Synaptic transmission between single tactile and kinaesthetic sensory nerve fibers and their central target neurones

Selective activation of single tactile or kinaesthetic afferent nerve fibers in conscious human subjects by means of the intraneural microstimulation procedure reveals quite marked differences among the different classes in their capacity for eliciting perceptual responses. This work, conducted largely by Swedish researchers, suggests that there may be differential transmission security for different fiber classes across synaptic linkages in the central tactile and kinaesthetic sensory pathways. In order to test this hypothesis we have developed an experimental paradigm in the anaesthetized cat, based upon paired, simultaneous recording from an individual afferent fiber in an intact peripheral nerve fascicle, and from the central target neurone of that afferent fiber within the dorsal column nuclei (DCN). Our results demonstrate, for all tactile and kinaesthetic fiber classes examined, that the minimum sensory input, a single impulse in one sensory fiber, can generate spike output from DCN target neurones. This remarkable security of transmission has been demonstrated for single Pacinian corpuscle (PC) fibers (associated with Pacinian Corpuscle receptors); single SAI and SAII fibers (the slowly adapting Type I and II tactile fibers associated, respectively, with Merkel and Ruffini receptor endings); the Hair Follicle Afferent fibers (HFA fibers); and kinaesthetic afferent fibers of both joint and muscle origin. The results demonstrate that the differential capacities of various tactile and kinaesthetic fiber classes to generate perceptual responses when activated singly in microneurography experiments do not appear to be explicable in terms of systematic differences in DCN transmission characteristics.