Physiology of Pain

Question 1

Define pain

Answer 1

An unpleasant sensory and emotional experience which is associated with actual tissue damage or described in terms of such damage

Question 2

What are the three forms of pain?

Answer 2

1. Nociceptive
2. Inflammatory
3. Pathological

Question 3

Which type of pain is not adaptive?

Answer 3

Pathological pain

Question 4

What is pathological pain?

Answer 4

Pain that outlives its biological or adaptive purpose

Question 5

What is adaptive pain?

Answer 5

Pain which serves as an early warning system to detect and minimise contact with damaging stimuli

(nociceptive and inflammatory pain)

Question 6

What threshold level does nociceptive pain have?

Answer 6

High

It is activated by intense stimuli

Question 7

Which type of pain is associated with pain hypersensitivity?

Answer 7

Inflammatory pain

Question 8

What is pain hypersensitivity?

Answer 8

An increased behavioural response to a constant painful stimulus in order to allow for healing

This is adaptive

Question 9

What is allodynia?

Answer 9

Increased sensitivity to painful region and when non-painful stimuli cause pain.

This is to ensure constact with the painful area is minimised to allow for healing

Question 10

Which two types of pathological pain exist?

Answer 10

1. Neuropathic pain
2. Dysfunctional pain

Question 11

What is neuropathic pain?

Answer 11

Pain caused due to nerve cell damage

Question 12

What is dysfunctional pain?

Answer 12

A potentially irregular processing of inoculous information in the CNS

Question 13

What is the name of the condition that results in complete absense of pain?

Answer 13

Congenital insensitivity to pain (CIP)

Caused by a loss of function mutation affecting voltage activated sodium channels which are highly expressed in nociceptive receptors

Question 14

What are the two types of nociceptor?

Answer 14

1. Aδ fibres
2. C fibres

Question 15

What are the 3 main differences regarding Aδ and C nociceptive fibres?

Answer 15

1. Aδ fibres are thinly myelinated, C fibres are not
2. Aδ fibres only respond to thermal or mechanical stimuli whilst C fibres respond to all noxious stimuli (polymodal)
3. Aδ fibres confer “fast” pain, whilst C fibres confer “slow” pain

Question 16

What is the pathway from the sensory receptor to the CNS for pain?

Answer 16

1. Nociceptor
2. Second order neurone
3. Spinothalmic or spinoreticulothalmic tracts

Question 17

Which type of nociceptive fibre has:

a) A really high heat threshold (53 degrees)
b) A more moderate heat threshold (43 degrees)

Answer 17

a) Aδ type I
b) Aδ type II

Question 18

What is the neurotransmitter associated with nociceptive receptors?

Answer 18

Glutamate

Question 19

How is nociceptive information sent to the CNS within the dorsal horn?

Answer 19

• Glutamate
• Peptides (substance P and neurokinin A)

Question 20

What is the efferent function of C fibres?

Answer 20

Pro-inflammatory mediator release from peripheral terminals (calcitonin and substance P)

Question 21

Calcitonin gene related peptide (CGRP) and substance P can contribute to what?

Answer 21

Neurogenic inflammation

Question 22

How do calcitonin gene related peptide and substance P cause increased sensitivity of nerve endings?

Answer 22

• Vasodilatation
• Extravation of plasma proteins
• Histamine release from mast cells
• Bradykinin and prostaglandin production promoted
• This increases the sensitivity of surrounding nerve endings of nociceptors

Question 23

Which receptors are found on the (post ganglionic) second order neurone?

Answer 23

1. AMPA (main one)
2. NMDA

Question 24

When will NMDA receptors become involved in transmission of Ca²⁺ ions at the second order neurone?

Answer 24

During intense noxious stimuli

Question 25

Why do NMDA receptors not participate as frequently at the second order neurone?

Answer 25

They are usually blocked by Mg²⁺

Substance P and CGRP prolong the depolarisation of the 2^nd order neurone causing an intense depolarisation

This aids in NMDA receptor activation by causing unbinding of Mg²⁺

Question 26

The laminae at the dorsal horn, where Aδ and C fibres synapse, have what collective name?

Answer 26

The Laminae of Rexed

Question 27

Which laminae of the dorsal horn will Aδ and C fibres synapse at?

Answer 27

I and II (also V for Aδ)

These are the superficial layers

Question 28

Which neurone will receive input from all 3 types of nociceptive nerve fibre?

Answer 28

Wide dynamic range neurone

Question 29

Which nerve fibre transmits proprioceptive stimuli?

Answer 29

Aβ fibres

Question 30

How many major nociceptie tracts are there?

Answer 30

2