Physiology of Pain Flashcards
Define pain
An unpleasant sensory and emotional experience which is associated with actual tissue damage or described in terms of such damage
What are the three forms of pain?
- Nociceptive
- Inflammatory
- Pathological
Which type of pain is not adaptive?
Pathological pain
What is pathological pain?
Pain that outlives its biological or adaptive purpose
What is adaptive pain?
Pain which serves as an early warning system to detect and minimise contact with damaging stimuli
(nociceptive and inflammatory pain)
What threshold level does nociceptive pain have?
High
It is activated by intense stimuli
Which type of pain is associated with pain hypersensitivity?
Inflammatory pain
What is pain hypersensitivity?
An increased behavioural response to a constant painful stimulus in order to allow for healing
This is adaptive
What is allodynia?
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
Which two types of pathological pain exist?
- Neuropathic pain
- Dysfunctional pain
What is neuropathic pain?
Pain caused due to nerve cell damage
What is dysfunctional pain?
A potentially irregular processing of inoculous information in the CNS
What is the name of the condition that results in complete absense of pain?
Congenital insensitivity to pain (CIP)
Caused by a loss of function mutation affecting voltage activated sodium channels which are highly expressed in nociceptive receptors
What are the two types of nociceptor?
- Aδ fibres
- C fibres
What are the 3 main differences regarding Aδ and C nociceptive fibres?
- Aδ fibres are thinly myelinated, C fibres are not
- Aδ fibres only respond to thermal or mechanical stimuli whilst C fibres respond to all noxious stimuli (polymodal)
- Aδ fibres confer “fast” pain, whilst C fibres confer “slow” pain
What is the pathway from the sensory receptor to the CNS for pain?
- Nociceptor
- Second order neurone
- Spinothalmic or spinoreticulothalmic tracts
Which type of nociceptive fibre has:
a) A really high heat threshold (53 degrees)
b) A more moderate heat threshold (43 degrees)
a) Aδ type I
b) Aδ type II
What is the neurotransmitter associated with nociceptive receptors?
Glutamate
How is nociceptive information sent to the CNS within the dorsal horn?
- Glutamate
- Peptides (substance P and neurokinin A)
What is the efferent function of C fibres?
Pro-inflammatory mediator release from peripheral terminals (calcitonin and substance P)
Calcitonin gene related peptide (CGRP) and substance P can contribute to what?
Neurogenic inflammation
How do calcitonin gene related peptide and substance P cause increased sensitivity of nerve endings?
- 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
Which receptors are found on the (post ganglionic) second order neurone?
- AMPA (main one)
- NMDA
When will NMDA receptors become involved in transmission of Ca2+ ions at the second order neurone?
During intense noxious stimuli
Why do NMDA receptors not participate as frequently at the second order neurone?
They are usually blocked by Mg2+
Substance P and CGRP prolong the depolarisation of the 2nd order neurone causing an intense depolarisation
This aids in NMDA receptor activation by causing unbinding of Mg2+
The laminae at the dorsal horn, where Aδ and C fibres synapse, have what collective name?
The Laminae of Rexed
Which laminae of the dorsal horn will Aδ and C fibres synapse at?
I and II (also V for Aδ)
These are the superficial layers
Which neurone will receive input from all 3 types of nociceptive nerve fibre?
Wide dynamic range neurone
Which nerve fibre transmits proprioceptive stimuli?
Aβ fibres
How many major nociceptie tracts are there?
2
