Lecture 11 - Intro To Pain Flashcards Preview

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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