Anatomy And Physiology Of Pain Flashcards
Definition of pain
Pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage
Nociception
Describes the neural processes involved in producing the sensation of pain
Noiciceptive pathways
Transduction in the periphery, through transmission to the dorsal horn of the spinal cord, then on to the brain
Normal physiological pain includes
Instant and acute pain
Abnormalities from processing from the stimuli to the CNS causes
Chronic pain
Acute
Pain < 12 weeks duration
Chronic
Continuous pain lasting > 12 weeks
Pain that persist beyond the tissue healing time- outlines the expected tissue healing time
Chronic non-cancer pain and chronic cancer pain
Nociceptive pain
Pain that arises from actual or threatened damage to non-neural tissue and is due to the activation of nociceptors
E.g. hitting your foot
Neuropathic pain- hard to explain/vocalise
Pain caused by a lesion or disease of the somatosensory nervous system
E.g. stinging, burning, tingling
Nociplastic pain/ other pain (pain which isn’t Nociceptive or neuropathic)
Pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors, or evidence for disease or lesion of the somatosensory system causing the pain
Allodynia
Pain due to a stimulus that does not normally provoke pain
E.g. light touch
Dysesthesia
An unpleasant abnormal sensation, whether spontaneous or evoked
Hyperalgesia
Increased pain from a stimulus that normally provokes pain
Hypoalgesia
Dismissed pain in response to a normally painful stimulus
The pain pathway
Peripheral receptor: to detect the relevant stimulus
1st order neuron: from the periphery to the ipsilateral spinal cord
2nd order neuron: which crosses to the contra lateral cord and ascends to the thalamus (through tracts in the white matter), the system’s integrative ‘relay station’
3rd order neuron: from thalamus to midbrain and higher cortical centres (somatosensory centre)
Must go to thalamus before the somatosensory centra as thalamus acts as a filter to outline stimuli that should be noticed, eg thalamus outlines that pressure of clothes on person does not need to responded to
Nociceptors- pain receptor responsible for transduction
Transduction: Changing physical stimulus into electrical stimulus
Physical stimulus leads †o action potential
Most stimuli are polymodal (thermal/chemical/mechanical)
The extent of the stimuli causes pain
Primary afferent neurones
Nociceptors are the free nerve endings of primary afferent neurones
-A delta fibres- faster conducting pain primary afferents
-C fibres- slower conducting afferents
Found in any area of the body that can sense pain either externally or internally
External: skin/cornea/mucosa
Internal: viscera/ joints/ muscles/ connective tissue (usually internal organs contain c fibre variety)
The cell bodies of these neurones reside in either
-Dorsal root ganglion (body)
-Trigeminal ganglion (face/ head/ neck)
Brain doesn’t have nociceptors
Dorsal root ganglion
This is a structure that holds collection of cell bodies of 1st order neurons
- Present on the dorsal root (sensory)
- Composed of cell bodies of nerve fibres that are sensory (afferent)
- First order neurons
- Pseudo-unipolar neurons
- Can be the source of pain pathology
- Trigeminal ganglion is the equivalent for the face / head
Shingles- pain from dorsal root ganglion
Types of nevre fibres
Table on slide 17
Dorsal horn
The dorsal horn is the posterior part of grey
matter in your spinal cords
Some primary afferents synapse directly with the secondary neurone whilst others first synapse with interneurones first
Spinothalamic tract (STT)
Ascending sensory tracts (tracts that’s travel through the white matter to the thalamus at the 3rd order neuron)
-Dorsal columns: fine touch, proprioception, vibration
-Ventral spinothalamic tract: light touch
-Lateral spinothalamic tract: pain and temperature
-Anterior spinothalamic tract: crude touch
Sensory pathway that carries pain, temperature and crude touch information from the body
2nd order neurons
Originate in the spinal cord (substantia gelatinosa and nucleus proprius)
Axons decussate at / few levels above the site of entry / spinal segment
Cross the midline in the anterior commissure
