This chapter discusses the organization and function of normal myelinated axons and demyelinated and dysmyelinated axons. The myelinated fiber consists of an axon and its surrounding myelin sheaths. As a result of its high electrical resistance and low capacitance, the myelin functions as an insulator which prevents current loss during action potential conduction. Action potential conduction is rapid, and occurs in a unidirectional manner because sodium channels close soon after activation and remain refractory for a short time. Occasional reports have purported that neuroelectric blocking factors or sodium channel blocking factors may contribute to axonal conduction block in neuro-inflammatory disorders. Oligodendrocyte and Schwann cell-mediated remyelination of central nervous system (CNS) axons can both enhance conduction along CNS axons. Although the relationship of axonal degeneration to demyelination is not yet clear, neuroprotection of axons has emerged as a major theme in recent MS research.