2021A05 Describe the peripheral and central nervous system pathways that lead to perception of pain following a laceration of the thumb.

 

List:

·       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:

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:

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

o  Neo-spinothalamic tract: to thalamus (VPL nucleus)

o  Paleo-/archi-spinothalamic tracts: to brainstem

 

Tertiary afferents:

Somatosensation

·  VPL thalamus -> primary somatosensory cortex

o  Localisation

Other aspects

·  Brainstem -> medial thalamus, hypothalamus, amygdala

o  Affective response

o  Autonomic response

 

Descending modulation:

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

 

 

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