Demyelination regions of nerve fibers in frog muscle spindle as studied by serial sections for electron microscopy

Abstract

The ultrastructure of different kinds of demyelination regions of small sensory myelinated nerve fibers located within the capsule of the frog muscle spindle has been studied by means of serial sections. The myelinated fiber has been observed to shed its myelin stepwise at nodes or branching points. Loosely wrapped myelin lamellae have been found in parts of the nerves studied. The organization of Schwann cell cytoplasm close to the demyelination areas and its filament-containing processes and their contact arrangements with the nodal or branching axon have been analyzed. In the axoplasm in these areas groups of small vesicles as well as true accumulations of tightly packed mitochondria were observed. Further, densely packed neurofilaments of different appearance were found. The physiological significance of some of the characteristic features found in these-sensory demyelinating nerve fibers is discussed.