Lecture 23: Pain Flashcards
What is pain?
an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
What are the sensory-discriminative aspects of pain?
location, intensity and duration
What is the motivational-affective dimension of pain?
unpleasant feeling associated with pain
What does pain evoke?
emotional as well as sensory experiences
What is nociception?
the neural process of encoding noxious stimuli
activation of cells throughout the entire nervous system
What are nociceptors?
cells that are physiologically activated by noxious stimuli
What is nociceptive pain?
pain that arises from actual or threatened damage to non-neural tissue and is due to the activation of nociceptors
How is nociceptive pain signalled to the brain?
different noxious stimuli can activate specific receptors and/or ion channels on peripheral nociceptors
What is inflammatory pain?
pain that is driven by inflammation
How is inflammatory pain signalled to the brain?
some inflammatory mediators directly activate peripheral nociceptors to produce pain
others produce changes in sensitivity of peripheral nociceptors to noxious stimuli
What is neuropathic pain?
pain caused by a lesion or disease of the somatosensory nervous system
changes in circuit sensitivity and CNS connections
What is the role of nociceptors?
transduce noxious stimuli into electrical potentials and take information to the CNS
How is pain signalled to the brain (in general)?
ascending pathways transmit information towards cerebrum -> pain perception occurs in cerebrum -> descending modulation of pain signalling can suppress pain
What do peripheral nociceptors respond to and what do they code for?
usually respond to high threshold stimulation
code for intensity of noxious stimulation
How fast do peripheral nociceptors conduct action potentials?
conduct slowly relative to low threshold sensory neurons
What is the structure of peripheral nociceptors?
usually described as having un-specialised, free nerve endings
How can peripheral nociceptors be classified?
according to the conduction properties of their axons
What are the two types of peripheral nociceptors?
Aδ-fibre axons and C-fibre
What are the properties of Aδ-fibre axons?
thinly myelinated
conduction velocities 5-30 m/s
fast (sharp) pain
What are the properties of C-fibre axons?
unmyelinated
conduction velocities <2 m/s
slow (burning) pain
What do mechanical nociceptors respond to?
noxious mechanical stimuli, eg. cutting, crushing, pinching
What do thermal nociceptors respond to?
noxious temperature
What do polymodal nociceptors respond to?
combinations of mechanical/thermal, and/or chemicals or inflammatory
mediators
What does the dorsal horn of the spinal cord receive input from? Where do second order neurons originate?
nociceptors
in different lamina and ascend toward the brain
What is the role of interneurons in the dorsal horn of the spinal cord?
communicate between lamina and contribute to processing
What is the main ascending pathway for discriminative aspects of pain / temperature for the body?
anterolateral system / spinothalamic tract -> ventral posterior lateral nucleus of the thalamus -> primary somatic sensory cortex
What will a lesion in the spinal cord lead to?
reduced sensation of two-point discrimination on the ipsilateral side and reduced sensation of temperature and pain on the contralateral side
How can pain be modulated at different levels of the neuraxis?
sensitisation, neurogenic inflammation, descending inhibition and common analgesics
What is sensitisation?
increased responsiveness of nociceptive neurons to their normal (noxious) input, and/or recruitment of a response to normally subthreshold (innocuous) inputs
What is hyperalgesia?
increased pain from a stimulus that normally provokes pain
What is allodynia?
pain due to a stimulus that does not normally provoke pain
e.g. sunburn causes pain to normally innocuous touch
What is peripheral sensitisation?
increased responsiveness and reduced threshold of nociceptive neurons in the periphery to the stimulation of their receptive fields
What is central sensitisation?
increased responsiveness of nociceptive neurons in the central nervous system to their normal or subthreshold afferent input
What is neurogenic inflammation?
when peripheral nociceptors release inflammatory mediators that produce or exaggerate local tissue inflammation
What is NGF?
an inflammatory mediator which sensitizes bone nociceptors and produces sensitivity to weight bearing
What is the role of descending systems in regards to pain?
can modulate the transmission of ascending pain signals e.g. inhibition of pain signal as it passes through the dorsal horn / pain can be shut down by higher centres
What are common analgesics?
paracetamol, opioids, NSAIDs and local anaesthetics
