The dorsal column system: II. Functional properties and bulbar relay of the postsynaptic fibres of the cat's fasciculus gracilis.

Microelectrode recordings were made from dorsal column postsynaptic (DCPS) fibres, in the fasciculus gracilis of the cat, at thoracic level Th 12, and from single cells in the nucleus gracilis. The sensitivity of the fibres (Th 12) and cells (bulbar level) to both gentle and noxious stimuli was studied and a classification of the units was made on the basis of their responses to these natural stimuli. The DCPS fibres have been classified into three groups: i) 16.3% of them were activated only by light mechanical stimuli, ii) a few responded to nothing but noxious mechanical stimuli, iii) 77% were characterized by their sensitivity to both gentle and noxious stimuli and constituted the polymodal group. The study of the nucleus gracilis cells revealed that 31.2% of the total number of cells investigated in the nucleus were also characterized by a high degree of modality convergence (polymodal cells). Besides their convergence, the polymodal DCPS fibres and the polymodal cells of the nucleus gracilis were functionally in contrast to the properties of the specific fibres and cells of the dorsal column system in other respects: i) they responded with a slowly adapting response to constant mechanical stimulation of the skin, ii) they were sensitive to noxious mechanical and thermal stimuli. It is argued that the polymodal cells could be the bulbar relay of the impulses conveyed through the DCPS fibres of the fasciculus gracilis. The possible role of the dorsal column system in nociception is discussed in the light of the results.