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



·        Definition

·        Nociceptive receptor and primary afferent

·        Secondary afferent and tracts

·        Tertiary afferents and projections

·        Descending modulation



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


(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:


·   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


·   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:


·   VPL thalamus -> primary somatosensory cortex

o  Localisation

Other aspects

·   Brainstem -> medial thalamus, hypothalamus, amygdala

o  Affective response

o  Autonomic response


Descending modulation:


·   Pathway: PAG or RVM -> dorsal horn via Lissauer’s tract

·   Mediators: noradrenaline -> serotonin

·   Effect: inhibit 1° afferent (pre-synaptic) and 2° afferent (post-synaptic)


·   Opioids: ↓activity of OFF cell -> ↓Inhibition of ON cell



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