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 wav y 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 the nerves causing th em to lengthen by 9.35 +/- 0.89%. The nerves cold-fixed in situ at tha t point showed the myelinated fibers to be beaded. On relaxation the n erves rebanded and the fibers were no longer beaded. The tension at wh ich unbanding occurred was better determined when the epineural-perine ural sheaths were slit longitudinally. Under these conditions, unbandi ng occurred at a mean tension of 0.59 +/- 0.08 g and the nerves length ened by 8.56 +/- 0.58%. The lengthening was not statistically differen t from that seen in sheathed nerves. In preparations with the epineura l-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 ne eded were significantly lower than that for the sheathed and slit-shea th nerve groups. When cold-fixed, when banding was lost, the fibers we re seen to be beaded. Banding of the desheathed nerves returned on rel axation of the nerves. However, after tensions of 8 g they showed plas ticity in which the ends of the nerves needed to be pushed together to initiate rebanding in comparison to sheathed or sheath-slit nerves wh ich rebanded spontaneously following relaxation after even higher tens ions 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 sign ificantly different before and after treatment with collagenase. The c ollagenase-treated fibers, however, showed hyperbeading where the cons tricted regions extended over longer lengths, and the expansions displ ayed a more bulbous form. We conclude that beading is triggered when t he fibers become straightened and this occurs with tensions well below the levels where loss of structural and functional integrity would be encountered.