Anodally focused polarization of peripheral nerve allows discrimination of myelinated and unmyelinated fiber input to brainstem nuclei |
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Authors: | Jeffrey C Petruska Charles H Hubscher Richard D Johnson |
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Institution: | (1) Department of Neuroscience, University of Florida, Gainesville, FL 32610, USA, US;(2) Department of Physiological Sciences, University of Florida, PO Box 100144, Gainesville, FL 32610–0144, USA e-mail: johnson@ufbi.ufl.edu, Tel.: +1-352-392-4700 ext. 3834, Fax: +1-352-392-5145, US |
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Abstract: | 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.
Received: 14 November 1997 / Accepted: 3 March 1998 |
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Keywords: | Selective conduction C-fibers Pain Dorsal column nuclei Medullary reticular formation |
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