Physiology of Pain 1 Flashcards
What is pain?
An unpleasant sensory experience that is associated with tissue damage.
What are nociceptors?
Primary sensory neurons that detect pain
What are the three classifications of pain?
- Nociceptive: Normal functioning of nociceptors
- Inflammatory: Pain in response to inflammation
- Neuropathic: Pain in response to injury to the nervous system
What sensory nerve fibres are involved in pain transmission?
- A-delta fibres
- C fibres
What kind of pain do A-sigma fibres induce?
Sharp, pricking pain
What kind of pain do C fibres induce?
- Dull aches
- Burning pain
How can sensory nerve fibres be classified?
Through diameter and myelin content
What are A-alpha fibres and A-beta fibres involved with?
- Light touch
- Proprioception
What are A-sigma fibres involved with?
- Temperature
- Nociception (pain)
- Light
What are C fibres involved with?
- Nociception
- Temperature
What feelings do nociceptors induce?
- A-delta fibres create sharp, pricking pains (well localised)
- C fibres create dull aches or burning pains (poorly localised)
What does C fibres being polymodal mean?
It means that they are able to respond to different types of sensory information (pressure, temperature and chemicals).
Therefore, the CNS has to decode the information it receives from them.
What tract does pain information ascend?
Spinothalamic tract
What is referred pain?
When a pain in the body is felt in a different place to where it originated.
This is due to the convergence of visceral and cutaneous nociceptors on the same second order neurons in the spinal cord.
The brain then perceives the pain to be cutaneous
What is an example of referred pain?
Myocaridal Infarction / Angina
Occurs because visceral nociceptors from the heart and cutaneous nociceptors from the skin over the left surface of the arm converge as they enter the dorsal horn.
What is the substantia gelatinosa?
A mass of gelatinous gray matter that extends through the whole spinal cord and into the medulla oblongata.
It is involved with pain transmission.
What do first order neurons form when they synapse into the dorsal horn?
Tract of Lissauer
What are the two important regions in the descending regulation of pain?
- Periaqueductal Grey Matter (PAG)
- Rostral Ventromedial Medulla
What is the effect of the RVM on the spinothalamic tract?
Has the ability to inhibit or excite the spinothalamic tract
Where do 3rd order neurons of the spinothalamic tract synapse onto, and what structures in the brain do they have projections do and what does this allow for?
Primary somatosensory cortex
Projections to the insula and cingulate cortex, which encodes the emotional component of pain
What is stress-induced analgesia and how does it occur?
Loss of sensation of pain in extreme stress (e.g. in a battle)
Due to the activation of the descending pathways involved in the inhibition of pain
What type of neurons are able to release opioids, and in what parts of the CNS does this occur?
Interneurons
- Midbrain (PAG)
- Medulla (RVM)
- Dorsal horn
How do the PAG and RVM interact with each other and what can the interactions lead to?
- PAG is able to excite RVM
- RVM is then able to excite or inhibit the spinothalamic tract
What does the Tract of Lissauer synapse onto?
Substantia gelatinosa
