Pain
Central Pathways
The major central pain pathways are the Spinothalamic System and the Trigeminal Pain & Temperature System. Central axons of the nociceptive neurones enter the spinal cord via the dorsal root ganglia (where the majority, but not all, of the cell bodies are localised). On reaching the Dorsal Horn of the Spinal Cord, the neurones branch into ascending and descending collaterals to form the ‘Dorsolateral Tract of Lissauer’ whose axons run up and down one or two spinal cord segments before penetrating the Dorsal Horn Grey Matter. Within the Dorsal Horn Grey Matter, the axons branch to contact neurones in several of Rexed’s laminae (the descriptive divisions of grey-matter taken in a cross-section).
Each neurone type innervates a specific or specific set of laminae. Both
A
and C fibres
have synapsing branches in the I & II laminae. Stimulation of neurones
in Laminae II results in stimulation of second-order projection neurones in
laminae IV, V and VI (some of the neurones here also receive direct input
from 1st order neurones. Axons of the 2nd-order neurones in these laminae
(Collectively known as the Nucleus Proprius) cross the mid-line and ascend
to the brainstem and thalamus in the anterolateral quadrant of the contralateral
half of the spinal cord. Collectively these neurones are referred to as the
Spinothalamic Tract and the overall system as the Anterolateral System.
Information from the face region is transmitted separately via the trigeminal system. First-order neurones from the trigeminal ganglion and ganglia linked to nerves VII, IX and X convey nociceptive information from the facial region to the CNS via the pons, medulla (spinal trigeminal tract) and go on to terminate in the pars interpolaris and pars caudalis (divisions of the trigeminal complex). Like the Spinothalamic System the second-order neurones involved do cross the midline and ascend to the contralateral thalamus. [18, 30, 40]
