Friday, October 28, 2011
Ultrastructure of the Myelin Sheath
The electron micrograph shows the ultrastructure of the myelin sheath: the axon is enclosed by a plasma membrane, the axolemma, which 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 irregular line resembling a string of pearls. 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. Accordingly, 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.