The Ultrastructure of the Nerve Axon Sheath

Abstract

Summary 1. In avoiding certain inherent indeterminacies in classical morphological methods and in obtaining further details regarding the microscopic and ultra‐microscopic structure of nerve axon sheaths, the methods of polarization optics and X‐ray diffraction are of great value. In the case of the myelin sheaths of vertebrate nerve fibres, for example, the optical and diffraction studies indicate the structure of the living fibre's sheath to be of smectic mixed fluid‐crystalline nature. The structure is, therefore, readily altered by chemical treatment to form the artifacts commonly observed in histological preparations. 2. A number of considerations suggest that the specific configuration of the lipoid and protein components of the myelin sheath is as follows. The proteins occur as thin sheets wrapped concentrically about the axon, with two bimolecular layers of lipoids interspersed between adjacent protein layers. While this means that in a radial direction within the cylindrical sheath there are alternate predominantly aqueous and predominantly hyirocarbon phases, the latter cannot be described as being entirely “non‐aqueous” 3. Polarization optical studies show that, contrary to the general view, invertebrate nerve fibres quite widely possess, aside from connective tissue investments, thin sheaths which are essentially similar in ultrastructure to the well‐defined myelin sheaths of vertebrate fibres. The demonstration of this fact involved a reinterpretation of the meaning of Gothlin's metatropic reaction, in which immersion of the fibre in media of high refractive index permits the (intrinsic) birefringence of lipoids present in the normal sheath in an oriented condition to become apparent by the reduction of the masking (form) double refraction of protein. Associated with the invertebrate metatropic axon sheaths are cells similar to the Schwann cells of vertebrate fibres. 4. Quantitative birefringence studies have disclosed that the axon sheaths of a wide variety of fibre types differ chiefly with respect to the relative amounts of oriented protein and lipoid present. This difference is observed not only between typical invertebrate and vertebrate fibres, but also when the fibres of a single vertebrate nerve are compared. For example, the curve obtained when sheath birefringence of frog sciatic fibres is plotted against fibre diameter shows wide variations in the magnitude of double refraction, changing continuously from birefringence due preponderantly to lipoids, in the case of the larger fibres, to that which, in the smallest fibres, results primarily from proteins. The transition from lipoid to protein predominance occurs at a fibre diameter of about 2μ., agreeing well with the division between “medullated” and “non‐medullated” fibres arrived at by histologists. It has been suggested that the low concentration of lipoid in the sheaths of small fibres is related to physical factors opposing the introduction of the lipoids into cylindrical structures of high curvature. 5. Examination of available inform...

Changes in axon birefringence during the action potential

“…Most of the resting retardation at the L. B. COHEN, B. HILLE AND R. D. KEYNES centre of the axon was apparently contributed by longitudinally oriented fibrils and tubules in the axoplasm, because removal of the axoplasm and perfusion with potassium fluoride solution reduced the retardation through the centre by about 90 %. At the edges of the axon the retardation was unaffected by perfusion, because of the appreciable contribution made by the form and intrinsic birefringence of the Schwann cell and connective tissue (Schmitt & Bear, 1939 The axon membrane itself probably contributes relatively little to the net resting birefringence. If it had a structure similar to that of an erythrocyte ghost, it would have a positive intrinsic birefringence and a negative form birefringence both with a radial optic axis, so that referred to the longitudinal axis its intrinsic birefringence would make a negative contribution and its form birefringence a positive one.…”

Adventures In Molecular Biology

“…A similar lipid-protein organization was shown to exist in invertebrate (unmyelinated) nerve, e.g. the squid giant nerve fiber, and lobster and other invertebrate fibers (36).…”

“…This led to the discovery of a 17 1-A period, representing the radial distance between parallel concentric layers, each continuing a double layer of mixed lipids, plus a monolayer of protein (36,37). Later studies by B.…”

Hypothermic decrease in microtubule density and birefringence in unmyelinated axons

“…Excellent reviews on the subject have been recently available (Cohen and Keynes, 1968;Cohen, Hille, and Keynes, 1970;Cohen, Hille, Keynes, Landowne, and Rojas, 1971;Von Muralt, 1975). The overall "resting" birefringence of some optically anisotropic fibrillar cells, chiefly nerve fibers, has been worked out in many valuable papers, particularly during the past thirty years (Schmitt and Bear, 1939). Recent data, in particular those concerning long-lasting birefringence changes, were presented by Watanabe and Terakawa (1976).…”