VOL. 4 {1950) MORPHOLOGY IN MUSCLE AND NERVE PHYSIOLOGY 73 



impulse propagation in nerve. To gain a perspective as to the promise of further inves- 

 tigations at or near the molecular level it will be useful to consider what information of 

 this sort is now at hand. The discussion will be centered around the two chief components 

 of the fibre, the axon (myelin) sheath and the axon (axis cylinder). 



THE AXON SHEATH 



The general features of the molecular architecture of the myelin sheath have been 

 deduced from polarized light and X-ray diffraction studies*^ Essentially the sheath 

 consists of lipid-protein layers about 180 A thick wrapped concentrically about the axon. 

 The lipid phases exist as smectic mesomorphic double layers of mixed lipids, the paraffin 

 chains being oriented normal to the planes of the layers, i.e., radially in the sheath. The 

 protein component is intercalated between double layers of lipids in thin sheets esti- 

 mated to have an over-all thickness of 25-30 A per period. This is presumably the protein 

 which, on fixation, gives rise to the neurokeratin network. In view of our ignorance of 

 the properties of this protein it is impossible to say anything about its configuration in 

 the very thin layers in the sheath. When nerve is dried the thickness of the layers is 

 reduced by about 25 A and a considerable fraction of the sheath lipids is extravasated 

 from the organized structure to form separate lipid phases. In the skeleton of the original 

 structure which remains it seems probable that a fraction of the lipid molecules is firmly 

 bonded to the thin protein layers and that this linkage maintains the structure in the 

 dried sheath. The nature of this linkage can only be surmised though one may suspect 

 that the acid groups of the cephalin molecules may be involved. 



Thus far electron microscopy has contributed little to our knowledge of sheath struc- 

 ture though advances in this direction may be expected when sectioning methods are 

 applied. From osmic acid fixed nerves disintegrated with sonic oscillations, Sjostrand 

 [unpublished) has observed fragments of very thin layers which may have been derived 

 from the myelin sheath. He had previously demonstrated with the electron microscope 

 that the outer limbs of the retinal rods consist of stacks of thin discs*^' ^^. This is in 

 agreement with the polarized light analysis which indicated that, like those of the myelin 

 sheath, the thin layers contain lipid and protein components oriented perpendicular and 

 parallel, respectively, to the planes of the layers. It has been suggested** on very 

 inadequate grounds, that the protein of the rod outer limbs may be a type of "neu- 

 rokeratin". De Robertis and the writer have also observed thin layers in preparations 

 from fragmented myelinated nerves. Curiously the fragmented layers frequently show 

 characteristic angular cleavage. If the layers actually derive from the sheath this type 

 of cleavage is unexpected since the sheath has thus far been considered to be uniaxial 

 with optic axes normal to the layers. Measurements of the thickness of the layered 

 fragments may help determine their origin since the over-all thickness of the sheath 

 layers is known from X-ray data. 



The X-ray and polarized light results concern only the highly organized lipid- 

 protein substance of the sheath. Determination of the detailed structure of the various 

 other sheath components which have been observed histologically must await electron 

 microscope study in thin sections. Among these structures are the boundaries of the 

 sheath at the incisures, the Golgi funnels and spirals of Rezzonico, the axolemma mem- 

 brane, the Schwann cell and the outer fibrous investiments. The structure at the node 

 will be particularly interesting because the limiting envelope of the fibre at this point 

 References p. 76I77. 



