Specific permeability properties of demyelinated rat nerve fibres.

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.