2021A Question 5
Describe the peripheral and central nervous system pathways that lead to perception of pain following a laceration of the thumb.
Examiner Report
92.8% of candidates achieved a pass in this question.
The pass rate for this question was 92% and there were many answers of high calibre.
The major domains assessed in this question were:
- Definition of pain
- Nociceptors and transduction
- Spinal cord synapse
- Ascending and supraspinal pathways
- Cortical processing
- Descending modulation
While a diagram was not required to pass the question, candidates who provided a simple, well-labelled diagram were able to better organise their answer and ensured that they addressed all the domains required. It is recognised that textbooks differ in the nomenclature of the supraspinal pathways and the detail of cortical processing functions, and allowance was made for this.
While peripheral nervous system and descending modulation pathways were generally well described, relatively few candidates recognised the emotional aspects and the autonomic responses associated with perception of pain.
Model Answer
Structure:
- Definition
- Nociceptive receptor and primary afferent
- Secondary afferent and tracts
- Tertiary afferents and projections
- Descending modulation
Definition
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or is described in such terms.
Primary Afferents
| Component | Description |
|---|---|
| Nociceptive receptor | - Free nerve ending on 1° afferent in the thumb - Dense innervation of epidermis - Ionotropic and metabotropic transduction - Activated by mechanical, thermal, chemical insults - Sensitised by inflammation |
Nociceptor (the neuron) |
- From periphery via median/radial nerve to the dorsal horn (C6 level) - Cell body in dorsal root ganglion - Ascent or descent 1-2 levels via Lissauer’s tract Aδ fibre: - Releases glutamate - Synapse at superficial layers. - Sharp, fast, well localized (i.e. somatic pain) C fibre: - Releases glutamate, substance P - Synapse at deeper layers. - Dull, slow, poorly localized (i.e. visceral pain) Silent nociceptor: - Conveys pain only when sensitized |
Secondary Afferents
| Component | Description |
|---|---|
| Neurons | - Nociceptive-specific: Superficial layers, variable threshold - Wide dynamic range: Deeper layers, high threshold, only when sensitized - Note also interneurons: Excitatory and inhibitory - Note inhibitory afferents: E.g. Aβ conveying light touch |
| Tracts | - Decussation in anterior commissure - Ascent in spinothalamic tracts - Neo-spinothalamic tract: To thalamus (VPL nucleus) - Paleo-/archi-spinothalamic tracts: To brainstem |
Tertiary Afferents
| Component | Description |
|---|---|
| Somatosensation | - VPL thalamus → Primary somatosensory cortex - Localisation |
| Other aspects | - Brainstem → Medial thalamus, hypothalamus, amygdala - Affective response - Autonomic response |
Descending Modulation
| Mechanism | Description |
|---|---|
| Physiology | - Pathway: PAG or RVM → Dorsal horn via Lissauer’s tract - Mediators: Noradrenaline → Serotonin - Effect: Inhibit 1° afferent (pre-synaptic) and 2° afferent (post-synaptic) |
| Pharmacology | Opioids: ↓ activity of OFF cell → ↓ Inhibition of ON cell |