Mechanisms of pain

Question 1

What are the three classifications of pain?

Answer 1

prickling pain, burning pain, deep pain

Question 2

What fibres are activated during ‘prickling pain’?

Answer 2

A-delta

‘fast’ pain transmitted to the CNS by these fibres

doesn’t elicit much of an autonomic response

Question 3

What pain fibre is myelinated?

Answer 3

all A fibres

action potential travels faster

Question 4

What are the two types of nociceptive pain?

Answer 4

somatic and visceral

Question 5

What is a nociceptor?

Answer 5

a sensory neuron that responds to potentially damaging stimuli by sending signals to the CNS

Question 6

What causes a ‘wind up’?

Answer 6

prolonged stimulation of C fibre causing build up of substance P in the spinal cord dorsal horn

Question 7

Why are C fibres termed ‘polymodal’?

Answer 7

respond to a variety of stimuli

noxious heat and chemicals like capsaicin

Question 8

What are the three stages of nociception?

Answer 8

transduction, transmission, interpretation

Question 9

What are ‘mechanically insensitive afferents’?

Answer 9

sensory neurons with extremely high thresholds, don’t respond to mechanical stimuli

Question 10

Describe how pain sensation travels from the nociceptors to the brain

Answer 10

sensory A and C fibres enter the dorsal horn of the spinal cord

synapse with second order neurons which cross over to the opposite side of the cord and pass up to the brain in tracts

spinothalamic tract carries fibres to the thalamus

third order neurons carry the information to a number of areas of the brain

Question 11

What are the two types of non-nociceptive pain?

Answer 11

neuropathic and sympathetic

Question 12

What causes neuropathic pain?

Answer 12

lesion or disease of the somatosensory nervous system

can be central (multiple sclerosis) or pereipheral (diabetic neuropathy)

Question 13

What is the Gold Standard method of measuring pain?

Answer 13

using a pain questionaire

e.g McGill

Question 14

What are A-beta fibres?

Answer 14

sensory neurons that respond to non-noxious stimuli, detect light touch and vibrations

Question 15

What is allodynia?

Answer 15

central sensitisation, non-nociceptive A-beta fibres evoke pain sensations

different from hyperalgesia where localised inflammation leads to an exaggerated response to pain

Question 16

What substances increase the sensitivity of nociceptive sensory endings following an injury?

(primary hyperalgesia)

Answer 16

histamine, prostaglandins, bradykinin

Question 17

What are the effects of substance P release?

Answer 17

release of histamine from mast cells

vasodilation

substance P is necessary for the perception of moderate to severe pain

Question 18

What is the ascending pathway used by visceral sensation?

Answer 18

spinoreticular tract

NOT spinothalamic

ends in reticular formation

Question 19

What is the gate theory of pain?

Answer 19

activation of non-nociceptive fibres can interfere with signals from pain fibres, therefore inhibiting pain

some areas in the dorsal horn receive input from A-delta, C and A-beta fibres (laminae)

A-beta fibres form an exitatory synapse with an inhibitory interneuron which lessens the response of the projectory neuron

A-beta fibres over ride pain messages

Question 20

What is the Neuromatrix theory of pain?

Answer 20

pain is a multidimensional experience

sensory component

affective

cognitive-evaluative

consists of a medial and lateral pain system originating from the medial and lateral thalamic nuclei

Question 21

Summarise the ascending pain pathways

Answer 21

A-delta - myelinated, sharp pain

C - non-myelinated, dull aching pain

A-beta - non-nociceptive, responds to non-noxious stimuli

synapse with secondary neurons in dorsal horn of spinal cord

ascend in spinothalamic (or spinoreticular) tract to brain

Question 22

What does the medial pain system consist of?

Answer 22

medial thalamic nuclei, anterior cingulate cortex (ACC) and insula

Question 23

What does the medial pain system do?

Answer 23

mediates affective-cognitive-evaluative aspects of pain

Question 24

What does the lateral pain system do?

Answer 24

mediates sensory/discriminative aspect of pain

Question 25

What does the lateral pain system consist of?

Answer 25

primary (SI) and secondary (SII) somatosensory cortices and lateral thalamic nuclei

Question 26

What is the substantia gelatinosa?

Answer 26

point where first order neurons of the spinothalamic tract synapse

Question 27

What is the difference between primary and secondary hyperalgesia?

Answer 27

primary - sensitivity to mildly painful stimuli within the injured area

secondary - sensitivity to pain outside the area of tissue damage

Question 28

What influences pain perception?

Answer 28

cognitive appraisal of the meaning of the sensation - pain intensity is reduced when the pain is perceived to be controllable

attentional orientating to painful sensation - distraction increases activity in prefrontal cortex, ACC and PAG

behavioral reactions - negative emotional states can increase pain

Question 29

What has been suggested as a possible cause of chronic pain?

Answer 29

dysfunctional descending inhibitory pain system (PAG)

or

enhanced descending facilatatory system

Question 30

What does electrical stimulation in the periaquaductal gray elicit?

Answer 30

analgesia

PAG is the primary control centre for descending pain modulation

Question 31

What are the brain’s own opiate neurotranmitters?

Answer 31

endorphins and enkephalins