VOL. 4 (1950) MORPHOLOGY IN MUSCLE AND NERVE PHYSIOLOGY 75 



anode and flattening at the cathode-^. More recently Flaig^" believed to have shown 

 that the viscosit}^ and turgor of the axoplasm of the squid giant fibre is considerably- 

 increased during activity. He suggested that excitation increases the viscosity by shifting 

 the sol-gel equilibrium. If Flaig's results are confirmed, careful investigation of the 

 light scattering by the axon might be warranted. The existence of elongate particles 

 in the fresh axon is demonstrated by the positive birefringence which, though weak, is 

 measureable in large axons such as in the squid giant fibre. Semi-quantitative analysis 

 of the positive form birefringence indicated that though the oriented fibrous structures 

 occupy a small portion of the axon volume, they must have a considerable degree of 

 regularity of internal structure, for their intrinsic birefringence is comparable with that 

 of myosin or collagen fibres^^. 



No change in molecular orientation in the axoplasm of squid giant fibres during 

 activity could be detected by polarization optical means^^. Using a sensitive photo- 

 electric method capable of recording small changes in birefringence without appreciable 

 time lag, it was concluded that if any change occurred it was less than 0.2% of the initial 

 birefringence for the spike process and less than o.oS°o for the slow recovery processes. 

 Unless more sensitive methods yield positive results it may be concluded that impulse 

 propagation is associated with little if any change in orientation of the elongate par- 

 ticles of the axon. 



From electron microscope studies, Richards, Steinbach, and Anderson"^ 

 described contorted fibrils composed of kinked elongate particles in a.xoplasm extruded 

 from squid giant fibres. They suggested that these structures may form the basis of 

 neurofibrils. De Robertis and Schmitt^* observed characteristically double-edged 

 fibrils in electron micrographs of material obtained by sonic fragmentation of frozen 

 sections of formalin fixed nerves of various types. Such structures had never before been 

 observed. For descriptive purpose the fibrils were tentatively called "neurotubules". 

 The dense material at the edges is for the most part removed by washing with water. 

 It is not yet clear to what extent this dense material is associated with the fibrils in the 

 natural state and to what extent it may have been deposited upon them during the 

 preparative procedure. 



After staining with phosphotungstic acid or shadowing with heavy metal the fibrils 

 have a cross-striated appearance. The axial period averages about 650 A and detailed 

 intraperiod structure has been observed. Since this period is similar to that of collagen^^ 

 and since nerve fibres are closely invested with connective tissue the possibility that 

 neurotubules may be collagen fibres invested with dense materials of undetermined 

 origin was carefully considered. The fragmentation technique employed makes it 

 difi&cult to determine the location of the neurotubules in the nerve fibre. All the evidence 

 was consistent with the view that they are of axonic origin. Important in the reasoning 

 was the fact that typical double-edged fibrils were not observed in preparations of nerves 

 which had been allowed to undergo degeneration in vivo (Wallerian) or in vitro^^. 

 However, in recent experiments on late degeneration, results at variance with those 

 previously described were obtained. Preparations from nerves degenerated for as long 

 as three weeks were not devoid of double-edged fibrils but contained them in considerable 

 abundance. The reason for this discrepancy is not clear. However, in view of the impor- 

 tance of the degeneration changes to the argument that fibrils are of axonic origin, the 

 entire matter is being reinvestigated. Speculation as to the possible role of the neuro- 

 tubules in nerve function would be premature at this time. 

 References p. yOjyy. 



