T2 L9: Physiology of Pain Flashcards
What is Nociceptive pain?
Pain in response to tissue injury involving nociceptors
What is Neuropathic pain?
Pain in response to injury to the NS
Describe the properties of Aα and Aβ fibres
Myelinated with a large diameter
30-75m/sec
What do Aα and Aβ fibres detect?
Light touch (Proprioceptive)
Describe the properties of Aδ fibres
Thinly myelinated with a medium diameter
5-20m/sec
What do Aδ fibres detect?
Light touch, temperature (Nociception)
What do C fibres detect?
Temperature (Nociception)
Describe the properties of C fibres
Unmyelinated with a small diameter
0.5-2m/sec
What do Meissner’s corpuscles (Aδ) detect?
Stroking/fluttering
What do Pacinian corpuscles (Aδ) detect?
Vibration
What do Merkel discs (Aδ) detect?
Pressure
What type of fibre are free nerve endings?
Aδ/C
What do Ruffini endings (Aδ) detect?
Stretch
What type of pain is felt when Aδ fibres are activated?
A sharp pricking pain. It’s well localised
What type of pain is felt when C fibres are activated?
A slow dull pain or burning. It’s poorly localised
What does Polymodal mean?
Responding to several different forms of sensory stimulation Eg. C-fibre nociceptors
What do TRPV1 detect and what is it’s agonist?
Detect hot temperature
Agonised by Capsaicin (Chilli’s)
What do TRPM detect and what is it’s agonist?
Detect cold temperature
Agonised by Menthol
What do TRPA1 detect and what is it’s agonist?
Detect very cold temperatures
Agonised by Cinnamon
Give examples of chemicals released during inflammation that have excitatory effects on nociceptors
ATP H+ Serotonin (From Platelets) Histamine (From Mast cells) Bradykinin Prostaglandin (From COX) Nerve growth factor
What does ATP bind to to activate nociceptors?
Purinergic receptors (P2X)
What do H+ bind to to activate nociceptors?
Acid sensing ion channels
What does Serotonin bind to to activate nociceptors?
5-HT3 receptors
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?
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
What is Hyperalgesia?
Noxious stimuli produce exaggerated pain responses
What is Allodynia?
Non-Noxious stimuli produce painful responses
What causes hypersensitivity?
Peripheral and central sensitisation
increased responsiveness and reduced threshold of nociceptive neurons driven by tissue injury or inflammation
What function do Bradykinin and NGF have in peripheral sensitisation?
They reduce the threshold of heat activated channels (TRPV1)
What function do Prostaglandins have in peripheral sensitisation?
They reduce the threshold of sodium channels making the cell more excitable
What does phosphorylation of a channel do?
It reduces its threshold making it more excitable
Which tract is used for pain information to ascend in the CNS?
The spinothalamic tract
What do first-order neurones to when they reach the spinothalamic tract?
They enter through the dorsal horn and form the tract of Lissauer and then synapse in the Substantia gelatonisa (In lamina I and II)
What causes referred pain?
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
Where do the 3rd-order neurones ascend to?
The primary somatosensory cortex in the brain
What is the sensory homunculus?
A topographic representation of the sensory distribution of the body found in the cerebral cortex
Lower body: medial
Upper body: lateral
What is the physiology behind the emotional component of pain?
The third-order neurones split and project into the insula and cingulate cortex and encode the emotional components
What is the effect of lesions on the insula?
They stop the person from crying out in pain even though they’re in pain
What is stress-induced analgesia?
When pain is supressed by activation of descending modulatory pathways Eg. battle victims, endurance athletes
What are the two descending regulation regions of pain and what do they do?
Periaqueductal grey matter (PAG) and Rostral ventromedial medulla (RVM)
They modulate activity of the spinothalamic tract (Can be excitatory or inhibitory)
How is pain inhibited?
The periaqueductal grey matter (PAG) neurones excite serotonergic neurones which excite inhibitory interneurons which then inhibit spinothalamic tract neurones
How do opioids inhibit pain?
They act on inhibitory metabotropic receptors
What effect do opioids have in the PAG and RVM?
They inhibit inhibitory interneurons
What effect do opioids have in the dorsal root?
They inhibit secondary-order neurones
