The Relation of Axon Sheath Thickness To Fiber Size In The Central Nervous System of Vertebrates

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Abstract
In a series of land vertebrates sheath thicknesses of myelinated nerve fibers in a variety of central tracts have been measured, and plotted against axon diameters. Preparations include monkey, cat, rat, mouse, mole, hedgehog, bullfrog, and green frog. Tracts examined include dorsal column cuneate and gracilis bundles, pyramidal tract, optic nerve and tract, trigeminal and auditory nerves, and for comparison, the saphenous nerve. Maximal sizes of axons (inside sheath diameter) in different preparations vary widely, from 12 μ or more to 2 μ, and most tracts have axon diameter ranges down to 0.3 μ. In the cat auditory nerve the axon size range is restricted (80% between 2 and 3 μ). All fibers in the mammalian central tracts are myelinated, with a wide scatter of values of the ratio of sheath thickness to axon diameter, as measured in a single cross section. The ratio varies with fiber diameter, becoming larger with decreasing diameter. Average ratios attain the value of 1/5 in different preparations at different axon diameters, generally from 3 μ or less in central tracts (in some preparations ratios remain just below 1/5 in the smallest fibers present) as compared to 5 μ in the cat saphenous nerve. In certain tracts (e.g., dorsal columns of spinal cord), the ratio is approximately 1/5 over the whole size range of the fibers present. In the frog optic nerve the myelinated fibers tend to have a uniform sheath thickness throughout the diameter range.