Lecture 15 : Pain Flashcards
What are nociceptors?
Free nerve endings, found everywhere in the body, except brain neural tissue
What do nociceptors respond to?
Noxious or potentially damaging stimuli
* Strong mechanical stimuli - piezo channels
* Extreme temperatures - TRP channels
* Chemicals released by damaged tissue - Inflammatory signals
- Some exogenous chemicals activate receptors directly, e.g. capsaicin
How does the diameter of an axon affect the conduction velocity?
Larger diameter = faster conduction velocity
What are “C” fibres?
- Smallest diameter, unmyelinated axons
- Slow conduction velocity
- Signal ongoing damage (or potential damage)
- Large receptive field
- Dull throbbing pain
What are “Aδ” fibres?
- Small diameter myelinated axons
- Faster conduction velocity
- Signal acute onset of painful stimulus
- Small receptive field
- Sharp pain
What are “Aß” fibres?
- Touch
- Largest diameter myelinated axons
- Fastest conduction velocity
- Mechanoreceptor fibres
Describe sensitisation in pain pathways:
Pain pathways become more sensitive following injury (hyperalgesia)
– Injury site and area around injury become tender
Describe the mechanism behind the sensitisation of pain pathways:
- Sensitisation of sensory endings by locally released factors and Piezo channel sensitisation
- Changes at CNS synapses - neurons become more excitable along the anterolateral pathway
What is peripheral sensitisation (inflammation)?
- Inflammatory signals cause ongoing pain receptor activation
- Histamines also activate pain receptors and cause itch
- Substances are released by nociceptors to activate mast cells for histamine release and induce vasodilation and blood flow to affected area
Describe the inhibition of pain pathways:
- Activation of the touch or cold sensory pathways can inhibit the secondary afferent neuron of the pain pathway, via inhibitory interneurons, and thereby suppress the transmission of the pain signal to the brain
What is the trans-epidermal nerve stimulation (TENS)?
- “Gating” of pain impulses by non-painful stimuli of nearby nerves
- Inputs from nearby non-pain nerves inhibits response of ascending pain fibres
- Low levels of currents will activate the axons of touch receptors which initiates the gate control mechanism to reduce pain
Describe how local anaesthetics work:
They block sodium channels which prevent the transmission of pain by blocking action potentials e.g. lidocaine, articaine, marcaine (bupivacaine)
What is local pain?
Pain experienced at the site of origin
What is referred pain?
Pain felt in a region other than the site of origin
- Usually visceral/internal organ pain felt at the body surface/skin
- Presumed due to convergence of visceral and somatic pain pathways
What is neurogenic pain?
- Pain felt along distribution of a nerve due to spontaneous action potentials generated by the damaged nerve
- Caused by damage or dysfunction of the nervous system, particularly the peripheral nerves
What is muscle pain?
- Can be neurogenic
- Can be referred from joints, ligaments, or tendons
- Can originate from poor blood supply due to lack of oxygen and nutrients
