Physiology and Pathophysiology of Pain Flashcards
What is pain?
An unpleasant sensory and emotional experience associated with, or resembling that association with, actual or potential tissue damage
What are the key notes for pain?
Is always a personal experience, pain and nociception are different phenomena, learn the concept of pain, may have adverse effects and can use verbal description to express
Describe nociception
Physiological process by which noxious stimulation is communicated through the peripheral and CNS
Where does perception of pain occur?
Somatosensory cortex
What neural pathways are involved in pain?
PNS - detection and transmission
Spinal cord - processing ang transmission to brain (thalamus)
Brain - perception, learning and response
Modulation by descending tracts
Describe nociceptors
Free nerve endings of A delta (faster pain as myelinated) and C fibres (slower)
Respond to thermal, chemical and mechanical noxious stimuli
Describe primary afferents
Cell body in dorsal root ganglion
First order neurons
Synapse at spinal cord
Where do the 1st order neurons synapse and what neurons receive this?
In Rexed lamina 1 and 2
Input to nociceptive specific, low threshold mechanoreceptive and wide dynamic range
After this axons continue as tracts
What is the main ascending tract for nociception?
Spinothalamic tract - arises in Rexed lamina 1,2 and 5, then becomes lateral spinothalamic tract
Also have spinoreticular, spinomesencephalic and spinohypothalamic tracts
What are the 2 systems of the thalamus?
Lateral and medial systems
What communicates with the thalamus?
Somatosensory cortex, spinal cord and basal ganglia in lateral system
Insula, brainstem and cingulate in medial system
Describe descending pathways for pain
Periaqueductal grey in midbrain assimilates the info
Effective via rosteroventral medulla
Usually decreases pain signals - DNIC
Noradrenergic system
What is the outcome of sensitisation?
Leftward shift of stimulus intensity curve towards innocuous (not harmful)
What causes spontaneous pain?
Spontaneous activity in nerve fibres
What are the changes in nociceptor to cause allodynia?
This is pain from stimulus which is not usually painful
There is decreased threshold for that response
What are the changes in nociceptor to cause hyperalgesia?
This is abnormally heightened sensitivity to pain
There is exaggerated response to normal and supra-normal stimuli
What can influence the primary afferent neuron?
Interneurons and descending neurons
What is central sensitisation?
Response of second order neurons in the CNS to both noxious and non-noxious stimuli
What are the 3 main components of central sensitisation?
Wind up
Classical
Long term potentiation
Describe wind up - central sensitisation
Involves only activated synapses
homosynaptic activity dependant progressive increase in response of neurons
Manifests over course of stimuli and terminates with stimuli
What is included in wind up?
Substance P and CGRP
Describe classical central sensitisation
Involves opening up of new synapses
Hetero-synaptic activity dependant plasticity
Immediate onset with appropriate stimuli and outlasts the initial stimuli
What receptors does classical sensitisation involve?
NDMA receptors
Describe long term potentiation
Involves mainly activated synapses
Occurs primarily for very intense stimuli
Involves NMDA and AMPA receptors
What are the mechanisms for pain?
Nociceptive
Neuropathic
Nociplastic
Describe acute pain
Physiological
Presence of noxious stimuli
Serves protective function
Usually nociceptive
Describe chronic pain
Pathological
No presence of noxious stimuli
Does not serve any purpose
Nociceptive, neuropathic and nociplastic
What is nociceptive pain?
A sensory experience that occurs when specific peripheral sensory neurons respond to noxious stimuli
Where is nociceptive pain and how long does it last?
Painful region is typically localised at site of injury
Usually time limited and resolves when damaged tissue heals
Can also be chronic
What does nociceptive pain usually respond to?
Conventional analgesics
What is neuropathic pain?
Pain caused by a lesion or disease of the somatosensory nervous system
What is neuropathic pain?
Painful region may not be the same as site of injury - in in neurological territory of affected structure - nerve, root, spinal cord, brain
Usually chronic
Describe nociplastic pain
Pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing activation of peripheral nociceptors or disease or lesion in somatosensory system
What can cause nociplastic pain?
Fibromyalgia
Chronic widespread pain
Painful physical symptoms of depression and anxiety
What are the characteristics of nociplastic pain?
Stimulus independent, no inflammation or injury, no structural neuronal damage
Due to central plasticity, no protective or adaptive function and pathological pain
