Anodally focused polarization of peripheral nerve allows discrimination of myelinated and unmyelinated fiber input to brainstem nuclei

We investigated the ability of a novel direct current (DC) polarization technique to block selectively the conduction in peripheral myelinated nerve fibers and allowing propagation in only unmyelinated fibers. In anesthetized adult rats, distal branches of the sciatic nerve (caudal cutaneous sural and tibial nerves) were exposed for electrical stimulation of A- and C-fibers. Two specially fabricated trough electrodes of different size and surface area were placed onto the sciatic nerve. Through these proximal electrodes a controlled ramped DC was timed to coincide with the arrival of A- and C-fiber action potentials, evoked electrically at the distal nerves or naturally from the foot or ankle, with the intent of blocking propagation in A-fibers while allowing C-fiber throughput. Neuronal recordings were made both peripherally (proximal sciatic nerve fascicles or L5 dorsal roots) and centrally (single cells in the nucleus gracilis or nucleus reticularis gigantocellularis). The DC polarization was shown to block conduction in myelinated A-fibers effectively, while allowing conduction in the unmyelinated C-fibers, without activation of fibers via the DC polarization itself. This was dependent upon the following factors: electrode polarity, onset rate of polarization, peak amplitude of polarization, distance between polarizing electrodes, size difference between polarizing electrodes, and gross nerve size. These experiments demonstrate that anodally focused DC polarization, applied utilizing two trough electrodes of different sizes, is capable of effectively, reversibly, and reproducibly blocking conduction in myelinated A-fibers evoked either electrically or naturally, while still allowing conduction to occur in the unmyelinated C-fiber population. In the context of experimental usage, we have demonstrated blocking of low-threshold A-fiber, but not C-fiber, mediated inputs to the caudal brainstem. This technique should find wide application in studies involving the processing of information conveyed centrally by the unmyelinated C-fiber afferent population, including discriminating afferent responses to peripheral stimuli, the role of C-fiber input in reflex activity, and the plasticity following injury or other manipulations.