Measurement of myelin sheath resistances: implications for axonal conduction and pathophysiology.

As commonly understood, the myelin sheath of axons insulates the internodal axolemma and essentially restricts transmembrane currents to nodal regions. However, recordings obtained from within the myelin sheath showed that its apparent resistance to current generated by action potentials is similar in magnitude to that of the internodal axolemma. This suggests that the sheath does not appreciably limit transmembrane current flow, presumably because there is a longitudinal shunt under the myelin and through the paranodal region. Thus, in some demyelinating diseases and other axonopathies, the safety factor for impulse conduction may be lowered by a loosening or a reduction in the number of paranodal axoglial junctions.