before projecting to the limbic forebrain. Part of the tract originating from the substantia gelatinosa (also known as Lissauer's tract) becomes continuous with the spinal tract of the trigeminal nerve before reaching the thalamic nuclei.

Supraspinal Control

Transmission from the dorsal horn is not only modulated by activity in the same and other segments, but also by descending tracts from the brain and brainstem.

Descending pathways originate in the cortical and diencephalic system, the peri-aqueductal gray matter, raphe nuclei and locus ceruleus and medullary dorsal horn. Descending control of pain involves, among other substances, 5-hydroxytryptamine (5HT), noradrenaline (NA), 7-amino-butyric acid (GABA) and opioids.

5HT pathways arise in the raphe nuclei and other nuclei in the pons and medulla, terminating in laminae I, II, IV and V. Depletion of 5HT from these neurones blocks the action of systemic opiates. Noradrenergic pathways descend from the locus ceruleus and other brainstem sites to laminae I, II, IV, VI and X. Both pathways act by inhibiting nociceptor neurones and interneurones in the dorsal horns.

Naturally occurring opioids (enkephalins, dynorphins and (3-endorphins) can be found at both spinal and supraspinal levels, causing modulation of both ascending and descending pathways. The mechanism of action is not clear, but may be by effects on the descending nora-drenergic and serotinergic pathways or by inhibiting release of primary afferent neuro-transmitters in the spinal cord [2,3].

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