ORDER-DISORDER PHENOMENA IN MYELINATED NERVE SHEATHS .5. EFFECTS OF TEMPERATURE ON RAT SCIATIC AND OPTIC NERVES, AND STRUCTURAL DIFFERENCESBETWEEN THE 2 TYPES OF NERVE

We describe in this work X-ray scattering and electron microscope stud ies of rat sciatic and optic nerves as a function of temperature. The scattering experiments were analyzed as described in the previous pape rs of this series: a variety of parameters were determined, some of wh ich characterize the lattice disorder, others the structure of the mot if. The main results are the following. All the parameters determined by the X-ray scattering study vary with temperature and the temperatur e-dependence is specific for the type of nerve (sciatic or optic). Mos t of the disorder related parameters display a minimum or a maximum in the vicinity of physiological temperature (38 degrees C in rat); this observation, strongly supported by the electron microscope study, sho ws that the degree of organization of myelin is highest near physiolog ical temperature. The structure of the motif, as revealed by the elect ron density profile, is fairly different in the two types of nerves (i n contrast with the assumption made by previous workers); the structur e also varies with temperature and the temperature-induced alterations are nerve-type specific. In the two types of nerve the thickness of t he Lipid bilayer varies with temperature as expected for a lipid-conta ining system with hydrocarbon chains in the disordered conformation. I n sciatic nerve the thickness of the (thinner) cytoplasmic polar layer , which is also the layer most affected by lattice disorder in this ty pe of nerve, decreases dramatically with increasing temperature. In op tic nerve, in which lattice disorder predominantly affects the extrace llular layer, the thickness of both the cytoplasmic and the extracellu lar layer is barely affected by temperature.