Myelin: Molecular Architecture of CNS and PNS Myelin Sheath

Abstract

Schwann cells and oligodendrocytes make the myelin sheaths of the peripheral nervous system (PNS) and central nervous system (CNS), respectively. Their myelin sheaths are structurally similar, consisting of multiple layers of specialized cell membrane that spiral around axons. Both kinds of myelin sheaths mainly consist of compact myelin, which is mostly composed of lipids as well as a few unique proteins. PNS myelin sheaths also have prominent regions of noncompact myelin, in which tight junctions, gap junctions, and adherens junctions join apposed layers of the myelin sheath. CNS myelin sheaths do not have adherens junctions, and they have a different arrangement of tight junctions and gap junctions. Despite their structural and molecular differences, the nodal regions of the PNS and CNS are similarly organized. The nodal axolemma contains high concentrations of voltage-dependent Na+ and K+ channels that are linked to the spectrin cytoskeleton by ankyrin-G. The paranodal membrane contains septate-like junctions, which are composed of both glial and axonal components. The juxtaparanodal and internodal regions lack distinct morphological specializations, but the apposed axonal and glial membranes have distinct molecular components that are probably organized by trans molecular interactions.