Somatosensation : pain Flashcards
Two major central pathways of the somatosensory system
- Dorsal column-medial lemniscal system
* Spinothalamic tract (anterolateral system)
Ascending pathways meditating sensory aspects of pain for body and face
2nd order neurons decussate and project to ventral-posterior nuclear complex of thalamus
• VPL: body
• VPM: face
What is pain?
Dual aspect model:
- Sensory-discriminative
- Affective-motivational
Sensory discriminative
- Location
- Intensity
- Duration
- Quality
Affective-motivational
- Unpleasantness – the painfulness of pain
* Effects on arousal, mood (affect), behaviour
Maintenance of nociceptor activity after injury
‘Inflammatory soup’ of cytokines, prostaglandins and small signalling molecules maintains depolarisation and sensitivity of C-fibre terminals after original stimulus
• Hyperalgaesia
• Allodynia
The anterolateral system (STT)
Dorsal horn interneurons
• Located in superficial and deep layers of dorsal horn
• Synaptic input from C- and A-δ fibres
• Axons cross and ascend in anterolateral white matter
• Some are multi-modal (receive convergent nociceptive and non-nociceptive inputs)
• Some receive convergent input from visceral afferents
Cortical representation of pain is complex
- STT projects to S1 via VP nuclei of thalamus (like DCML system)
- however, STT and DCML axons do not converge on same thalamic neurons – pathways are parallel
- S1 is necessary for the localization of pain, but stimulation of S1 gives rise to referred tactile, not painful, sensations
- Additional areas are involved in pain sensations
Two systems in central pain processing
- Diverge at the level of the thalamus
- Lateral system (do not confuse with the anterolateral system)
- Medial system
• Lateral system (do not confuse with anterolateral system)
- VP nuclei of the thalamus, in parallel with DCML system
- Primary and secondary somatosensory cortex (SI and SII)
- Sensory-discriminative
- Project via specific somatosensory thalamic nuclei
Medial system
- Midline nuclei of the thalamus (intralaminar)
- anterior cingulate and insular cortex
- Affective-motivational
- Project to different cortical areas via (non-specific) midline thalamic nuclei
Descending modulation of pain pathways
- Analgaesic properties of opium known for centuries
* Endogenous opioids (enkephalins, endorphins) and opioid receptors discovered 1970s-80s
Treatment of pain
Successful at treating pain as neurophysiological response to tissue damage
• NSAID
• Opiate drugs
Chronic pain
• antidepressants (e.g., amitriptyline, duloxetine)
• At lower dosage and in absence of diagnosis of clinical depression
Chronic pain
- > 3 months
- Alarmingly high prevalence
- May be due to nerve damage from prior injury: neuropathic pain
- However, increasingly accepted that pain can be dissociated from tissue damage
Pain dissociated from tissue damage
• Phantom limb pain
• Central pain
If you have a pain in a tissue that hasn’t existed for years this alone suggests that pain is in the mental image of the body rather than the body itself.
