Pain-signalling systems in the dorsal and ventral spinal cord

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Abstract
A review of the literature on pain-signalling systems in the spinal cord provided convincing evidence that nociceptive information is transmitted by multiple ascending systems. These systems include the postsynaptic fibers of the dorsal columns, which relay in the rostral dorsal column nuclei; the spinocervical tract, which relays in the lateral cervical nucleus; the neo-spinothalamic tract, which ascends directly from cord to thalamus; the paleo-spinothalamic tract, which projects to the midline-intralaminar thalamic regions; and the spinoreticular systems, which sends fibers throughout the brain stem reticular formation. The diffuse, polysynaptic, propriospinal systems may also carry pain-related information. The data indicate that the first 3 systems, which comprise the lateral group, are generally similar to each other on at least 4 dimensions: modalities represented, conduction velocities, loci of origin and termination areas. Despite their overall similarities, there are clear differences, particularly in the precise modes of termination and routes of ascension. They appear to be controlled differently by the inhibitory systems descending from the brain. The other 3 systems, which constitute the medial group, appear to have the same modality spectrum. However, they differ from the lateral group on other dimensions: they conduct more slowly; their cell bodies tend to be located more deeply in the spinal grey matter; and, most importantly, their termination patterns are grossly different from those of the lateral group. Several speculative proposals were presented to explain the functions and possible interactions among the various systems. These proposals emphasize 2 main factors: the possible influence of on-going behavioral sequences on the relative activity among the systems, and the possible effects of existing tissue damage on the mediation of subsequent noxious stimuli. Although these proposals are speculative, they are amenable to experimental investigation. Implications for the treatment of pain were discussed.