Adjustment of the myelin sheath to changes in axon caliber

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Abstract
“Chronic” axon swellings were produced by applying a snug ligature around the sciatic nerve of rats by the fourteenth postnatal day, allowing the nerve to compress itself by its subsequent growth. The technique results in swelling of the entire myelinated fiber population proximal to the constriction, and in hypoplasia or arrested growth distal to it. “Chronic” axon swellings differed strikingly from the “reactive” axon swellings in the stumps of transected nerves in having normal or slightly below normal concentrations of axoplasmic organelles, including mitochondria and smooth endoplasmic reticulum.Swelling of the axons was associated with attenuation of their sheaths having normal interperiod width but fewer turns of myelin lamellae than found in normal fibers of comparable calibers. Release of the compression resulted in rapid restitution of axon swellings; the excess axoplasm was drained from the fibers and the sheaths rapidly acquired normal thickness. The magnitude of all observed changes in the sheaths of swollen and of restituted fibers was consistent with a rearrangement of the existing turns of myelin lamellae without change in the total amount of myelin. The myelin sheath appears capable of adjusting to both increases or decreases in axon size by slippage of its lamellae.