Relation between myelin sheath thickness, internode geometry, and sheath resistance.

The thickness of the myelin sheath of nerve fibers was traditionally assessed solely as a function of axon caliber. Studies concerning the additional effect of variation in internode length are of relatively recent date. Carefully calibrated measurements of sheath thickness and internode geometry were used in this study to define an equation to predict the approximate number of lamellae from axon caliber and internode length, for normal and regenerated peripheral nerve fibers, and for fibers from hypomyelinated murine mutants. The definition of sheath thickness thus obtained was compared with different assumptions on the biophysical nature of myelin sheath resistance. The observed relations between sheath thickness and internode geometry were not compatible with an effective adjustment of sheath thickness to a radial flow of current across the sheath. Conversely, sheath thickness was found to vary in such a way that the resistance of the spiral path between the lamellae was matched precisely to axonal current density. The calculated resistance of the spiral leakage path, furthermore, was equal to measured sheath resistance. This new concept reconciles low sheath resistance with a high resistance of the myelin leaflet, yielding, at the same time, a fine tuning of sheath resistance to variations of internode geometry.