Physiology and Pathophysiology of Pain

Question 1

What is pain?

Answer 1

Pain is an unpleasant sensory and emotional experience which we primarily associate with tissue damage or describe in terms of such damage or both

Question 2

What is noiciception?

Answer 2

The detection of tissue damage by specialized transducers connected to A - delta and C fibers

Question 3

What part of the noiciceptor detects pain?

Answer 3

Free nerve endings

Question 4

What type of stimuli do noiciceptors respond to?

Answer 4

Thermal

Chemical

Mechanical

Noxious stimuli

Question 5

Which type of nerve fibre is responsible for fast pain? Slow pain?

Answer 5

Fast pain - A delta

Slow pain - C fibres

Question 6

What are the three types of neurone found predominantly in the grey matter?

Answer 6

Low threshold mechanoreceptive neurons (layer 3 and 4) - receive input from A beta fibres

Noiciceptive specific neurons (layer 1 and 2) receiving input from C and A delta fibres

WDR (wide dynamic range) (layer 5) - main input is A beta responds to both noxious and non-noxious stimuli via interneurons.

Question 7

Where do primary noiciceptive afferent impulses primarily end?

Answer 7

In the dorsal horn

Question 8

What is the anterior spinothalamic tract responsible for?

Answer 8

Sensation of simple touch

Question 9

What is the lateral spinothalamic tract responsible for?

Answer 9

Conveys fast and slow pain

(pain and temperature sensation)

Question 10

Which rexed lamina does the spinothalamic tract arise in?

Answer 10

Arises in the rexed lamina 1,2 and 5

Question 11

Where does the lateral spinothalamic tract end?

Answer 11

Terminates in the ventroposterior thalamic nuclei

Question 12

Where does the ventroposterior thalamic nuclei feed into?

Answer 12

The somatosensory cortex to facillitate the spatial, temporal and intensity discrimination of painful stimuli

Question 13

Where does the ventral spinothalamic tract project?

Answer 13

To the medial thalamic nuclei

Question 14

Where does the medial thalamic nuclei project to?

Answer 14

Cortical regions such as anterior cingulate and insular cortex as well as other parts of the limbic system

Question 15

Summary

Answer 15

Question 16

Where does pain perception occur?

Answer 16

In the somatosensory cortex

Question 17

Where do descending pathways arise from that usually descrease the pain signal?

Answer 17

Arise from the periaqeductal grey matter

travel from the brain to the dorsal horn

Question 18

What is hyperalgesia?

Answer 18

Increased perception of pain or even perception of non-noxious stimuli as noxious stimuli

Question 19

When does hyperalgesia happen?

Answer 19

Happens when there is a tissue injury and inflammation

Question 20

What do the terms primary hyperalgesia and secondary hyperalgesia mean?

Answer 20

Primary - hyperalgesia at the site of injury

Secondary - hyperalgesia in surrounding uninjured tissue

Question 21

Define allodynia

Answer 21

Central pain sensitization following normally non-painful stimulation

Allodynia can lead to the triggering of a pain response from stimuli which do not normally provoke pain.

Question 22

What is the difference between allodynia and hyperalgesia?

Answer 22

Allodynia - decreased threshold for response

Hyperalgesia - Exaggerated response to stimuli

Question 23

How does spontaneous pain compare to allodynia and hyperalgesia?

Answer 23

Spontaneous pain doen’t affect the sensation of pain - it results in spontaneous activity in nerve fibres

Question 24

What is the main difference between peripheral sensitization and central sensitization?

Answer 24

Central sensitization happens at the level of the spinal cord

Peripheral sensitization causes sensitivity to heat

Question 25

What are the three components of central censitization?

Answer 25

Wind-up Classical Long-term potentiation

Question 26

What happens during Wind up?

Answer 26

C fibres are influenced by substance P (their threshold is lowered) C fibres are now more sensitive and are more likely to depolarize There is also an increase in the receptive field of C fibres

Question 27

Which neurones are involved in wind up?

Answer 27

Only the neurones taking part in the synapses with primary afferent input

Question 28

When does wind up manifest and when does it terminate?

Answer 28

Manifests over the course of a stimulus and terminates with that stimulus

Question 29

How does the classical - central sensitisation mechanism work?

Answer 29

Involves opening up of new synapses as a result of a strong enough stimuli to elicit the response. If the intensity is strong enough then it occurs immediately with the stimuli and can outlast the duration of the stimuli Results in secondary hyperalgesia Once activated, it can be maintained even by low intensity of the offending stimuli

Question 30

What neurotransmitters and receptors are associated with classical sensitization?

Answer 30

NMDA receptor activation by glutamate is known to trigger a series of changes resulting in classical central sensitization

Question 31

Which synapses are involved in long-term potentiation?

Answer 31

Involves mainly the activated synapses

Question 32

What type of stimuli does long - term potentiation result from?

Answer 32

Results from very intense stimuli

Question 33

What are the receptors and transmitters for long-term potentiation?

Answer 33

The mechanism is involves both AMPA and NMDA receptor activation by glutamate.

Question 34

What is the result of suprasegmental central sensitization of pain?

Answer 34

Question 35

What is the difference between chronic pain and acute pain?

Answer 35

In chronic cases the presence of noxious stimuli is not essential and the source can be nociceptive, neuropathic or mixed

Question 36

What is nociceptive pain?

Answer 36

A sensory experience that occurs when specific peripheral sensory neurones (nociceptors) respond to noxious stimuli

Question 37

When does nociceptive pain normally end?

Answer 37

Usually resolves when damaged tissue heals

Question 38

What is neuropathic pain?

Answer 38

Pain initiated or caused by a primary lesion or dysfunction in the somato-sensory nervous system

Question 39

Where does neuropathic pain occur?

Answer 39

•The painful region may not necessarily be the same as the site of injury – pain occurs in the neurological territory of the affected structure (nerve, root, spinal cord, brain)

Question 40

Is neuropathic pain more likely ot be acute or chronic?

Answer 40

Chronic Ulinke nociceptive pain it responds poorly to conventional analgesics

Question 41

How can we reduce transduction of pain?

Answer 41

NSAIDS Ice Rest LA blocks

Question 42

How do we reduce transmission of nociception?

Answer 42

Nerve blocks Drugs 1. Opioids 2. Anticonvulsants Surgery 1. **DREZ** - dorsal root entry zone (The surgery destroys the area where damaged nerves join the central nervous system, thereby interfering with inappropriate pain messages from nerves to the brain.) 2. **Cordotomy** - cutting nerves of spinal cord

Question 43

How do we alter the perception of pain?

Answer 43

Education Cognitive behavioural therapy Distraction Relaxation Graded motor imagery Mirror box therapy

Question 44

How can we manipulate descending modulation?

Answer 44

Placebos Drugs * Opioids * Antidepressants Surgery * Spinal cord stimulation