T2 L9: Physiology of Pain

Question 1

What is Nociceptive pain?

Answer 1

Pain in response to tissue injury involving nociceptors

Question 2

What is Neuropathic pain?

Answer 2

Pain in response to injury to the NS

Question 3

Describe the properties of Aα and Aβ fibres

Answer 3

Myelinated with a large diameter

30-75m/sec

Question 4

What do Aα and Aβ fibres detect?

Answer 4

Light touch (Proprioceptive)

Question 5

Describe the properties of Aδ fibres

Answer 5

Thinly myelinated with a medium diameter

5-20m/sec

Question 6

What do Aδ fibres detect?

Answer 6

Light touch, temperature (Nociception)

Question 7

What do C fibres detect?

Answer 7

Temperature (Nociception)

Question 8

Describe the properties of C fibres

Answer 8

Unmyelinated with a small diameter

0.5-2m/sec

Question 9

What do Meissner’s corpuscles (Aδ) detect?

Answer 9

Stroking/fluttering

Question 10

What do Pacinian corpuscles (Aδ) detect?

Answer 10

Vibration

Question 11

What do Merkel discs (Aδ) detect?

Answer 11

Pressure

Question 12

What type of fibre are free nerve endings?

Answer 12

Aδ/C

Question 13

What do Ruffini endings (Aδ) detect?

Answer 13

Stretch

Question 14

What type of pain is felt when Aδ fibres are activated?

Answer 14

A sharp pricking pain. It’s well localised

Question 15

What type of pain is felt when C fibres are activated?

Answer 15

A slow dull pain or burning. It’s poorly localised

Question 16

What does Polymodal mean?

Answer 16

Responding to several different forms of sensory stimulation Eg. C-fibre nociceptors

Question 17

What do TRPV1 detect and what is it’s agonist?

Answer 17

Detect hot temperature

Agonised by Capsaicin (Chilli’s)

Question 18

What do TRPM detect and what is it’s agonist?

Answer 18

Detect cold temperature

Agonised by Menthol

Question 19

What do TRPA1 detect and what is it’s agonist?

Answer 19

Detect very cold temperatures

Agonised by Cinnamon

Question 20

Give examples of chemicals released during inflammation that have excitatory effects on nociceptors

Answer 20

ATP
H+
Serotonin (From Platelets)
Histamine (From Mast cells)
Bradykinin
Prostaglandin (From COX)
Nerve growth factor

Question 21

What does ATP bind to to activate nociceptors?

Answer 21

Purinergic receptors (P2X)

Question 22

What do H+ bind to to activate nociceptors?

Answer 22

Acid sensing ion channels

Question 23

What does Serotonin bind to to activate nociceptors?

Answer 23

5-HT3 receptors

Question 24

When one branch of a nociceptor is activated by inflammation, release of which substances is triggered from other nociceptors and what does this lead to?

Answer 24

Substance P and Calcitonin gene related peptide (CGRP) leading to vasodilation and activation of mast cells which release more histamine

This is the process of neurogenic inflammation

Question 25

What is Hyperalgesia?

Answer 25

Noxious stimuli produce exaggerated pain responses

Question 26

What is Allodynia?

Answer 26

Non-Noxious stimuli produce painful responses

Question 27

What causes hypersensitivity?

Answer 27

Peripheral and central sensitisation | increased responsiveness and reduced threshold of nociceptive neurons driven by tissue injury or inflammation

Question 28

What function do Bradykinin and NGF have in peripheral sensitisation?

Answer 28

They reduce the threshold of heat activated channels (TRPV1)

Question 29

What function do Prostaglandins have in peripheral sensitisation?

Answer 29

They reduce the threshold of sodium channels making the cell more excitable

Question 30

What does phosphorylation of a channel do?

Answer 30

It reduces its threshold making it more excitable

Question 31

Which tract is used for pain information to ascend in the CNS?

Answer 31

The spinothalamic tract

Question 32

What do first-order neurones to when they reach the spinothalamic tract?

Answer 32

They enter through the dorsal horn and form the tract of Lissauer and then synapse in the Substantia gelatonisa (In lamina I and II)

Question 33

What causes referred pain?

Answer 33

The convergence of visceral and cutaneous nociceptors on the same second-order neuron in the spinal cord. The brain will then perceive the pain as cutaneous

Question 34

Where do the 3rd-order neurones ascend to?

Answer 34

The primary somatosensory cortex in the brain

Question 35

What is the sensory homunculus?

Answer 35

A topographic representation of the sensory distribution of the body found in the cerebral cortex Lower body: medial Upper body: lateral

Question 36

What is the physiology behind the emotional component of pain?

Answer 36

The third-order neurones split and project into the insula and cingulate cortex and encode the emotional components

Question 37

What is the effect of lesions on the insula?

Answer 37

They stop the person from crying out in pain even though they're in pain

Question 38

What is stress-induced analgesia?

Answer 38

When pain is supressed by activation of descending modulatory pathways Eg. battle victims, endurance athletes

Question 39

What are the two descending regulation regions of pain and what do they do?

Answer 39

Periaqueductal grey matter (PAG) and Rostral ventromedial medulla (RVM) They modulate activity of the spinothalamic tract (Can be excitatory or inhibitory)

Question 40

How is pain inhibited?

Answer 40

The periaqueductal grey matter (PAG) neurones excite serotonergic neurones which excite inhibitory interneurons which then inhibit spinothalamic tract neurones

Question 41

How do opioids inhibit pain?

Answer 41

They act on inhibitory metabotropic receptors

Question 42

What effect do opioids have in the PAG and RVM?

Answer 42

They inhibit inhibitory interneurons

Question 43

What effect do opioids have in the dorsal root?

Answer 43

They inhibit secondary-order neurones