Nociception and thermoception Flashcards
What is pain?
“An unpleasant sensory and emotional experience, associated with actual tissue damage or described in terms of such damage”
What three types of pain are there?
Nociceptive
Inflammatory
Pathological
What are some characteristics of nociceptive pain?
Adaptive (an immediate protective response, short-lived)
What are some characteristics of inflammatory pain?
Adaptive (assists in healing, persists over days, possibly weeks)
What are some characteristics of pathological pain?
Maladaptive (no physiological purpose, persists over months, years, or even a lifetime)
What are nociceptors?
Peripheral primary sensory afferent neurons normally activated by intense stimuli (e.g. thermal, mechanical, chemical) that are noxious
What kind of fibres are nociceptors?
Ad and C
What noxious stimuli do Ad fibres respond to?
Mechanical and heat- First pain (stabbing/pricking)
What noxious stimuli do C fibres respond to?
All- secondary pain (Burning/throbbing)
What are the three broad categories of nocireceptor?
Mechanical
Thermal
Chemical
Give some examples of thermal receptors
TRPA1, TRPC3 and TRPV1 (all noxious heat)
Give some examples of chemical receptors
ASIC- H+
P2X and P2Y- ATP
B2- Bradykinin
Where are the somas of nociceptors found?
DRG
Where do nociceptors synapse and decussate?
Posterior horn
Decussate immediately
What are Peptidergic Polymodal Nociceptors?
Subset of C fibres with afferent and efferent function
What is the afferent function of Peptidergic Polymodal Nociceptors?
Transmit nociception via glutamate and substance P
What is the efferent function of Peptidergic Polymodal Nociceptors?
Release proinflammatory mediators such as substance P to increase inflammation. Neurogenic inflammation.
What can long term stimulation of nociceptors do?
Increase excitability leading to hyperalgesia and allodynia.
What is neurogenic inflammation?
Inflammation induced by local release of inflammatory mediators from efferent neurons (Peptidergic Polymodal Nociceptors)
Describe neurogenic inflammation
Peptides (SP and CGRP) released from free nerve ending of peptidergic nociceptor due to tissue damage, or inflammatory mediators.
What effect does Substance P have on tissue?
Vasodilation and extravasation of plasma proteins (promotes formation of bradykinin and prostaglandins) Release of histamine from mast cells
Sensitizes surrounding nociceptors
What effect does CGRP have on tissue?
Vasodilation
What are the important NT in the primary synapse of nociception?
Glutamate (Na influx)- AMPA and NMDAR
Substance P and CGRP- During high Hz stim to prevent voltage dependent Mg+ block
Where do Ad and C fibres mainly terminate?
Dorsal horn
Lamina of Rexed I and II
What two types of second order neuron can primary nociceptive neurons synapse onto?
Nociceptive specific
Wide dynamic range neurons
What three fibre types do WDR neurons receive input from?
Ab
Ad
C
Where does visceral pain originate from?
Tissues and walls of hollow organs
What causes visceral pain?
Stretching, twisting, inflammation and ischaemia – but not cutting, or burning
How do visceral afferents reach the SC?
By following the sympathetic tract
What nerve fibres can visceral afferents converge with?
Skin afferents
What can this convergence of visceral and skin afferents cause?
Referred pain
What is visceral pain?
Perceived at a distance from the affected organ (AKA. referred pain)
Whats is viscerosomatic pain?
Sharp and well localised.
Inflammatory exudate from a diseased organ contacts a somatic (body wall) structure
What is the difference between pain and nociception?
Pain is awareness of suffering
Nociception is activation of nociceptors
What is gate theory?
Nerve impulses, evoked by injury, are influenced in the spinal cord by other nerve cells that act like gates, either preventing the impulses from getting through, or facilitating their passage.
Give a simple example of gate theory
Ab and C fibres both processed in same area of SC. Increased activation of Ab decreases activation of C.
Through which two tracts do nociceptors project to the brain?
Spinothalamic tract
Spinoreticular tract
What nociceptors are found in the STT and where do they terminate?
Ad fast fibres from lamina I and WDR from lamina V
Decussate immediately and terminate in the posterior thalamus
What nociceptors are found in the SRT and where do they terminate?
C fibres to reticular nucleus in BS
