L2 Somatosensation 2 Flashcards
(34 cards)
A - beta
- big
- fat
- myelinated
e.g. discriminative touch
A- delta
- small
- myelinated
- free nerve endings
e.g. fast pain localised, crude localisation of touch
C fibres
- unmyelinated
- non-localising
- slow pain
What does fast pain depend upon?
- Small myelinated axons - A delta
- esp high-threshold mechanoreceptors that
- respond selectively to cutting or pinching
What does slow pain depend on?
- signals in unmyelinated axons - C fibres
- usually Polymodal
- respond to warmth and touch as wel as noxious stimuli
- some respond to chemicals released in trauma
What are nociceptors?
Specialised high-threshold detectors of damagins stimuli
Multi modal nociception
Mechanical
High threshold mechanoreceptors, Aδ
– Tissue damage
Multi-modal Nociception
Thermal
- Extreme cold, C-fibres
- Extreme heat, Aδ
Multi-modal Nociception
Chemical
- ATP, purinergic receptors (PTX3)
- Reduction in pH
- Bradykinin
- Direct and indirect effects
Multi-modal Nociception
another type?
axon reflexes
Multi-modal Nociception
TTX-insentive Na+ channel
what does this cause?
No or extreme pain
Mutations in SCN9A cause this
Why are chillies hot and painful?
Activate Capsaicin and TRP channels
What can TRP channels respond to?
Temperatur AND Chemicals
- TRPV1 specific to nociceptors:
- capsaicin and heat , current enhanced by reduction in pH
- TRPV2 found in A δ:
- high temperature
- TRPM8:
- menthol and low temperature
What does Capsaicin activate
A depolarizing ion-channel that is also activated by noxious heat
TRP is a type of depolarizing channel
Nociception
Dorsal Horn Inputs
Differential laminar inputs define physiology of dorsal horn laminae
There is lots of capacity for modulating receptors!
- Lamina 1
- Nociception-specific neurons specific neurons
- Cold-specific
- Wide-dyanmic range neurons
- Lamina II
- Variety of interneuons
- Lamina III/IV
- Interneurons and supraspinal projection neurons: innocuous
-
Lamina V
- Projection neurons: innocuous and nociceptive
*
- Projection neurons: innocuous and nociceptive
Nociception
Dorsal Horn Projections
where do they arise/cross/ascend to?
- arise from superficial and deep laminae
- cross the midline
- ascend in white matter to terminate subcortically
What are the important targets for Dorsal horn Projections?
- Thalamus
-
Periaqueductal Grey (PAGP)
- important! crucial in modulating pain transmission
-
Reticular formation
- activates attention, awareness
- nociception information goes here
What do nociceptor afferents co-release?
What other information is there?
GLUTAMATE AND PEPTIDES
- Substance P, CGRP , somatostatin
- Dense-cored vesicles
- Long range diffusion
- Substance P via NK1: slow epsps
What do some interneurons release?
Enkephalin - endogenous, naturally occurring opiate
- µ-opiod recceptor on dorsal horn neurons and sensory afferent terminals
- Presynaptic action reduces transmitter release
Where does the Spino-thalamic pathway decussate?
Immediately upon entry to the spine
WHere does the DCML pathway decussate?
At the medulla
Why does referred pain occur?
Nociceptors from the vicera converge on the dorsal horn neurons that also receive input from cutaneous nociceptors
Hence the brain gets confused and perceives pain from the organs as being from the skin
hence why referred pain symptoms from the heart are at T1-4 dermatomes etc
nociceptive aspect of pain goes via?
SPINOTHALAMIC TRACT
- Goes to VP thalamus
- And then to Somatosensory cortex (s1,s2)
(more info):
- Inputs to somatosensory cortex
- Localisable pain
- Lesions and pain
- Phylogenetically recent
The affective/emotional side of pain goes via the:
MIDLINE THALAMIC NUCLEI
to the ACC and Insular cortex
(more info):
Projection to medial nucleus of thalamus
- Input from deep dorsal horn laminae
- Projection to basal ganglia and cortex
- Phylogenetically old