Then form the anterolateral tract
lateral STT (pain & temperature) and
anterior STT (crude touch)
Terminate in the thalamus
(ventral posterior lateral nucleus)
Some axons terminate in the reticular
formation and midbrain
Thalamus
Midline, paired symmetrical structure in the brain
Approx 6 X 3 cms long
All sensations (except olfactory) relay/ pass through
Multiple nuclei- those bothered about most in pain transmission are
-VPL (Ventral posterior lateral nucleus of thalamus)
-Medial group
Reciprocal connections to all parts of the cortex
3rd order neuron cell bodies are found in thalamus
Sensory cortex
Broadmann area 3, 1, 2
Every area on the body is represented in a spatial fashion sensory homonculus
Pain matrix
Areas in brain involved in pain processing
Unique to every individual but there areas which are common in every person when pain is inflicted
Include insula, amygdala, cingulate cortex, Periaqueductal gray
Insula
This is where the degree of pain (experienced or imagined) is judged
Contributes to the subjective aspect of pain perception
Plays a role in perception, motor control, self awareness and interpersonal experience
May also play a part in addiction
Severity of pain is identified
Amygdala
plays a key role in learned emotional responses (fear, anxiety, depression)
important brain center for the emotional-affective dimension of pain and for pain modulation
hyperactivity in the of the central nucleus of the amygdala accounts for pain-related emotional responses and anxiety-like behavior
Cingulate cortex
Located on the medial aspect of the cerebral hemispheres
Intricately linked with the limbic system which is associated with emotion formation and processing, learning and memory
Maintains reciprocal connections with other pain processing areas
Has recently been shown to be one of the areas activated by acupuncture
Peri-aqueductal gray- beginning of descending pathways
Grey matter located around the cerebral aqueduct in brainstem
Receives input from cortical and sub-cortical areas
Projects onto neurons in the dorsal horn
-Modulate afferent noxious transmission
Neurons bear opioid receptors
(Look at next slide)
Pathways also include noradrenergic and
serotonergic neurones
Stimulus of the PAG can result in profound analgesia (no pain felt)
Reason why exercise makes you feel better and helps with pain
We have fibres that go down from the periaqueductal gray matter down to the spinal cord and modulate or alter how were processing the pain signal that’s going up
Chronic pain
Abnormal processing of pain
Complex processing of pain
Biopsychosocial factors- on slide
Treatment of pain
Stimulate descending inhibitory pathway
Gate control
Pharmacotherapy
Gate control theory
If you’ve got more pain inputs than other things, pain will get transmitted as a priority
However, if you tip the balance away from pain towards other inputs, in theory, you should get less pain transmission
The way the gate control theory is described is as a gate in the spina cord and the dorsal horns of the spinal cords- so if the gate is open, pain will be transmitted. If the gate is closed entirely, pain will not be transmitted
The gate can be shut slightly at the paraqueductal gray down though the descending pathway
This can be done by peripheral stimulation of other fibres (eg rubbing area of pain will activate the A beta fibres and close the gate)
The C fibre is activating the second order neurone and allowing pain signals to be transmitted
The C fibre also has a branch that is inhibiting the inhibitory neurone in the dorsal horn, therefore its not doing any action
Then the A beta fibres are activated by rubbing the area
A beta fibres are larger faster myelinated fibres and they stimulate inhibitory interneurons so they will block the forward transmission of pain
Opioids
Used to treat ACUTE pain
Problems with long term opioids consumption
Tolerance
Immunosuppression
Affects hormones
Not taking medicines
Increases pain
Decreases side effects
Initiated on opioid
Moderate pain and side effects
Tolerance
Increased side effects
Increased pain
OIH: Opioid induced Hyperalgesia
Whole body increase to stimuli/ pain
OIH: Opioid indicted Hyperalgesia
Side effects reduced
Moderate pain
