Anatomy And Physiology Of Pain

Question 1

Definition of pain

Answer 1

Pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage

Question 2

Nociception

Answer 2

Describes the neural processes involved in producing the sensation of pain

Question 3

Noiciceptive pathways

Answer 3

Transduction in the periphery, through transmission to the dorsal horn of the spinal cord, then on to the brain

Question 4

Normal physiological pain includes

Answer 4

Instant and acute pain

Question 5

Abnormalities from processing from the stimuli to the CNS causes

Answer 5

Chronic pain

Question 6

Acute

Answer 6

Pain < 12 weeks duration

Question 7

Chronic

Answer 7

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

Question 8

Nociceptive pain

Answer 8

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

Question 9

Neuropathic pain- hard to explain/vocalise

Answer 9

Pain caused by a lesion or disease of the somatosensory nervous system

E.g. stinging, burning, tingling

Question 10

Nociplastic pain/ other pain (pain which isn’t Nociceptive or neuropathic)

Answer 10

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

Question 11

Allodynia

Answer 11

Pain due to a stimulus that does not normally provoke pain

E.g. light touch

Question 12

Dysesthesia

Answer 12

An unpleasant abnormal sensation, whether spontaneous or evoked

Question 13

Hyperalgesia

Answer 13

Increased pain from a stimulus that normally provokes pain

Question 14

Hypoalgesia

Answer 14

Dismissed pain in response to a normally painful stimulus

Question 15

The pain pathway

Answer 15

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

Question 16

Nociceptors- pain receptor responsible for transduction

Answer 16

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

Question 17

Primary afferent neurones

Answer 17

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

Question 18

Dorsal root ganglion

Answer 18

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

Question 19

Types of nevre fibres

Answer 19

Table on slide 17

Question 20

Dorsal horn

Answer 20

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

Question 21

Spinothalamic tract (STT)

Answer 21

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

Question 22

Thalamus

Answer 22

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

Question 23

Sensory cortex

Answer 23

Broadmann area 3, 1, 2

Every area on the body is represented in a spatial fashion sensory homonculus

Question 24

Pain matrix

Answer 24

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

Question 25

Insula

Answer 25

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

Question 26

Amygdala

Answer 26

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

Question 27

Cingulate cortex

Answer 27

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

Question 28

Peri-aqueductal gray- beginning of descending pathways

Answer 28

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

Question 29

Chronic pain

Answer 29

Abnormal processing of pain

Complex processing of pain

Biopsychosocial factors- on slide

Question 30

Treatment of pain

Answer 30

Stimulate descending inhibitory pathway

Gate control

Pharmacotherapy

Question 31

Gate control theory

Answer 31

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

Question 32

Opioids

Answer 32

Used to treat ACUTE pain

Question 33

Problems with long term opioids consumption

Answer 33

Tolerance

Immunosuppression

Affects hormones

Question 34

Not taking medicines

Answer 34

Increases pain

Decreases side effects

Question 35

Initiated on opioid

Answer 35

Moderate pain and side effects

Question 36

Tolerance

Answer 36

Increased side effects

Increased pain

Question 37

OIH: Opioid induced Hyperalgesia

Answer 37

Whole body increase to stimuli/ pain

Question 38

OIH: Opioid indicted Hyperalgesia

Answer 38

Side effects reduced

Moderate pain