Mechanisms of pain Flashcards
What are the three classifications of pain?
prickling pain, burning pain, deep pain
What fibres are activated during ‘prickling pain’?
A-delta
‘fast’ pain transmitted to the CNS by these fibres
doesn’t elicit much of an autonomic response
What pain fibre is myelinated?
all A fibres
action potential travels faster
What are the two types of nociceptive pain?
somatic and visceral
What is a nociceptor?
a sensory neuron that responds to potentially damaging stimuli by sending signals to the CNS
What causes a ‘wind up’?
prolonged stimulation of C fibre causing build up of substance P in the spinal cord dorsal horn
Why are C fibres termed ‘polymodal’?
respond to a variety of stimuli
noxious heat and chemicals like capsaicin
What are the three stages of nociception?
transduction, transmission, interpretation
What are ‘mechanically insensitive afferents’?
sensory neurons with extremely high thresholds, don’t respond to mechanical stimuli
Describe how pain sensation travels from the nociceptors to the brain
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
What are the two types of non-nociceptive pain?
neuropathic and sympathetic
What causes neuropathic pain?
lesion or disease of the somatosensory nervous system
can be central (multiple sclerosis) or pereipheral (diabetic neuropathy)
What is the Gold Standard method of measuring pain?
using a pain questionaire
e.g McGill
What are A-beta fibres?
sensory neurons that respond to non-noxious stimuli, detect light touch and vibrations
What is allodynia?
central sensitisation, non-nociceptive A-beta fibres evoke pain sensations
different from hyperalgesia where localised inflammation leads to an exaggerated response to pain
What substances increase the sensitivity of nociceptive sensory endings following an injury?
(primary hyperalgesia)
histamine, prostaglandins, bradykinin
What are the effects of substance P release?
release of histamine from mast cells
vasodilation
substance P is necessary for the perception of moderate to severe pain
What is the ascending pathway used by visceral sensation?
spinoreticular tract
NOT spinothalamic
ends in reticular formation
What is the gate theory of pain?
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
What is the Neuromatrix theory of pain?
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
Summarise the ascending pain pathways
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
What does the medial pain system consist of?
medial thalamic nuclei, anterior cingulate cortex (ACC) and insula
What does the medial pain system do?
mediates affective-cognitive-evaluative aspects of pain
What does the lateral pain system do?
mediates sensory/discriminative aspect of pain
What does the lateral pain system consist of?
primary (SI) and secondary (SII) somatosensory cortices and lateral thalamic nuclei
What is the substantia gelatinosa?
point where first order neurons of the spinothalamic tract synapse
What is the difference between primary and secondary hyperalgesia?
primary - sensitivity to mildly painful stimuli within the injured area
secondary - sensitivity to pain outside the area of tissue damage
What influences pain perception?
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
What has been suggested as a possible cause of chronic pain?
dysfunctional descending inhibitory pain system (PAG)
or
enhanced descending facilatatory system
What does electrical stimulation in the periaquaductal gray elicit?
analgesia
PAG is the primary control centre for descending pain modulation
What are the brain’s own opiate neurotranmitters?
endorphins and enkephalins
