Pain Mechanisms Flashcards
Hyperalgesia vs. Allodyna
- primary & secondary
Hyperalgesia = increased sensitivity to noxious stimuli
Allodyna = feeling of pain from non-noxious stimuli
Primary = pain @ site of injury
Secondary = pain away from site of injury
**referred pain
Sensitization
- peripheral vs. central
Sensitization = increased responsiveness or decreased threshold of neurons
Peripheral: dysfunction in PNS
- pain @ site of injury in response to painful stimulus (aka primary hyperalgesia)
Central: dysfunction in CNS
- pain away from site of injury b/c signals are being sent from the periphery to the SC or brain (aka secondary hyperalgesia OR allodynia)
3 aspects of pain
- sensory discrimination - perceiive location, magnitude, duration & quality
- motivational-affective - suffering of pain, emotional aspect
- cognitive evaluative - pain perceived based on past experiences
Lateral pain system
- which afferent fibers
- function
Alpha delta (group III)
Function: sharp, noxious, thermal pain (sensory discrimination)
- LOW threshold, FAST pain
Medial pain system
- which afferent fibers
- function
C fibers (group IV)
Function: dull, noxious, thermal & mechanical (motivational, affective pain & emotional memory)
- HIGH threshold, SLOW pain
Peripheral termination of 1st order neuron
- receptors
- functions (2)
terminates in skin, viscera, joints, or muscle
Nociceptors - sensory receptor that transduces painful stimulus in the PNS
Two main functions:
- Tranduction: detection of noxious or damaging stimuli
- Transmission: passage of resulting sensory input from peripheral terminals to Sc
Central termination of 1st order neuron
- termination sites
Terminates in dorsal horn of SC and synapses w/ 2nd order neuron
- first place where centralization can occur
- *release glutamate & substance P into cleft where it is taken up by dorsal horn neuron
- A delta: lamina I and IV
- C fibers: lamina II
Second order neuron in pain pathway
- what is it
- 3 classifications
aka the spinothalamic tract
- projection = excitatory
- Interneurons = inhibitory
- Wide dynamic range = excitatory or inhibitory
- receives info from BOTH sides & multiple sites
- place of CONVERGENCE: input can be confused which can lead to misinterpretation aka referred pain
- contributes to chronic pain
Tracts w/n the Medial Pain system (3)
- Spinoreticular - interferes w/ sleep
- Spinomesencephalic - activates descending inhibitory pathway b/c projects to PAG
- Spinolimbic - emotional memory of pain
3rd order neuron of pain pathway
thalamocortical neuron
from VP nuclei to S1, limbic system and cingulate cortex
Descending pain pathways
- where do they originate
- termination
- chemicals involved
Originate in cortex or midbrain (PAG, RVM most common)
Terminate in the dorsal horn of the SC connecting to central terminal axon of 1st order neuron
Chemicals involved: opioids, serotonin, GABA, norepinephrine
Central sensitization in the SC
- due to
due to sensitization of DH neurons @ SC level b/c
- up-regulation of receptors (glutamine specifically)
- sprouting of axons/dendrites into lamina I, II, IV
- production of new genes
- cell death of interneurons
- decreased inhibition of descending pathways
- increased responsiveness of DH neurons
Central sensitization in the Brain
- due to
- overexpression of NMDA receptors
- changes in homonculus
- decrease pain chemicals essential to brain health
- increase functional activity of pain matrix from fMRI studies
- loss of gray matter volume
Gate Control Theory
- what is it
- factors involved in action of gate
Can modulate activity of the DH neurons by opening and closing the neural gate, which can override the stimulus of the pain (trick the brain)
Factors involved in action of the gate:
- amt of activity in nociceptive fibers
- amt of activity in other peripheral fibers
- signals from descending pathways
Opening of the gate due to…
INC activity of nociceptors & DEC activity of descending inhibitory pathways
This is bad, causing more pain
due to… more extent of injury, anxiety, depression, focusing on pain
