Unit 3- Pain & nociception Flashcards
what is pain?
an unpleasant sensory experience associated with nociception?
what is nociception?
information indicating tissue damage
what are the two types of nociceptors?
A-delta fibre: simple naked nerve ending
- thin myelination (5-30 m/s)
- first, sharp well localised pain
responds to intense mechanical stimulus
c fibre: polymodal nociceptor - naked nerve ending
- no myelination = slow conduction velocity (0.5 - 3 m/s)
- slow, poorly localised pain (secondary)
why do c fibres respond to capsaicin?
opens same non-selective cation channel - Trpv1 as high temperature
what is the consequence of high doses of capsaicin?
desensitise pain because of depletion of substance P
what stimulates the release of substance P and what is its effect?
axons carry high frequency trains of APs causes neuropeptide release - stimulus perceived as moderate to intense pain
- substance P released peripherally from sensory neurons which induces mast cells to release histamine + inflammatory 5HT (serotonin) - activates nociceptors and causes vasodilation
what molecules increase nociceptor sensitivity?
histamine
prostaglandin
bradykinin
what is primary hyperalgesia?
increased sensitivity to pain to area of injury
how do painkillers work?
block cyclooxygenases that produce prostaglandins
what is the consequence of chronic stimulation of nociceptors due to trauma?
accumulation of substance P in dorsal horn leads to increase in neuronal responsiveness - secondary hyperalgesia - pain to area around injury
low threshold nociceptors respond to light touch (non-noxious stimuli) and generate pain responses - allodynia
what is the principle of the gate theory of the pain (melzack and wall 1965)?
pain messages travel through the periphery through “nerve gates” in spinal cord and up to brain - gates open - more pain
what factors open nerve gates?
- injury/ inactivity/ poor body mechanics
- cognitive factors - focussing on pain/ emotional/ stress
- high frequency APS along spinothalamic interneurones
what factors close nerve gates?
- increasing activity/ pain killers
- cognitive factors to cope: distraction/ attitude/ emotional state - serotonin act on GABAergic neurons to block signals
- mechanical stimulation to area of pain
- blocking release of substance P
why does mechanical stimulation to area of pain help to close the nerve gate?
spinothalamic interneurones are contacted by both nociceptive and touch fibres so during painful stimuli, rubbing the region activates touch fibres which inhibits the pain signal
what substances can block the release of substance P?
endorphins/ enkephalins - natural opiate-like peptides - analgesic effect