Beading of myelinated nerve fibers

The phenomena of beading upon stretching was studied in nerve fibers of freeze-substituted nerves. Beading appears rapidly along the length of the mammalian nerve. Nerve fibers of frogs kept at low temperatures have a crenated shape in cross section and show wrinkling in longitudinal section, but no beading is produced upon stretching them. Frogs kept at higher temperatures have nerve fibers which are more cylindrical in shape and a moderate degree of beading can then be excited by stretching. The beading is due to constrictions along the length of the fibers. Electron micrographs show an increased concentration of neurofilaments in the constricted part of the fiber, a change interpreted as squeezing of the fluid part of the axoplasm into the expanded parts of the beads.