The structure and function of myelin: from inert membrane to perfusion pump.

The goal of this overview is to propose a novel structure/function model of central nervous system myelin. Although myelin is known to be a compact multilamellar structure that wraps around axons, the biologic role this structure plays in the nervous system remains an enigma. One means of ascertaining myelin's biologic role is by analyzing its structure. The recent discovery of tight junctions in myelin may be the key that unlocks the mysterious black box of myelin structure/function. Tight junctions in other cell types are invariably adjacent to adherens junctions, with both of these junctional plaques playing critical roles in paracellular barrier function, i.e., adhesion of cell membranes, signal transduction, and fluid movement between cells via aqueous pores and channels. The application of current knowledge about junctional plaques to myelin is an original concept. This knowledge, taken together with evidence from studies of normal and pathologic myelin, supports the possibility that a primary function of junctional plaques in myelin is to perfuse the periaxonal space.