442 The Molecular Basis of Nerve Conduction /24 : 2 



TABLE I 

 Cathode Anode 



Swelling Shrinkage 



Decreased opacity Increased opacity 



Decreased light scattering Increased light scattering 



"Looser" structure Tighter structure 



Lower threshold for spike formation Higher threshold for spike formation 



Decrease of [Ca + + ]/K + Increase of [Ca + + ]/K + 



The swelling at the cathode and the opposite effect at the anode may- 

 be due to electro-endosmotic movement of water. (When a current 

 flows in a limited region, water as well as ions may be transported 

 because of potentials generated along the boundaries.) Likewise, the 

 decrease of the ratio [Ca ++ ]/[K + ] could also cause the swelling. 

 Various investigators have hypothesized peristalsis-like waves which 

 might travel along the axon, giving rise in themselves to the action 

 potential. All the mathematical schemes indicate such waves would 

 have to be continually supplied with additional energy. Nonetheless, 

 this peristalsis-like motion might contribute to the spike potential or its 

 rate of conduction. 



The "loosening" of the protoplasmic structure and the decreased 

 scattering of light could be interpreted as a breaking up of the protein 

 structure of the axon membrane in the region, mimicking "about to be 

 excited." The Ca ++ and K + changes also tend to support this view. 

 The clotting of both mammalian blood and sea urchin eggs depends on 

 the presence of Ca + + , and is decreased by K + . Thus, one may think 

 of the area near the cathode as being in some sense "solvated" and that 

 near the anode as "clotted." 



The ions Ca ++ and K + have "antagonistic" effects not only on 

 protein action but also on nerve conduction in general. An excess of 

 K + (or an absence of Ca + + ) in the external medium tends to lower the 

 threshold for the production of a spike potential. If the excess K + is 

 carried to an extreme, the axon fires repeatedly without any stimulus; 

 it thus produces tetany and eventually blocks conduction. An excess of 

 Ca + + (or the absence of K + ) tends to raise the threshold for excitation, 

 and in the extreme it blocks conduction of spike potentials. Thus, the 

 ion changes shown by the model appear highly significant. 



Several valid criticisms can be raised regarding the simplified system 

 discussed here. First, none of the chemical or mechanical effects 

 presented lead to an all-or-none type of spike potential. Further, they 

 are inherently incapable of revealing a threshold and are not connected 

 in an obvious way to any energy-supplying mechanism. The observa- 

 tions made with this model tell us nothing about the behavior of the 



