Lecture 11 - Intro To Pain Flashcards
(17 cards)
Pain definition
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
Distinct classes of nociceptors
- thermal: superficial, respond to extreme heat or cold
- mechanical
- polymodal: can be both termal and mechanical
- silent: inflammatory agents that switch from silent to polymodal
Nociceptor classes and where they are found in the body
- superficial: thermal and mechanical
- throughout the body: polymodal
- muscles, joint and viscera: silent
Type of pain receptor fibers
- Adelta: myelinated, travel 5-30 m/sec, respond to pain and temperature, found on body surface. Associated with “first pain”
- C fibers: 0.5-2 m/sec, respond to temperature, itches and pain. Unmyleinated and found throughout the entire body. Associated with “second pain”
Activation of nociceptors
- tissue damage triggers local release of bradykinin, PGs and K+
Increased swelling and sensitisation of nociceptors
- retrograde activation of collarerals triggers release of substance P - mast cell degranulate and release histamine
Plasma extravasation which can sensitise nociceptors
Retrograde activation of collaterals triggers release of substance P which acts on blood vessels to promote blood flow at injury site
- turgor and color
Primary hyperalgesia
- Sensitisation of the nociceptors on peripheral fibres can result in primary hyperalgesia/allodynia
Allodynia vs hyperalgesia
- allodynia: previously inocous stimuli are now perceived as painful
- hyperalgesia: noxious stimuli are now perceived as more noxious
Nociceptive receptors project to spinal chord
- A-delta and C-fivres from the body project into spinal cord
- spinal cord is a laminated structure
- lamina is designated 1-10
- each lamina has a specific function: neurons in laminae 1, 2, 5 and 10 each receive inputs from the nociceptors from distinct body tissues
Pain fibres and dermatomes
- each spinal nerve caries nociceptor fibres that go through specific body areas
- A and delta fibres are carried in each spinal nerve
- each spinal segment processes noxious signals arising from each dermatome
- the maps of noxious input are not highly accurate
Where do A-delta and C fibres project
- A-delta fibres project in lamina 1 and 5
- C fibres project into lamina 2
Central sensitisation
- glutamate is a neurotransmitter that can bind to both AMPA and NMDA receptors
- AMPA receptor shows a very quick response and firing
- NMDA receptor is activated upon repetitive firing by AMPA -> this induces plasticity in neural chord -> central sensitization -> intense activation in response to mild stimulus -> neuropathic pain, hyperalgesia
- prolonged activatin of the spinal neurons by nociceptive input can trigger cell death int he spinal chord -> inhibitory type most vulnerable
Referred pain mechanism
- convergence of primary afferent fibres onto common pools of spinal neurons can result in perceptual mistake
- fibres from spinal lamina 1,2,5,10 cross the midline in the spinal chord and ascend towards higher brain regions -> travel in anterolateral quadrant/tract to primary sensory cortex which contains a body map
Spinothalamic tract
Some fibres ascent directly to the thalamus -> part of the cortex that controls the emotional partr of pain
Ventral trigeino-thalamic tract
Pain from the head region is relayed through the trigeminal nerve
How does morphine act
- morphine alters descending modulation -> Originates from midbrain regions, which receive nociceptor inputs