Pain physiology Flashcards
What are the three classifications of pain?
Nocioceptive
Inflammatory
Pathological
What is the definition of pain?
An unpleasant sensory and emotional experience.
What are nocioceptors?
Peripheral primary specific sensory afferent neurones activated by intense noxious stimuli e.g. Chemical, thermal, mechanical etc.
Are nocioceptors first or second order neurones?
First order
What kind of neurones do nocioceptors relay information to and where are they?
Second order neurones in the CNS
Is nocioceptive pain adaptive or maladaptive?
Why?
Adaptive as it is an early warning system that detects and minimises contact with damaging stimuli.
Is nocioceptive pain high or low threshold?
High threshold, it is provoked only by intense stimuli.
What happens when a nocioceptor is activated by noxious stimuli?
It overrides other CNS activities and initiates a withdrawal reflex. It also initiates an emotional response that allows us to remember this and avoid the stimuli in future.
Is inflammatory pain adoptive or maladaptive?
It is adaptive and protective
What other two things can inflammatory pain result in?
Pain hypersensitivity and allodynia (innocuous stimuli now elicit pain)
What is the benefit of inflammatory pain?
Assists in healing of damages area by: discouraging physical contact and movement.
Is inflammatory pain high or low threshold?
Low threshold in order for tenderness to promote healing.
What is adaptive pain?
Helpful pain that will encourage behaviour that aids healing.
Is pathological pain adaptive or maladaptive?
Maladaptive, it has no protective function
What are the two categories of pathological pain?
Neuropathic and dysfunctional.
What is neuropathic pain?
Caused by damage or disease affecting the somatosensory nervous system.
What two things is pathological pain associated with?
Spontaneous pain and pain hypersensitivity.
What is dysfunctional pain?
A pain where there is no obvious neural lesion or inflammatory pain.
What causes congenital insensitivity to pain?
Rare condition resulting in loss of function mutations e.g. Missense in gene that encodes Na channels that are highly expressed in nocioceptive neurones
What are the symptoms of CIP?
Lip and tongue injury, bruises and cuts, multiple scars, bone fractures, joint deformity and premature mortality.
What are the two subtypes of nocioceptor?
A delta (incomplete figure of 8 sign) C-fibres
What type of stimuli do A delta nocioceptors respond to?
Mechanical and thermal stimuli.
Are A delta nocioceptors myelinated?
Yes but only thinly
What type of pain do A delta nocioceptors mediate?
First or fast pain e.g. Stabbing prickling sensations.
Are c fibres myelinated?
No they are unmyelinated so have slower conduction velocity.
What type of stimuli do C fibre nocioceptors respond to?
All types of stimuli collectively.
What type of pain do C fibres mediate?
Second or slow pain e.g. Burning, throbbing cramping aching sensations.
How does a stimulus in sensory neurones illicit an action potential?
They open ion channels in the nerve terminal.
What is the potential created by stimuli at a sensory nerve terminal called?
Depolarising receptor or generator potential.
What affects the generator potential amplitude?
The intensity of the stimulus, the generators potential is graded to this.
What effect does the generator potential amplitude have on the resulting action potential?
Action potential are triggered at a frequency proportional to the amplitude of the receptor potential.
Where do nociceptor nerves enter the spinal cord?
Where are their cell bodies?
What happens to the after they enter the spinal cord?
Enter through the dorsal horn
Bodies in the dorsal root ganglion or trigeminal ganglion.
They cross over the spinal cord and ascend to brain up the anterior horn.
What is a subset of C fibres that have afferent and efferent functions?
Peptidergic polymodal nociceptors.
What peptides do afferent peptidergic polymodal nociceptors release in the dorsal horn?
Substance P and neurokinin A
What substances do efferent peptidergic polymodal nociceptors release?
Inflammatory mediators e.g. Calcitonin gene-related peptide (CRGP), substance P etc.
What can chemical release from efferent peptidergic polymodal nociceptors contribute to?
Neurogenic inflammation.
What does release of substance P cause?
Vasodilation and extravasation of plasma proteins
Release of histamine from mast cells
Sensitises surrounding nociceptors
What does CRGP do?
Induces vasodilation
What stimuli can trigger release of chemicals from petidergic nociceptor efferents?
Tissue damage or inflammatory mediators.
What is the primary transmitter released from the primary afferent in the dorsal horn?
Glutamate
What is the name for the area in the dorsal horn where primary afferent axons terminate?
Laminae of rexed
Where do nociceptive C and A delta fibres mostly terminate?
Superficially in laminae 1 and 2 (and also 5 for A delta)
What do nociceptive specific cells synapse with?
C and Adelta fibres.
What type of cells only receive input from Abeta fibres?
Proprioceptive
What type of neurones receive input from C fibres Adelta and Abeta fibres?
Wide dynamic range neurones.
Where do second order neurones ascend the spinal cord?
In the anterolateral system
What does the anterolateral system in the spine mostly consist of?
The spinothalmic tract and the spinoreticular tract.
What types of neurones does the spinothalmic tract project?
What lamina do they come from?
Where does it terminate?
Fast fibre Adelta neurones
Lamina 1
Posterior nucleus of the thalamus
WDR neurones
Lamina 5
Terminates in posterior and ventroposterior nucleus of the thalamus.
What does pain proprioception require?
Simultaneous firing of both pathways in the spinothalmic tract
What does the spinoreticular tract mostly transmit?
Slow C fibre pain neurones
Where does the spinoreticular tract connect to in the brain?
Reticular nuclei in the brain stem (periaqueductal grey - PAG) and parabrachial nucleus (PBN)
What responses to pain is the spinoreticular tract involved in?
Autonomic responses e.g. Emotional arousal, fear etc.
