Background The biological process underlying axonal myelination is complex and often prone to injury and disease. The ratio of the inner axonal diameter to the total outer diameter or g-ratio is widely utilized as a functional and structural index of optimal axonal myelination. Based on the speed of fiber conduction, Rushton was the first to derive a theoretical estimate of the optimal g-ratio of 0.6 [1]. This theoretical limit nicely explains the experimental data for myelinated axons obtained for some peripheral fibers but appears significantly lower than that found for CNS fibers. This is, however, hardly surprising given that in the CNS, axonal myelination must achieve multiple goals including reducing conduction delays, promoting conduction fidelity, lowering energy costs, and saving space. Methodology/principal findings In this study we explore the notion that a balanced set-point can be achieved at a functional level as the micro-structure of individual axons becomes optimized, particularly for the central system where axons tend to be smaller and their myelin sheath thinner. We used an intuitive yet novel theoretical approach based on the fundamental biophysical properties describing axonal structure and function to show that an optimal g-ratio can be defined for the central nervous system (approximately 0.77). Furthermore, by reducing the influence of volume constraints on structural design by about 40%, this approach can also predict the g-ratio observed in some peripheral fibers (approximately 0.6). Conclusions/significance These results support the notion of optimization theory in nervous system design and construction and may also help explain why the central and peripheral systems have evolved different g-ratios as a result of volume constraints.

https://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0007754&type=printable

What is the optimal value of the g-ratio for myelinated fibers in the rat CNS? A theoretical approach.

The combined electron-microscope and biochemical evidence regarding mitochondria and mitochondrial fractions, although still preliminary in character, indicates that all the functional enzymatic components of the electron-transport chain are compactly arranged in the "elementary particle," which may therefore be regarded as the ultimate unit of mitochondrial function. New data have also been obtained on the hydrated lipoprotein matrix, the recently isolated structural protein, solubilized pure lipid fractions, cytochromes, and other constituent elements of the respiratory-enzyme assemblies.

Cell-membrane ultrastructure. Low-temperature electron microsopy and x-ray diffraction studies of lipoprotein components in lamellar systems.

Low-temperatare preparation techniques are described which are reported to give better morphologic and cytochemical preservation of biological specimens than do the standard freeze-drying and freeze-substitution methods. (C.H.)

Low-temperature preparation techniques for electron microscopy of biological specimens based on rapid freezing with liquid helium II.

Publisher Summary This chapter describes the physical structure and behavior of lipids and lipid enzymes. The resolving power of the electron microscope and X-ray diffraction camera has required biologists to reconsider the physical and structural properties of lipids in the organization of a living cell. The reason for this is that lipids, in the presence of water, tend to aggregate together and arrange themselves in a highly characteristic pattern of bimolecular sheets or cylinders. Their dimensions are within the resolving power of the electron microscope and any tendency toward layering of the sheets or concentric replication of the cylinders is amenable to interpretation by X-ray diffraction. However, the principles of surface chemistry have been applied with some success to the study of extracellular lipid behavior, particularly, in fat digestion and absorption.

H. Fernández-Morán

Papers

Fine Structure of Biological Lamellar Systems