On the molecular architecture of myelinated fibers

Schwann cells and oligodendrocytes make the myelin sheaths of the PNS and C NS, respectively. Their myelin sheaths are structurally similar, consisting of multiple layers of specialized cell membrane that spiral around axons, but there are several differences. (1) CNS myelin has a "radial component" composed of a tight junction protein, claudin-11/oligodendrocyte-specific p rotein. (2) Schwann cells have a basal lamina and microvilli. (3) Although both CNS and PNS myelin sheaths have incisures, those in the CNS lack the s tructural as well as the molecular components of "reflexive" adherens junct ions and gap junctions. In spite of their structural differences, the axona l membranes of the PNS and CNS are similarly organized. The nodal axolemma contains high concentrations of voltage-dependent sodium channels that are linked to the axonal cytoskeleton by ankyrin(G). The paranodal membrane con tains Caspr/paranodin, which may participate in the formation of axoglial j unctions. The juxta-paranodal axonal membrane contains the potassium channe ls Kv1.1 and Kv1.2, their associated beta 2 subunit, as well as Caspr2, whi ch is closely related to Caspr. The myelin sheath probably organizes these axonal membrane-related proteins via trans interactions.