6.4 Pain Pathways Flashcards
Aβ
- Diameter (mm): 5-10
- Conduction velocity (m/s): 40-75
- Myelination: ___________
- Modality: _______________
- Threshold: _______
Thick;
Mechanoreception (light touch), Proprioception, Nociception (sometimes in chronic pain);
Low
Aδ
- Diameter (mm): 2-5
- Conduction velocity (m/s): 10-35
- Myelination: _________
- Modality: __________
- Threshold: ________
Thin;
Thermoreception (cold), Mechanoreception, Nociception;
Intermediate
C
- Diameter (mm): 0.5- 2
- Conduction velocity (m/s): 0.5 -2
- Myelination: _________
- Modality: __________
- Threshold: _______
None;
Thermoreception (warm), Nociception;
High
In the epidermis, dermis and subcutaneous tissue, there are free nerve endings from _____________
• Express receptors that detect noxious stimuli (nociceptors) → found in numerous tissues such as skin, muscle, joints, bone, visceral organs and meninges
• 4 classes of nociceptors which include _______________
unmyelinated C fibres and thinly myelinated Aδ fibres
mechanical, thermal, polymodal and silent
Mechanical receptor
- type of fiber: __________
- detects: _______
Free nerve endings of Aδ fibres;
Intense pressure
Thermal receptor
- type of fiber: __________
- detects: _______
Free nerve endings of Aδ fibres;
Extreme heat
Polymodal receptor
- type of fiber: __________
- detects: ______
Free nerve endings of C fibres;
Range of stimuli (noxious mechanical, thermal, chemical)
Silent receptor
- type of fiber: __________
- detects: ______
Free nerve endings of C fibres;
Mechanical and thermal stimuli (once chemically sensitised by inflammatory mediators)
Heat
- Receptor
- Receptor subunit
- Transient receptor potential (TRP)
- TRPV1 (capsaicin-sensitive), TRPV2, 3, 4
Cold
- Receptor
- Receptor subunit
- Transient receptor potential (TRP)
- TRPM8 (menthol-sensitive)
pH (acidic)
- Receptor
- Receptor subunit
- Acid sensing ion channel (ASIC), Transient receptor potential (TRP)
- TRPV1
Purine
- Receptor
- Receptor subunit
- Purine receptors
- P2X
Mechanical
- Receptor
- Receptor subunit
- Two-pore potassium channels
- K2P
During injury, there is quick onset of ________________ pain (lasting for short period of time) mediated by faster Aδ fibres:
• Later superseded by a second slower appearing _____________ pain mediated by slower C fibres
The origin of referred pain is usually distant from where it is perceived → commonly generated in a visceral organ but perceived on distant skin or muscle (e.g. neck pain in MI or shoulder pain in cholecystitis).
sharp and localised;
dull and poorly localisable
Where are nociceptor specific neurones found? What fibers do they synapse with? What do they respond to?
- I and II (mostly), deeper laminae (some)
- Synapse with Aδ and C fibres
- Only respond to painful stimuli
Where are wide dynamic range neurones found? What fibers do they synapse with? What do they respond to?
- III to VI (mostly)
- Synapse with all three types
- Respond to full range of possible painful and non-painful stimuli
Both nociceptive specific cells and wide dynamic range neurones are _____________ neurones → conduct the incoming nociceptive signal up to higher brain centres:
• Numerous projection neurones with different targets exist → most important ones are those projecting to the thalamus (spinothalamic tracts)
o Thalamus acts as the ______________, but pain pathways may also address other nuclei
• Some tracts address the brainstem (e.g. spinoreticular, spinomesencephalic tracts)
second order (projection);
main relay station
From the thalamus, 3rd order neurones originate to transmit information to ________________ (pain matrix), and can use either a medial pathway originating from the medial thalamus or a lateral pathway coming from the lateral thalamus:
Destination:
- Medial pathway: ______________
- Lateral pathway: ________________
certain cortical areas;
Parts of the amygdala, Anterior cingulate cortex, Insular cortex;
Parts of the amygdala, S1 and S2
Primary hyperalgesia
- where does it occur
- what is it caused by
Occurs only in the close vicinity of the damage and is caused by peripheral sensitization
Secondary hyperalgesia
- where does it occur
- what is it caused by
Detected further away from the incision (covering a bigger area) and is caused by central sensitization
The main cause of peripheral sensitization is the release of many inflammatory mediators as a consequence of the traumatic cell damage:
• Causes attraction of ____________ to the injury site and activation of the ______________
• Further mediator production and release → conglomerate of mediators (mediator soup) recruits more ____________ and lowers their activation threshold → increased number of impulses reaches the spinal cord and higher centres
mainly immune cells;
local autonomic nervous system;
nociceptors
What substances are found in the mediator soup causing peripheral sensitisation?
- TNFa
- O*
- Il- 6
- H+
What receptors do glutamate activate and what are their involvment?
- AMPA (α-amino-3-hydroxy-5-methyl-4 isoxazeloproprionic acid) –> Acute pain transmission
- NMDA (N-methyl-D-aspartate) –> Ongoing pain, Central sensitisation
- Kainate
What receptors do Substance P activate?
- Neurokinin-1 –> Ongoing pain, Central sensitisation
