Pain and Thermosensation Flashcards
Define pain
An unpleasant sensory and emotional experience, associated with actual tissue damage or described in terms of such damage
What are the 3 forms of pain ?
- Nociceptive (acute) pain (e.g. pin prick, visceral distension)
- Inflammatory (prolonged) pain (e.g. sunburn, inflamed wound)
- Pathological (neurogenic) pain (e.g. IBS, fibromyalgia, arthritis, diabetes, cancer, AIDS)
What are the characteristics of pain in each of the following areas:
- Skin
- Muscle
- Viscera
Skin pain:
- Well localised
- Pricking
- Stabbing
- Burning
Muscle pain:
- Poorly localised
- Aching
- Soreness / tenderness
- Cramping, stabbing, burning
Visceral pain:
- Poorly localised (often referred to a somatic structure)
- Dullness
- Vagueness
- Fullness
- Nausea
- Often associated with autonomic features (e.g. nausea, vomiting, sweating, pallor)
What are nociceptors?
- They are specific peripheral primary sensory afferent neurons which are activated by intense stimuli (e.g. thermal, mechanical, chemical) that are noxious or damaging
- Nociceptors are first order neurones that relay information to second order neurones in the CNS by chemical synaptic transmission
What are the main subtypes of nociceptors ?
- Ad-fibres these are mechanical/thermal nociceptors which respond to first/ fast pain e.g. stabbing, pricking sensations etc
- C-fibres are nociceptors which respond to all noxious stimuli, they mediate second/ slow pain
What is the main reason that Ad-fibres respond much faster than C-fibres ?
A gamma-fibres are myelinated and C-fibres are unmyelinated
How does the rate of AP’s produced from nociceptors correspond to the intensity of the applied stimulus ?
Rate of AP produced increase with the intensity of the applied stimulus
Where are the cell bodies of Nociceptive C- and Aδ-fibres located and where do there axons terminate ?
- Cell bodies located in the dorsal root ganglia (simply the dorsal root ganglion in the dorsal root of a spinal nerve)
- Axon terminates centrally in the dorsal horn of the spinal cord in various laminae of Rexed (I-V)
Recall that General somatic information from the anterior head (including: the oral and nasal cavities, paranasal air sinuses, teeth, intracranial dura and cerebral arteries) is mediated by two trigeminothalamic* pathways
Impulses originate at the terminals of the two trigeminal nerves (CN V) each of which have three divisions:
- V1 - ophthalmic
- V2 - maxillary
- V3 - mandibular
Describe the neurotransmission between the primary afferent and second order neurones in the dorsal horn (or spinal nucleus in the trigeminothalamic system):
- Primary transmitter is glutamate producing a fast e.p.s.p. (—-) and neuronal excitation by activating primarily postsynaptic AMPA receptors with NMDA receptor participation (when afferent input is intense)
- Peptides (substance P and CGRP) also participate (particularly during high frequency stimulation) causing a slow and prolonged e.p.s.p. that facilitates activation of NMDA receptors by relieving voltage-dependent block by Mg2+
Define visceral pain
This is pain originating from nociceptors covering tissues (e.g. peritoneum, pleura), or walls of hollow organs. Originates from stretching, twisting, inflammation and ischaemia – but not cutting, or burning
Define referred pain
This is when the brain ‘interprets’ the nociceptive information arising from the viscera as originating from an area of skin that may be distant to the internal organ
What is the main tract which second order neurons of nociceptive tracts ascend in ?
- The spinothalamic tract
- Note they also ascend via spinoreticular tract (SRT) (think this is part of the rectuclar formation) much less important
Describe the involvement of the spinothalamic tract (fast pathway) in sensation of pain
Acts as a ‘warning system’ by signalling the exact location and severity of the injury and the duration of pain. Also analyses the features of the pain – is it burning, sharp, pricking, aching?
Its signals are relayed to the primary somatosensory cortex (post central gyrus)
Describe the involvement of the spinoreticular tract (slow pathway) in sensation of pain
Signals to the intralaminar nuclei of the thalamus indirectly through the brainstem reticular formation via reticulothalamic tracts
Signals from the thalamus are relayed by thalamocortical neurones to limbic areas of the forebrain (e.g. cingulate and insular cortices, amygdala)
Registers the emotional/motivational component of pain, its inherent unpleasantness, anguish and suffering
- ‘I don’t like it!’
- Stop it!’
