Ultrastructure of the Myelin Sheath
The electron micrograph shows the axon enclosed by a plasma membrane, the ax-olemma; it is surrounded by a series of regularly spaced, concentric dark and light lines (period lines). The width of each lamella from one dark line to the next measures 120 Å on average (1 Å = 0.1 nm), with the dark line taking up 30 Å and the light line 90 Å. As seen at higher magnification, the light lines are subdivided by a thin irregularline resembling a string of pearls (G7). We thus distinguish a dense major period line and a fainter intraperiod line. Studies using polarized light and X-rays have shown that the myelin sheath is made up of alternating layers of protein and lipid molecules. Ac-cordingly, the dark lines (major period line and intraperiod line) are regarded as layers of protein molecules and the light lines as layers of lipid molecules.