The synaptic linkage for tactile and kinaesthetic inputs to the dorsal column nuclei

Our sensory abilities in touch and kinaesthesia depend upon approximately eight major classes of receptors and sensory nerve fibres. When individual fibres of these different kinaesthetic and tactile fibre classes are selectively activated in conscious human subjects by means of the intraneural microstimulation procedure there are quite marked differences observed among the classes in their capacity to generate perceptual responses. These differences may be attributable to differential transmission characteristics for different fibre classes at synaptic junctions within the sensory pathways. To test this hypothesis we have employed a paired, simultaneous recording paradigm in which we have examined the efficacy of transmission across the dorsal column nuclei (DCN) in a one-to-one synaptic linkage between single, identified tactile or kinaesthetic afferent fibres and their central DCN target neurons. These studies demonstrate a remarkable security of transmission for all fibre classes examined. For all classes, the minimum sensory input, a single impulse in one sensory fibre, can generate spike output from DCN target neurons. The results demonstrate that the differential capacities of various tactile and kinaesthetic fibre classes to generate perceptual responses when activated singly, do not appear to be explicable in terms of systematic differences in DCN transmission characteristics.