--with large fiber nerves, the axons are still the pathway or conduit for the transmission of signals; the myelin sheathing covers the axons and acts as a focuser and insulator, keeping those signals moving at proper speeds and to proper destinations.

Demyelination will cause the axonal signals to degrade and become patchy, slow, or unreliable.


B12 deficiency can cause both meylin and the axon to degrade, though typically the myelin sheathing goes first--and the myelin is much more suscpetible to repair. If one gets to the point of axonal degredation, one has a much longer and more difficult road back.