Physiology of pain Flashcards
What is pain?
Unpleasant sensory & emotional experience associated w/ actual or potential tissue damage
A conscious experience arising from activity in multiple sensory & emotional centres of brain
Dynamic, subjective, plastic & emotional product of complex neural processing
What is nociception?
- The neural process of encoding noxious stimuli
- It is a physiological process involving transduction, transmission & modulation of neural signals from the periphery to the CNS
- Physiological response to tissue damageNOTE: Nociception doesn’t require consciousness & is protective in nature
What are nociceptors? What do they do?
Pain receptors
Unmyelinated small diameter nerve fibre endings in skin that sense actual or potential tissue damage.
- They alert us to injury by detecting injury-related chemicals, temp & pressure
Why do we feel pain?
- Learning experience - pain memory = stops us doing the damaging action again.
- Enables healing & recovery = makes us lie down and stop everyday activities.
What are the 4 parts to transmitting pain? Define each
- Transduction- process by which a physical or chemical stimulus is transformed into an electrical signal (action potential) that can be carried to the CNS.
- Transmission - transmission of the action potential along sensory afferent neurones from the periphery to the CNS.
- Perception - the awareness of pain associated with a specific area of the body.
- Modulation - refers to the up and down regulation of pain signals through out the spinal chord & brain.
Give 3 examples of primary afferent nociceptive nerve fibres, explain their conduction speed & the type of pain they cause
Aδ fibres:
- Lightly myelinated & fairly small diameter
- Sharp well-localised pain- reflex response in acute pain
- Middle conduction speed
C fibres:
- Unmyelinated & smallest diameter
- Slowly burning pain- poorly localised
- Very slow conduction speed
Aβ fibres:
- Highly myelinated & larger diameter
- sense touch & pressure. Non-pain sensing
- Fastest conduction speed
Name some neurotransmitters that primary nociceptors use.
Substance P
L-glutamate
GABA
VIP
CCK-8
Explain how transaction occurs
- Damaged cells cause immune cells to arrive.
- Immune cells release cytokines.
- Prostoglandins are released by nearly all cells as a response to inflammation
- Sensory nerve fibres (1st order neurone) are all over the body & they detect the prostoglandins
- This stimulates an action potential to go to the dorsal horn in the spinal cord.
How is an action potential stimulated? Give examples of ion channels depending on the mechanism of injury e.g. heat
- The AP is stimulated by an influx of Na⁺ into the nociceptor through different ion channels depending on the injury.
- Ca²⁺ & 2nd messengers induce central sensitisation in the dorsal horn = amplifies the nociceptive input after an injury.
Heat- TRPV 1- C-fibres & Aδ fibres
Cold- TRPM8- C-fibres
ATP (tissue damage)- P2X3
Noxious mechanical- TRPA1- Aδ fibres & C-fibres
Transmission- explain the ascending pathways
Inflammatory mediators (cytokines) are released locally due to inflammation/injury- produce pain.
Initial pain is sensed by nociceptors – free nerve endings in the skin, muscle and other tissues
Pain is transmitted by primary sensory neurons to the dorsal horn of the spinal cord via myelinated A-delta fibres for fast, acute pain & non-myelinated C-fibres for slow, throbbing/dull pain
Pain is transmitted via ascending pathway
A fibres:
-At the dorsal horn, the primary sensory neuron synapse w/ second order neuron of the spinothalamic tract- substance P is released as neurotransmitter
- This second order neuron immediately decussates & passes up to the thalamus- important for localised pain
C- fibers:
- Enter dorsal root
- synapse w/ axon that pass through anterior commissure to contralateral side
- pass up spina-reticulo-thalamic tract- important for emotional aspect of pain
Both- In the thalamus, second order neurons synapse w/ third order neurons leading to the somatosensory cortex (sensory cortex a limbic system)- register pain & mediate emotional components
Perception- how does body perceive pain?
different areas of the somatosensory cortex correlate to different areas of the body so the 3rd order neurone tells the brain the location of the pain.
This helps the brain perceive the pain in a specific location
pain is perceived in the opposite side of the brain to the stimulus because the 2nd order neurones cross the spinal cord.
Define modulation
How the pain is inhibited in the body
I.e. gate control theory
Explain the Gate Control Theory
-Involves the inhibition of pain at the dorsal horn
- e.g. bumping your elbow against a wall leads you to rub it.
- The action of rubbing your elbow reduces the pain because it activates Aβ fibres, which activates the inhibitory interneuron in the dorsal horn.
- This leads to the inhibition of Aδ & C fibres so the pain impulses can no longer be sent to the brain = gate closed.
- The serotonin & noradrenaline neurone from the descending pathway release: serotonin & noradrenaline.
- These chemicals both inhibit the release of substance P (neurotransmitter) from the 1st order neurone entering the dorsal horn.
- The chemicals also stimulate interneurones to release an opioid, enkephalin.
- Enkephalin inhibits the pre-synaptic 1st order neurone from releasing substance P & it will inhibit the post-synaptic neurone for repolarising.
Psychological factors that may influence patient’s chronic pain
Anger
depression
anxiety/ fear
Pharmacological interventions to manage pain
NSAID- consider first
Combine w/ PPI e.g. omeprazole
If using alone, then opioids should be considered
