Peptide- and non-peptide-containing unmyelinated primary afferents: the parallel processing of nociceptive information.

Primary afferent C fibres can be subdivided into a number of subgroups on the basis of cytochemistry or receptor binding characteristics. Numerous peptides have been localized to dorsal root ganglia, yet these appear to be only found in approximately 50% of small perikarya. A large proportion of the remaining small cells do not contain peptides but are identifiable in rodents by their content of a fluoride resistant acid phosphatase. Attempts have been made to correlate particular biochemical types with particular receptive field profiles, with rather modest success. As an alternative we suggest, principally from an analysis of skin afferents, that peptide- and non-peptide-containing afferents are two distinct C fibre pathways innervating similar peripheral structures and conveying similar information, but to different areas within the dorsal horn. Morphological evidence also suggests that these two subsystems form either glomerular or simple synaptic arrangements in the dorsal horn. The significance of parallel pathways for the processing of nociceptive information is briefly discussed.