74 F- O. SCHMITT VOL. 4 (1950) 



has especial physiological significance. Technical difficulties make it hard to study this 

 surface structure with polarized light. 



From polarized light studies it has been suggested that all nerve fibres may possess 

 a lipid-protein sheath having the same type of architecture as that of the myelin sheath*^. 

 Such a sheath has been demonstrated in several types of invertebrate fibres though the 

 investigation has not yet been extended to the so-called naked fibres such as the Remak 

 fibres. In the limiting case the naked fibre may possess a surface structure no more 

 complex than the plasma membrane itself. The polarized light method is probably 

 sufficiently sensitive to detect molecular orientation in such paucimolecular layers. 

 However, the bearing of such data on the problem of impulse propagation would still 

 remain to be shown. 



No direct connection between sheath ultrastructure and physiological properties 

 has been demonstrated, although a correlation has been pointed out between sheath 

 birefringence, e.g., essentially lipid concentration, and velocity of impulse propagation*^. 

 This correlation is at best only rough when applied to the fibres of a particular type of 

 nerve but seem more suggestive when fibres of widely different types of nerves are con- 

 sidered. For several types of vertebrate and invertebrate fibres having approximately 

 equal conduction velocities, Taylor*^ found that the product of fibre diameter and 

 sheath birefringence is roughly constant. 



THE .\XON 



The most interesting structures in the axon are, of course, the neurofibrils. Only in 

 exceptional cases can these objects be observed in the fresh fibres, the chief lore of the 

 literature being concerned with fixed and stained preparations. The neurofibrils may 

 approach the limit of microscopic resolution in fixed and stained preparations. Hence 

 it is readily understandable that, if they pre-exist in the fresh axon, they may not be 

 visible, particularly if refractive index relations are unfavourable. In the dark field 

 microscope Ettisch and Jochims*^ observed no structure in the fresh axon though very 

 fine collagen fibrils of the connective tissue were clearly visible, indicating a fundamental 

 difference in the two types of fibres. After treatment with reagents such as CaClg or 

 fixatives, neurofibrils immediately appear. Apparently only slight colloidal alterations 

 suffice to make them visible. It was concluded by Peterfi^^ that the fresh axon is a 

 rodlet sol capable, under very slight chemical provocation, of forming a fibrous system. 

 He suggested that the mutual interaction of the elongated micelles may be intimately 

 associated with impulse propagation. 



Electrical studies have failed to indicate any direct role of axoplasm except as a 

 passive conductor of current. An electrode may be inserted into the axon of the squid 

 giant fibre without blocking conduction. But if the inner surface of the cell membrane 

 is injured conduction ceases^' *^. However, Curtis and Cole's*^ statement that "This 

 makes it seem rather unlikely that there is an internal structure in the axon which 

 takes a prominent part in the active mechanism of propagation" must be accepted with 

 caution since there is no evidence that the manipulation mentioned disrupted any 

 axonic structures which might be present as it did the membrane structure. 



Changes in the colloidal organization of the axon with activity have been sought, 

 but thus far the experimental techniques have been very crude. It has been claimed 

 that the fibre exhibits changes in contour with electrical polarization, swelling at the 

 References p. 76lyy. 



