Physiology of Pain Flashcards
What is pain?
an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
What are the 2 components of pain?
- sensory (nociception)
- emotional overlay
What are nociceptors?
somatosensory primary afferents
peripheral endings are the sensory receptors
cell body is located in the dorsal root ganglion
synapse onto the second order neuron in the dorsal horn
(within laminae I, II, and V)
What is the difference in location of touch receptors and nociceptors?
touch receptors: synapse in brain
nociceptors: synapse in the spinal cord
Where is the cell body of nociceptors (1st order neuron)?
in the dorsal root ganglion
In which layers of the dorsal horn do the nociceptors primary afferents synapse?
Layer I, II, V
What are the 2 broad classes of nociceptors?
- A-delta fibres
- C fibres
What are A-delta fibres?
- type of somatosensory (nociceptive) primary afferent
- causes immediate, sharp pain (1st pain)
- axon is thin and myelinated
- conducts pain impulses quickly (5-30m/s)
- NT is glutamate
- responds to: mechanical trauma and noxious heat
What is noxious heat?
> 45C
temperature at which heat starts to become damaging to tissues
What are C fibres/
- type of somatosensory (nociceptive) primary afferent
- causes delayed aching pain (2nd pain)
- axon is thin but unmyelinated
- slower speed of pain transmission (0.5-2m/s)
- multiple NTs: glutamate, substance P etc
- responds to inflammatory mediatiors, mechanical trauma, noxious heat/cold (stimuli that don’t change very quickly)
How does activation of A-delta fibres lead to pain perception?
- > Noxious stimuli
- > activation of A-delta nociceptors
- > info conducted to spinal cord
- > Protective segmental reflex (e.g. WITHDRAWAL)
- > transmission via anterolateral spinal tract to cerebral cortex
- > perception (sharp pain)
How does activation of C fibres lead to pain perception
[secondary/slower response]
- > damaged tissue releases K+
- > activation of C fibres, info transmitted to spinal cord
- > protective reflexes (e.g. IMMOBILISATION)
- > transmission via anterolateral spinal tract to cerebral cortex
- > perception of aching pain
- > informs learning and memory (hippocampus)
- > aversion memory (won’t do same action in future as associated with fear)
How does infection modulate pain transmission?
bacterial release immunogenic chemicals
recruitment of immune cells
- inflammation
- can also directly depolarise nociceptors, which indirectly causes inflammation
What problems might having no nociception have?
e.g. CIPA (no pain, very rare)
repeated tissue damage due to lack of protective mechanisms
can develop osteoarthritis at early age
What is the lateral pain pathway?
controls sensation an perception
SENSATION
awareness event has occurred
“pain”
PERCEPTION nature of pain where? what? meaning? response?
What is the course of the ‘main somatosensory/touch pathway’?
- Primary afferent (periphery)
- enters spinal cord and ascends in dorsal column (ipsilateral side)
- reaches medulla where it synapses onto 2nd afferent (in dorsal column nuclei)
- 2nd neuron decussates in medulla
- ascends to thalamus via medial lemniscus
- 2nd neuron synapses in the VP nucleus of thalamus
- 3rd neuron ascends from thalamus to cortex via the internal capsule
- terminates in the primary SS area (parietal area)
What is the primary SS pathway responsible for?
SS = somatosensory
conscious perception and touch
What is the lateral pathway responsible for?
pain pathway
=> nociception
What is the course of the ‘lateral pain pathways’?
- primary afferents (periphery)
- enters spinal cord and synapses in dorsal horn
- 2nd neuron decussates in dorsal horn and ascends via anterolateral tract (mainly)
- 2nd neuron travels to the VP nucleus of thalamus where is synapses
- 3rd neuron travels from VP nucleus to cortex via the internal capsule
- 3rd neuron terminates in the primary SS area (parietal lobe)
Where do the ‘lateral pain’ and main SS pathways join in their course to the cortex?
at the level of the thalamus
[2nd neurons both synapse at the VP nucleus)