Pain Flashcards
(38 cards)
1
Q
why is pain needed?
A
- to be aware of minor injuries
- unless carefully managed, injuries could become infected leading to diseases such as leprosy
- people who are congenitally insensitive may be unaware of injuries and could end up causing more damage
pain is essential for survival
2
Q
what illnesses can cause loss of pain sensation?
A
- leprosy
- diabetes mellitus
due to nerve damage
3
Q
how does pain differ from classical senses such as vision?
A
- it is both a discriminative sensation and a graded motivation (behavioural drive)
4
Q
what strange symptoms can pain cause?
A
- allodynia: sensitisation to normally innocuous stimuli
- hyperpathia: hysterical responses to small pain
- temporal augmentation
- after-sensations
- emotional variability
5
Q
what is convergence theory?
A
- suggests that pain is an integrated, plastic state represented by a pattern of convergent somatosensory activity within a neuromatrix
6
Q
what is specificity theory?
A
- pain is a distinct sensation, detected and transmitted by specific receptors and pathways to distinct pain areas of the brain
7
Q
what are pain receptors?
A
free nerve endings called nociceptors
8
Q
how are nociceptors classified?
A
- according to the activating stimulus, the fibre type and the conduction velocity
9
Q
what are the A-delta nociceptor fibres?
A
- lightly myelinated
- fast: 20m/s
- mechano-sensitive and mechanothermal-sensitive
10
Q
what are the C nociceptor fibres?
A
- unmyelinated
- slow: 2m/s
- polymodal: mechanical, thermal and chemical sensitive
11
Q
what are nociceptors?
A
- a subset of afferents with free nerve endings that respond specifically to pain
12
Q
how can mechanothermal-sensitive nociceptors be identified to specifically respond to pain by heat?
A
- in heat responses, afferents whose activity correlates with pain perception can be identified
- thermoreceptor activation has already saturated before pain is perceived
- this means pain is not due to the ramping up of normal receptors, and is instead perceived by separate nociceptors
13
Q
what are the two categories of pain fibres?
A
- Fast/first pain: sharp and immediate
- mimicked by stimulation of A-delta fibres - Slow/second pain: delayed, diffuse and longer-lasting
- mimicked by stimulation of C fibres
14
Q
how can specificity of nociceptors be determined compared to other fibres?
A
- stimulation of A-alpha (proprioceptive) and A-beta (mechanoreceptive) fibres NEVER elicits pain sensation
therefore there is a distinct set of A-delta and C nociceptor fibres that are specifically used in pain detection
15
Q
what are molecular pain receptors?
A
- specific molecular receptors associated with nociceptor nerve endings are activated by heat
- capsaicin receptor (TRPV1) is activated in nociceptive A-delta and C fibres at 45C, and capsaicin which is the active component in chillies
- other molecular receptors are activated in A-dela fibres at higher thresholds (52C)
these molecular receptors are specifically heat detection machines
16
Q
how can nociceptors also detect chemicals?
A
- capsaicin from chillies mimics endogenous vanilloids that are released by stressed tissues
- therefore nociceptors can detect the release of chemicals from stressed cells
17
Q
what are the 2 central pain pathways?
A
- sensory discriminative pathway
- signals location, intensity and type of stimulus
- involves the spinothalamic tract/anterolateral system - Affective-motivational pathway
- signals unpleasantness and enables autonomic activation of fight or flight response
18
Q
how do spinothalamic projections preserve topology?
A
measuring the activity of the somatosensory cortex indicates:
- this region responds to painful stimuli which correlates to pain intensity
- this is spatially mapped
19
Q
how is pain cortically represented?
A
- MRI of cortical activation of C fibres (pain stimuli) and A-beta fibres (mechanical stimuli) show that pain stimuli activate the same region in the somatosensory cortex as non-painful mechanical stimulation applied to the same area
- pain however has a distinct response that includes other regions
20
Q
what other regions in the cortex does pain activate?
A
- insula
- cingulate cortex
- both regions are connected to the limbic system and are involved in emotional responses
- part of the affective-motivational response to pain
21
Q
what is the affective-motivational pathway?
