Physiology and Pathophysiology of Pain

Question 1

What is nociception?

Answer 1

Detection of tissue damage by specialized transducers connected to A(delta) and C fibres

Question 2

Where are the cell bodies of primary afferent nerves located? Where do these nerves synapse?

Answer 2

Cell bodies in the dorsal root ganglion

Synapse at the spinal cord

Question 3

Which nerve fibres are responsible for detecting fast/sharp pain? Which fibres sense secondary slow/dull pain? Why?

Answer 3

• A(delta) fibres detect first/sharp pain because they are myelinated with medium diameter
• C fibres detect slow/dull pain because they are unmyelinated with small diameter

Question 4

Cell bodies of spinothalamic tract neurones mostly located in which rexed lamina? What is a rexed lamina?

Answer 4

rexed lamina 1, 2 & 5

Rexed laminae is a system of labelling portions of grey matter in the SC according to their cellular composition. Similar to Brodmann areas

Question 5

Which nuclei of the thalamus is involved in the relay of nociceptive information?

Answer 5

Ventroposterior thalamic nuclei and the medial dorsal nucleus

Question 6

To what parts of the brain does the thalamus relay pain information?

Answer 6

• Cortex
• Limbic System
• Brainstem

Question 7

What is the primary control centre for descending pain modulation?

Answer 7

PAG

Periaqueductal grey matter (located around the cerebral aqueduct)

Question 8

What is hyperalgesia and when does it occur?

Answer 8

It is increased perception of pain or even perception of non-noxious stimuli as noxious

Occurs whenever there is tissue injury or inflammation

Question 9

Difference between primary and secondary hyperalgasia?

Answer 9

Primary: hyperalgesia at the site of injury

Secondary: hyperalgesia in the surrounding tissue

Question 10

What is allodynia?

Answer 10

Reduction in the threshold at which stimuli becomes noxious, often following non-painful stimulation

Question 11

Examples of peripheral sensitization?

Answer 11

• Allodynia: reduced threshold
• Hyperalgesia: exaggerated responses
• Spontaneous pain: often without stimuli

Question 12

What is central sensitization?

Answer 12

When the second order neurones in the CNS start inputting noxious and non-noxious stimuli as pain

Question 13

3 Main components of central sensitization?

Answer 13

• Wind up
• Classical
• Long-term potentiation

Question 14

What is wind up? When does it manifest?

Answer 14

Winding up the response to input

Smaller inputs create larger responses due to wound up system, manifests over the course of the stimulus and terminates with the stimulus

Question 15

What is classical central sensitization? How long does it last in relation to the activating stimulus?

Answer 15

• New synapses are opened up in the dorsal horn, only respond to significant stimuli
• When significant stimuli activates new pathways, pain response tends to outlast the stimuli and can be maintained at even low levels of ongoing stimuli

Question 16

How is long term potentiation central sensitization activated?

Answer 16

• Involves mainly the activated synapses, occurs primarily for very intense stimuli

Question 17

Differences between segmental and suprasegmental central sensitization?

Answer 17

Segmental - nerve injury in SC, pain response is segmented/localized

Suprasegmental - nerve injury in brain, pain is felt all over the body (fibromyalgia/chronic widespread pain)

Question 18

Differences between acute and chronic pain?

Answer 18

• Acute lasts for <1 month, chronic for >3-6 months
• Acute pain resolves upon healing
• Chronic pain doesn’t usually serve a protective function like acute pain
• Acute pain occurs in presence of noxious stimuli, chronic pain doesn’t require stimuli

Question 19

What is nociceptive pain?

Answer 19

Sensory experience in response to specific peripheral sensory neurones respond to noxious stimuli

Question 20

Can nociceptive pain be chronic?

Answer 20

• Usually time limited and resolves

- Can be chronic, but still in response to valid stimuli (eg. osteoarthritis)

Question 21

What is neuropathic pain?

Answer 21

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

Usually chronic, painful region may not be the same as site of injury

Question 22

How do neuropathic and nociceptive pain tend to respond to analgesics?

Answer 22

• Nociceptive tends to respond to conventional analgesics

- Neuropathic does not

Question 23

Four phases at which you can attempt to treat pain?

Answer 23

• Transduction (NSAIDS/Ice)
• Transmission (opiods/surgery)
• Perception (Behavioural therapy)
• Descending modulation (opiods/spinal cord stimulation)

Question 24

What is a cordotomy?

Answer 24

• Surgical procedure that disables selected pain conducting tracts in the spinal cord

(stops pain at transmission stage)