Somatosensation ll (neuro)

Question 1

2 major central pathways of the somatosensory system

Answer 1

• Dorsal column-medial lemniscal system (DCML)

- Spinothalamic tract (anterolateral system) (STT)

Question 2

Ascending pathways

Answer 2

• Ascending pathways mediating 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

Question 3

positions of DCML and STT

Answer 3

• DCML and STT cross the midline at different sites

- Differential loss of temperature/pain vs fine tactile discrimination

Question 4

What is pain?

Answer 4

Dual aspect model:
1. Sensory-discriminative
• Location  
• Intensity
• Duration 
• Quality
2. Affective-motivational
• Unpleasantness – the painfulness of pain
• Effects on arousal, mood (affect), behaviour

Question 5

Nociceptors

Answer 5

• Nociceptors are neurons specialized for detection of painful stimuli
• A-delta and C-fibres contribute different aspects of pain sensation
• The TRPV1 receptor is involved in transduction of noxious heat

Question 6

Maintenance of nociceptor activity after injury

Answer 6

• ‘Inflammatory soup’ of cytokines, prostaglandins and small signalling molecules
  maintains depolarisation and sensitivity of C-fibre terminals after original stimulus:
  • Hyperalgaesia
  • Allodynia

Question 7

The Anterolateral system (STT)

Answer 7

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

Question 8

Cortical representation of pain

Answer 8

• 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

Question 9

Two systems in central pain processing

Answer 9

• Diverge at level of thalamus
• Lateral system (do not confuse with anterolateral system):
  • VP nuclei of thalamus, in parallel with DCML system
  • Primary and secondary somatosensory cortex (SI and SII)
• Medial system:
  • Midline nuclei of thalamus (intralaminar)
  • Anterior cingulate and insular cortex

Question 10

Lateral and medial pain systems

Answer 10

- Lateral:
• Sensory-discriminative
• Project via specific somatosensory thalamic 
nuclei
- Medial:
• Affective-motivational
• Project to different cortical areas via (non-
specific) midline thalamic nuclei

Question 11

Descending modulation of pain pathways

Answer 11

• Analgaesic properties of opium known for centuries

- Endogenous opioids (enkephalins, endorphins) and opioid receptors discovered 1970s-80s

Question 12

Treatment of pain

Answer 12

• Successful at treating pain as neuro-physiological 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

Question 13

Chronic pain

Answer 13

• > 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

Question 14

Pain dissociated from tissue damage

Answer 14

• Phantom limb pain

- Central pain

Question 15

Anterior cingulotomy for intractable

pain

Answer 15

• Targeted lesion to disconnect anterior cingulate cortex on both sides
• Used for decades as last resort