ION-CHANNEL REDISTRIBUTION AND FUNCTION DURING DEVELOPMENT OF THE MYELINATED AXON

The development of myelinated axons represents one of the most complex interactions among different cell types in the nervous system. Striki ng changes occur in both morphology and function in the early postnata l period. Myelination effectively isolates electrically most of the ax olemma and dramatically alters the pathways for current flow that are required for rapid, reliable, and efficient conduction. Correspondingl y, ion channels must be directed to and stabilized at their required s ites. In the case of Na+ channels, this requires a 25-fold increase in density within nodes of Ranvier, and, in mammalian fibers, a virtuall y complete spatial separation from voltage-dependent K+ channels. Node s must also be properly spaced to ensure a high conduction velocity an d energy efficiency without compromising the safety factor for reliabl e propagation. In this review, we consider the events responsible for axon development, emphasizing the involvement of ion channels, We disc uss the current state of research in this area, including some controv ersies regarding mechanisms of neuron-glial communication. (C) 1998 Jo hn Wiley & Sons, Inc.