Unit 3- Pain & nociception

Question 1

what is pain?

Answer 1

an unpleasant sensory experience associated with nociception?

Question 2

what is nociception?

Answer 2

information indicating tissue damage

Question 3

what are the two types of nociceptors?

Answer 3

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)

Question 4

why do c fibres respond to capsaicin?

Answer 4

opens same non-selective cation channel - Trpv1 as high temperature

Question 5

what is the consequence of high doses of capsaicin?

Answer 5

desensitise pain because of depletion of substance P

Question 6

what stimulates the release of substance P and what is its effect?

Answer 6

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

Question 7

what molecules increase nociceptor sensitivity?

Answer 7

histamine
prostaglandin
bradykinin

Question 8

what is primary hyperalgesia?

Answer 8

increased sensitivity to pain to area of injury

Question 9

how do painkillers work?

Answer 9

block cyclooxygenases that produce prostaglandins

Question 10

what is the consequence of chronic stimulation of nociceptors due to trauma?

Answer 10

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

Question 11

what is the principle of the gate theory of the pain (melzack and wall 1965)?

Answer 11

pain messages travel through the periphery through “nerve gates” in spinal cord and up to brain - gates open - more pain

Question 12

what factors open nerve gates?

Answer 12

• injury/ inactivity/ poor body mechanics
• cognitive factors - focussing on pain/ emotional/ stress
• high frequency APS along spinothalamic interneurones

Question 13

what factors close nerve gates?

Answer 13

• 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

Question 14

why does mechanical stimulation to area of pain help to close the nerve gate?

Answer 14

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

Question 15

what substances can block the release of substance P?

Answer 15

endorphins/ enkephalins - natural opiate-like peptides - analgesic effect

Question 16

what are the types of pain?

Answer 16

• cutaneous (skin)
• somatic (within body - bones/muscles/ligaments)
• visceral (autonomic sensation from organs - spinoreticular tract)
• referred pain (pain from viscera appears on surface of body)

Question 17

how is a neuroma caused and what is its effect?

Answer 17

nerve damage - axons sprout from cut end of nerve so axons become tangled and sensitive to pressure and temperature - large impulse discharges = severe pain