Further observations on the structure of myelin

Abstract

The changes in X-ray diffraction pattern which accompany modifications of the myelin sheath of frog sciatic nerve (details of which have been published previously) have been reconsidered in detail. It is noted that some modifications lead to increases in the diffraction spacings of the complex structure, and in the case of the dried specimens this is explained in terms of variations in the angle of tilt of lipid molecules within the layered lipoprotein structure. Approximate layer dimensions are deduced from the variation of the diffraction spacings of modified nerve, and the interpretation summarized diagrammatically. In the case of fresh nerve myelin the necessary condensation of the lipid layer thicknesses is achieved by having the longest lipid molecules stabilized individually in a curled position by association with cholesterol molecules rather than by tilting them. This enables the myelin components to retain their radial orientation, and suggests a specific function for the free cholesterol in myelin structure.