Pain and thermosensation Flashcards
What is pain?
Sensory and emotional experience, associated with actual tissue damage or described in terms of such damage
What are the 3 forms of pain?
Nociceptive; immediate, protective response. ADAPTIVE
Inflammatory; assists in healing, persists over days - weeks ADAPTIVE
Pathological; no physiological purpose, persists over months, years or a lifetime. MALADAPTIVE
What are nociceptors?
Specific peripheral primary sensory afferent neurones which are normally activated preferentially by intense stimuli (thermal, mechanical and chemical)
What fibres make up nociceptors?
A-delta
C fibres
What noxious stimuli will A-delta fibres respond to?
Mechanical/ thermal
What noxious stimuli will C fibres respond to?
Polymodal; all noxious stimulus
Which pain will A-delta fibres mediate?
First; lancinating, stabbing, pricking
Which pain will C-fibres mediate?
Second; burning, throbbing, cramping, aching
What is the difference between A-delta and C fibres in terms of action potential conduction?
A-delta: thinly myelinated, quicker conduction velocity
C-fibres: unmyelinated, slower conduction velocity
What receptors are utilized in thermal stimulus?
TRP family
What receptors are common in chemical noxious stimulus?
H+ activates ASICs
ATP activates P2X and P2Y
Bradykinin activates B2
What is the difference between type 1 and type 2 A-delta fibres?
Type 1 = very hot temperatures at 53 °C
Type 2 = less hot temperatures at 43 °C
In which area will noxious axons enter the spinal cord?
Dorsal horn
Which neurotransmitters are involved in afferent noxious information?
Glutamate
Peptides; substance P and neurokinin A
What can noxious stimulation in the long term result in?
Hyperalgesia
Allodynia
What can efferent noxious action potentials result in?
Pro-inflammatory mediator (CGRP, substance P) release from free nerve endings contributing to neurogenic inflammation
What is the action of substance P?
Vasodilation and extravasation of plasma proteins
Promotes formation of bradykinin
Release of histamine from mast cells
Sensitizes surrounding nocicpetors
What is the action of CGRP?
Induces vasodilation
Is glutamate a slow or fast neurotransmitter?
Fast EPSP and neuronal excitation
Which receptors will glutamate act on?
AMPA and then NMDA when afferent input is intense
Describe the action of NMDA receptors
Usually silent as the pore is blocked by a magnesium
When the cell becomes depolarised via AMPA allowing Na+ inside the cell, the magnesium will be released and the NMDA will conduct calcium
What is the impact of calcium entering the cell via NMDA receptors?
Changes in gene transcription resulting in increased sensitivity of post synaptic cells to glutamate
What is the action of peptides in NMDA activation?
Cause a slow and prolonged EPSP that facilitates the activation of NMDA
Which laminae of rexed do the cell bodies of noxious afferents lie in?
Laminae 1 and 2
5 for A-delta also
With what cells will C and A-delta fibres synapse with in the laminae of rexed?
Nociceptive Specific (NS) cells
Which cell is present in laminae 5 and what does it receive input from?
Wide Dynamic Range (WDR) which receives input from A-beta, A-delta and C fibres; NOXIOUS AND NON-NOXIOUS STIMULUS
Which pathway will visceral pain afferents follow?
Symp pathways before entering the dorsal horn but do NOT synapse in the sympathetic ganglion
How does referred pain occur?
Visceral and skin afferent converge upon the same spinothalamic neurones (all cells with a visceral receptive field also have a separate cutaneous RF)
What is the segmental dermatomes for the heart?
T1-5
What is the segmental dermatomes for gallbladder pain?
C4
What is the gate control theory?
Pain evoked by nociceptors can be reduced by simultaneous activity in beta fibres via closing of the substantia gelatinosa
Where will projection neurones from lamina 1 of the spinothalamic tract terminate?
Posterior nucleus of thalamus
Where will projection neurons from the WDR nucleus of laminae 5 terminate?
Posterior and ventroposterior nucleus of the thalamus
What is required for pain perception?
Simultaneous firing of WDR and laminae 1 neurons to allow for location and intensity
Which fibres will the spinoreticular tract transmit?
Slow C-fibre pain
With what nuclei will the spinoreticular tract communicate?
Reticular nuclei in the brain stem; periaqueductal grey and parabrachial nucleus
What is the spinoreticular tract involved with?
Autonomic responses to pain, arousal, emotional responses and fear of pain
What are thermoreceptors?
Neurones that are specialised to respond to small changes in temperature
Do all thermoreceptors respond to all types of temperature?
NO - there are hot sensitive and cold sensitive spots which are innervated by different neurons
