ABSTRACT
The myelin sheath is normally regarded as an electrical insulator. However, the typical picture which we have from electron microscopy of fi xed myelin sheath is rather misleading. Studies of native myelin by X-ray and neutron diffraction have demonstrated that the myelin lamellae are not tightly compacted, but separated by cytoplasmic and extracellular spaces of about 4−5 nm, respectively, so that up to half the volume fraction of myelin is taken up by water (Kirschner and Caspar 1972, Kirschner et al. 1984). Moreover, the water in both aqueous layers contains salts that can be washed out or exchanged with the extracellular solution. The calcium ions in the extracellular compartment have a special role in maintaining myelin structure by electroctatic interaction with fi xed negative charges on the membrane (Ropte et al. 1990), but other ions can be shown to
diffuse readily throughout both aqueous compartments (Blaurock 1971). Consequently, the myelin sheath, taking into account its aqueous layers, can be regarded as an electrical conductor. The hypothesis that the aqueous layers endow the myelin sheath with longitudinal conductance much greater than its radial conductance was previously demonstrated by our multi-layered myelin sheath model of the human motor nerve fi bre (Stephanova 2001). In this study, it was found that conduction velocity of the action potential depends on the longitudinally conducting aqueous layers which assist action potential propagation. The conduction velocity was appreciably faster (by 8.6%).
