What is allodynia
- Non-noxious stimuli produce a painful response
39
What is hyperalgesia
- Noxious stimuli produce an exaggerated pain response
40
What is peripheral sensitization
- Increase in responsiveness of the peripheral ends of nociceptors
41
What is peripheral sensitisation driven by
- Driven by tissue injury or inflammation
- Bradykinin and NGF reduce the threshold of heat activated channels(TRPV1)
- Prostaglandin reduces the thresholds of sodium channels
42
Common example of peripheral sensitisation
- Sunburn
43
Mechanism of action - TRPV1
Bradykinin
- Binds to receptor(metabotropic - G protein-coupled)
- Activation of protein kinase phosphorylates TRPV1
- Phosphorylation of channel reduces its threshold(ie it fires more easily)
44
TRPV1 agonist and temp
- Chilli(capsaicin)
| - HOT
45
TRPM agonist and temp
- Menthol
| - Cold
46
TRPA1 agonist and temp
- Cinnamon
| - V Cold
47
What do first order neurons in the spinothalamic tract release to excite second-order neurons
- Glutamate and substance P to excite second-order neurons
48
Cortex for lower body projections in spinothalamic tract
- Medial somatosensory cortex
49
Cortex for upper body projections in spinothalamic tract
- Lateral somatosensory cortex
50
How does descending regulation of pain occur
- Stress-induced analgesia
| - eg: battle victims with no pain
51
Inhibition of pain pathway from RVM to dorsal horn
RVM --serotonergic neuron --> inhibitory interneuron --> nociceptor in dorsal horn
