Spinal dorsal horn neurone targets for nociceptive primary afferents: do single neurone morphological characteristics suggest how nociceptive information is processed at the spinal level.

It has become increasingly clear that nociceptive information is signalled by several anatomically distinct populations of primary afferents that target different populations of neurones in the spinal cord. It is probable that these different systems all give rise to the sensation pain and hence, an understanding of their separate roles and the processes that they employ, may offer ways of selectively targeting pain arising from different causes. The review focuses on what is known of the anatomy of neurones in LI-III of the spinal dorsal horn that are implicated in nociception. The dendritic geometry and synaptic input of the large LI neurones that receive input from primary afferents containing substance P that express neurokinin 1 (NK(1)) receptors suggests that these neurones may monitor the extent of injury rather than the specific localisation of a discrete noxious stimulus. This population of neurones is also critically involved in hyperalgesia. In contrast neurones in LII with the morphology of stalked cells that receive primary afferent input from glomerular synapses may be more suitable for fine discrimination of the exact location of a noxious event such as a sting or parasite attack. The review focuses as far as possible on precisely defined anatomy in the belief that only by understanding these anatomical relationships will we eventually be able to interpret the complex processes occurring in the dorsal horn. The review attempts to be an accessible guide to a sometimes complex and highly specialised literature in this field.