The relation of the beading of myelinated nerve fibers to the bands of fontana

The bands of Fontana, appearing as spirals or irregular light and dark strips crossing the surface of unstretched nerves, are due to the wavy disposition of nerve fibers within the epineural-perineural sheaths. A mean tension of 2.7 ± 0.23 (S.E.M.) g applied to segments of rat tibial nerves straightens the fibers and unbands thé nerves causing them to lengthen by 9.35 ± 0.89%. The nerves cold-fixed in situ at that point showed the myelinated fibers to be beaded. On relaxation the nerves rebanded and the fibers were no longer beaded. The tension at which unbanding occurred was better determined when the epineural-perineural sheaths were slit longitudinally. Under these conditions, unbanding occurred at a mean tension of 0.59 ± 0.08 g and the nerves lengthened by 8.56 ± 0.58%. The lengthening was not statistically different from that seen in sheathed nerves. In preparations with the epineural-perineural sheaths removed, banding was lost with tensions of 0.20 ±0.03 g and the nerves lengthened by 12.1 ± 1.04%. The tensions needed were significantly lower than that for the sheathed and slit-sheath nerve groups. When cold-fixed, when banding was lost, the fibers were seen to be beaded. Banding of the desheathed nerves returned on relaxation of the nerves. However, after tensions of 8 g they showed plasticity in which the ends of the nerves needed to be pushed together to initiate rebanding in comparison to sheathed or sheath-slit nerves which rebanded spontaneously following relaxation after even higher tensions of 40 g. At the highest tensions the nerves remained extended and could not be forcibly rebanded. Lengthening of desheathed nerves and sheath-slit nerves to the point where banding disappeared was not significantly different before and after treatment with collagenase. The collagenase-treated fibers, however, showed hyperbeading where the constricted regions extended over longer lengths, and the expansions displayed a more bulbous form. We conclude that beading is triggered when the fibers become straightened and this occurs with tensions well below the levels where loss of structural and functional integrity would be encountered.