The primary emphasis of this research concerns the mechanism of conduction in normal, demyelinated, and remyelinating axons. Nerve fibers are demyelinated by chemical or immune means. Following times ranging form 1 day to several months single axons are isolated for electrophysiological or optical analysis. Two new approaches are used for the determination of ionic channel distributions and the measurement of impulse conduction in these cells. A patch voltage clamp technique allows the detection of Na+ and K+ currents from small, well defined loci on the axolemma. Combining this method with external stimulation proximal to the patch site provides a measure of membrane current resulting from decremental conduction as action potentials invade the demyelinated zone. Several hypotheses concerning mechanisms of ionic channel redistribution during recovery from demyelination will be tested. New axolemmal Na+ channels might, for example, originate in proliferating Schwann cells, or could arrive via axoplasmic transport following synthesis in the soma. Ionic channel densities in Schwann cells and in axons at points of contact with Schwann cell processes will be measured. Changes in the pattern and timing of ionic channel redistribution with block of Schwann cell division or of axoplasmic transport will be sought. Optical measurements on axons stained with voltage-dependent dyes will provide information on the behavior of propagating signals as they enter a demyelinated segment. Coupled with ionic channel profiles and morphological measurements these results will be used to calculate detailed models of conduction. Experiments will begin with amphibia and will then include mammalian preparations. Attempts will also be made to work with CNS axons from optic nerve. Autoimmune lesions induced by exposure to peripheral or central myelin will be studied in an attempt to determine more clearly the causes of conduction delays and block in these models for demyelinating disease.
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