A
- no topographic mapping
- neurons in parabrachial nucleus respond to painful stimuli from anywhere on skin
- input into limbic and hypothalamic systems
- activity in cingulate cortex corresponds with unpleasant feeling of pain
- about emotion, not intensity (discriminative pathway)
22
Q
what conclusions have been made that pain perception is aligned with specificity theory?
A
- both cellular and molecular receptors that respond specifically to pain
- A-delta fibres, C fibres and TRPV1 - there are specific pathways that convey pain messages
- discriminative and affective-motivational - regions in the CNS are specifically activated in response to pain
- limbic system, insula, cingulate cortex and somatosensory cortex
23
Q
which phenomena of pain do not fit with the specificity theory?
A
- pain perceived is not always proportional to stimulus intensity
- pain is modulated by other stimuli e.g. acupuncture
- phantom limbs
- referral of pain from viscera to skin
- placebo
24
Q
what is hyperalgesia?
A
increased response to a painful stimulus
- hypersensitivity of damaged skin to a normally tolerable painful stimulus e.g. skin prick
- result of lowered nociceptor thresholds which heightens pain response
25
what is allodynia?
painful response to a normally innocuous stimulus
| - painful sensitivity of sunburnt skin to gentle mechanical stimulus or mild temperature
26
what is the inflammatory response?
- tissue damage releases inflammatory substances which affect nerve function, recruit mast cells and neutrophils, and increase blood flow
- bradykinin directly affects function of nociceptive molecular receptors (TRPV1)
- prostaglandins lower threshold for AP generation
27
how do aspirin and ibuprofen affect the inflammatory response?
- aspirin and ibuprofen are analgesics (painkillers)
| - they act on cyclooxygenase (COX), an enzyme important in prostaglandin biosynthesis
28
what is central sensitisation?
- result of activity-dependent local release of prostaglandins from nociceptive dorsal horn neurons
- prostaglandins lower thresholds for AP generation for neurons relaying nociceptive info, leading to hyperalgesia
- these neurons become sensitive to non-nociceptive inputs, so innocuous stimuli can be perceived as painful - allodynia
29
what is hyperpathia?
- results when there is fibre loss/damage (centrally or peripherally) that causes raising of detective threshold
- when the detection threshold is exceeded, the excitability is much greater, causing explosive pain
30
when can central sensitisation also occur (not inflammatory response)?
- when central pathways are damaged
- e.g. in diabetes mellitus, shingles, multiple sclerosis or after stroke
- neuropathic pain can be experienced after limb amputation
31
what is phantom limb pain?
- patients have illusion that the limb that has been amputated is still present
- indicates that central representation of the body persists in the absence of peripheral input
- children born without limbs can have phantoms, so the map may be preformed
- amputees experience phantoms, suggesting pain may be a representation of what we expect it to be
32
what is referred pain?
- pain due to damage of viscera is perceived as coming from certain locations on skin
- e.g. heart attack is preceded by pain in left arm
- reflects convergence of visceral afferents onto same pathways as cutaneous afferents in CNS
33
how is pain centrally modulated?
- pain perception varies according to context
- mechanisms exist, voluntary or involuntary, to over come severe pain
- e.g. soldiers with severe wounds experienced no pain
- placebo effect of relieving pain
34
what is the molecular mechanism in modulating pain?
- stimulation of periaqueductal grey activates brainstem nuclei that modulate dorsal horn activity
- descending inputs from the dorsal horn activate enkephalin
- enkephalin releases interneurons which presynaptically inhibit nociceptive fibres
35
what are enkephalins?
- members of a family of endogenous opioid peptides that include endorphins and dynorphins
- they release interneurons which presynaptically inhibit nociceptive fibres
36
how can pain modulation occur locally?
- rubbing on an injury causes A-beta fibres (mechanoreceptors) to inhibit nociceptive C fibre inputs into spinal cord
- known as gating of pain
- pain perception is the result of integration of convergent sensory info
37
how do we know that pain is not another somatic sense?
Nociceptive information is carried to areas of the limbic system whose activity correlates with the ‘experience’ that pain is unpleasant
38
why does Hyperpathia differ from hyperalgesia and allodynia?
it involves a reduction in sensitivity due to a raising of the pain detection threshold
