302 Physiology of pain Flashcards
What are different types of pain?
- Nociceptive (somatic or visceral)
- Inflammatory
- Neuropathic
What is nociceptive pain?
A physiological response to real or threatened non-neural tissue damage
-Threat is thermal, chemical, or mechanical
-Reversible pain once the insult is removed
Which fibres are stimulated in nociceptive pain?
C fibres and Aδ fibres on peripheral nerves
What is somatic nociception?
Activation of nociceptors in skin, muscles, bones, joints, and connective tissues
Transmitted along A-delta and C fibres
Somatic pain – sharp or dull pain. Exacerbated by movement.
What is visceral nociception?
-Activation of nociceptors in internal organs
-Transmitted along autonomic fibres
Visceral pain – poorly localised, deep, squeezing, cramping pain, dull, sickening.
Associated autonomic symptoms – nausea, vomiting, sweating
What is inflammatory pain?
-Response of the somatosensory nervous system to tissue damage and inflammation.
In the periphery, increased inflammatory mediators (cytokines and chemokines) sensitize local nociceptors:
Lowers threshold for responsiveness (peripheral sensitization).
Results in activation of pathways after innocuous input and in exaggerated responses to noxious stimulation.
The plasticity that underpins these changes is rapid (occurring in minutes).
Inevitable consequence of surgery and tissue trauma.
Upregulation of nociception normally resolves as wound healing occurs.
What is neuropathic pain?
A lesion or disease of the somatosensory nervous system.
Impacts on function and causes structure changes in the somatosensory nervous system.
The result is a combination of sensory loss and increased responsiveness to both noxious and innocuous stimuli.
What is allodynia?
Pain after non-painful stimuli
What is hyperalgesia?
Heightened pain after painful stimuli
What is hyperpathia?
An eruptive pain extending beyond the duration of a stimulus
What does neuropathic pain feel like?
Electric shock
Burning
Cramping
Constant
Fleeting
Provoked
Name some nociceptors
Free nerve ending
Merkel disk
Meissner’s corpuscle
Pacinian corpuscle
Hair follicle receptor
What does a Merkel disk detect?
Gentle touch
What do Meissner’s corpuscles detect?
Vibrations
Most sensitive to low frequency vibrations
What do Pacinian corpuscles detect?
Vibrations transmitted to the skeleton
What does a free nerve ending detect?
Temperature, mechanical stimuli (touch, pressure, stretch) or danger (nociception)
What is the difference between nociception and pain?
Nociception: how signals about possible tissue damage get from the site of injury to the brain.
Pain: an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such.
What factors feed in to create pain from nociception?
Beliefs
Psychological factors
Cultural issues
Other illnesses
Personality
Social factors
Describe the nociceptive pathway
4 steps:
Periphery
Spinal cord
Brain
Modulation
What happens physiologically in tissue injury?
-Release of chemicals soup
-Stimulates nociceptors.
-Sensitizes nociceptors.
-Signal travels in Aδ or C nerve to spinal cord
What does ‘group A-C’ nerve fibre classification refer to?
(The level of myelination)
Group A – heavily myelinated. Carry quick, sharp pain
Group B – moderately myelinated
Usually general visceral afferent fibres and preganglionic nerve fibres of the autonomic nervous system
Group C – unmyelinated. Carry slow, achy pain
What is the effect of myelin on nerves?
Myelination allows the formation of nodes of ranvier that speeds up the activity along the axon via saltatory conduction
What happens when the pain signal reaches the spinal cord?
- Dorsal horn is the first relay station.
- Aδ or C nerve synapses with second order neurone.
- Second order neurone travels up opposite side of spinal cord
What is the tract of lissauer for?
Afferent fibres reach the spinal cord and use it to ascend or descend
Where does the spinothalamic tract terminate?
At the thalamus
Where does each of the pain fibres synapse?
The grey matter of the dorsal horn that is divided into rexed laminae
A-delta fibres
terminate in Lamina 1 and 5.
Synapse directly with the 2nd order neurones that make up the ascending tracts.
C-fibres
predominantly synapse with neurones in Lamina 2 (substantia gelatinosa).
often synapse indirectly via interneurones (which can be wide dynamic range neurones)
How do pain signals get up the spinal cord?
After crossing to the contralateral cord (decussation), 2nd order neurones ascend the spinal cord in several tracts.
Most important are spinothalamic tract and the spinoreticular tract
Describe the divisions of the spinothalamic tract
Lateral - Neo (new)-spinothalamic tract.
It’s the discriminative aspect of pain perception.
Medial - paleo (primitive)-spinothalamic tract
It’s the autonomic and unpleasant emotional component of the pain experience
Where is the ventral posterior lateral (VPL) nucleus found ?
In the thalamus
Where does the lateral spinothalamic tract terminate?
VPL nucleus in the thalamus
Where does the medial spinothalamic tract terminate?
It’s a polysynaptic pathway that sends projections to PAG (periaqueductal grey) matter, hypothalamus and reticular system in the midbrain before reaching the medial thalamus
Where does the spinoreticular tract terminate?
In reticular formation of medulla and pons and then from there to the thalamus
Project diffusely to entire cerebral cortex. Pain reaches conscious level and promotes behavioural arousal.
What happens when the nociception reaches the brain?
It becomes pain
The thalamus sends signals to:
Sensory cortex
Limbic system
Brainstem
How does the brain create a perception of pain?
3rd-order neurones project from thalamus to somatosensory cortex.
Primary somatosensory projection (S1) - mediate localisation of the pain stimulus
Secondary somatosensory projection S2 - ?stimulus intensity
What is the role of the limbic system in pain?
Involved in creation of emotion, behaviour, and memory.
Insula
major hub for visceral nociceptive inputs
assessment of nociceptive stimulus intensity
Anterior Cingulate Cortex (ACC)
the frontal part of the cingulate cortex that resembles a “collar” surrounding the frontal part of the corpus callosum)
Functions: attention and response
ACC has connections to deep ancient structures:
Amygdala
Hippocampus
Hypothalamus
How are nociceptive signals modulated?
-Melzack and Wall’s gate control theory
-Descending inhibitory pathways
How does the Melzack and Wall’s gate control theory of pain work?
Non-painful input closes the “gates” to nociceptive input prevents AP traveling to the CNS
What happens in descending inhibitory pathways?
Originate from supra-spinal structures:
periaqueductal gray
reticular formation
nucleus raphe magnus
Endogenous opioid receptors heavy expressed here.
Serotonin and NA are key neurotransmitters in this pathway.
Inhibit transmission of nociception at level of DH.
How can anti-depressants modify the nociceptive pathway?
Because serotonin and NA are key transmitters here – TCAs and SNRIs increase the levels of these transmitters and ‘turn up’ your descending inhibitory action
