X‐ray diffraction studies on the structure of the nerve myelin sheath

Abstract

ONE FIGUREI n a previous paper (Schmitt, Bear and Clark, '35, hereinafter referred to as I) the diffraction patterns of a variety of nerves were described, and it was shown that the patterns of all the fresh nerves studied were due to the myelin sheath. Characteristic long-spacing equatorial reflections which are orders of a fundamental spacing of 171 A were shown to be due to lipide molecules whose long chains are directed radially in the sheath. The ineridionally accentuated shortspacing ring at 4.7 A was attributed to the interchain distance of these lipides in the tangential direction.Since publication of this paper considerable additional diffraction data have been obtained and further information has come from polarization optical studies. The newer information has necessitated revision of earlier views of myelin sheath structure in some details, and progress reports have been incorporated in recent reviews (Schmitt, '36 ; Schinitt and Bear, '39; Schmitt, '39; Schmitt and Palmer, '40). The conclusioit was reached that the myelin sheath, like tubular myelin forms, is composed of cylindrical smectic fluid-crystalline lipide layers wrapped concentrically about the axon. The sheath is more cornplex than such myelin forms in having thin sheets of neurokeratinogenic protein intercalated between lipide layers. For a more precise description detailed information was required concerning the configurations and dimensions of the lipide molecules as pure components and as mixtures both in the dry and wet condition. This information has now been at least partially supplied (Bear, Palmer and Schmitt, '41; Palmer and Schmitt, '41) and in the present paper the structure of the sheath is discussed in the light of these new facts. E X P E R I M E N T A L METHOD

Myelin structure transformed by dimethylsulfoxide.

“…Measurement of absolute electron density (9) indicates that these spaces are occupied by a low concentration of protein in water (8). The total water content in the myelin sheath has been estimated to be in the range 30-50% (4,8,10,11 during the appearance of the contracted lattice. This structure formed in Me2SO appears to be a distinct phase derived from native myelin.…”

“…Whatever the mechanism of these effects, removal of Me2SO restores normal nerve conduction (1)(2)(3), and our studies show that the myelin sheath also recovers its normal structure. Native myelins in a variety of nerves have repeat periods in the range 150-185 A (4,5). The repeating unit is the membrane pair derived by joining cytoplasmic surfaces of the extension from a Schwann or glial cell plasma membrane (6,7).…”

“…Measurement of absolute electron density (9) indicates that these spaces are occupied by a low concentration of protein in water (8). The total water content in the myelin sheath has been estimated to be in the range 30-50% (4,8,10,11 The exact period of the contracted phase is a function of Me2SO concentration and time of treatment. When first detected, the period is 7-9 A greater than the final value when the contraction has come to equilibrium (Fig.…”

Adventures In Molecular Biology

“…A large series ofXRD studies was also made of aqueous dispersions of pure lipids, mixtures of lipids, and lipid-protein complexes. These revealed the presence of parallel layers of mixed lipids separated by layers of water that, in certain preparations, had thickness greater than that occupied by the lipid bimolecular layers (38). The nature of the forces in volved in this process was not obvious from existing theories oflong-range attraction or repulsion.…”

Models of Structure of the Envelope of Influenza Virus

“…The thicknesses of lamellar protein-lipid bilayers are found to be in the region of 60 to 70 A. 36 The volume of nonspike envelope protein is about 1.07 X 108 A3 per particle. This gives a protein layer about 33 A thick on either side of the bilayer, and an envelope thickness of about 130 A, much thicker than actually observed.…”