The specific permeability properties of isolated rat fibres with diphtheria toxin induced paranodal demyelination were investigated in potential clamp experiments. Nodal widening (4 to 6 micrometers) and paranodal demyelination (20 to 90 micrometers) were associated with a very large increase in the normally low delayed K-permeability (greater than 10 times). The resting K-permeability (which was comparatively large in the rat fibres) was similarly increased, which decreased action potential amplitude and excitability. Fibres with paranodal demyelination had, in addition, a large increase in the maximum peak Na-permeability (2 to 5 times the normal), which indicated that excitability was upheld by formation of new Na-permeability sites. The conduction defects in demyelinated fibres can therefore not be related to the size of the demyelination, but to the specific permeability changes in the demyelinated segment.
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Tom Brismar (1981) studied this question.