DEMARCATION CURRENT OR CURRENT OF INJURY 215 



its normal steady state only through a continuous expenditure of oxida- 

 tive energy. 



The electrolytic composition on the inside of the fiber differs from 

 that on the outside primarily by its potassium-ion content. This 

 difference as estimated by Fenn is about 65 times larger inside than out- 

 side. Such a difference in a concentration cell, according to Gasser's 

 [1938] calculations, would produce about 100 millivolts. That such 

 large differences of potential do exist was verified by Hodgkin [1939] 

 from a single nerve fiber of a crab preparation. Such potentials may be 

 accounted for by assuming the existence of a concentration gradient or of 

 a diffusion gradient across the cell membrane in which the mobility of 

 the potassium cation is the dominant factor. 



A mode of exploring the cause of such a difference of potential is to 

 investigate whether or not most of the potential may arise as the result 

 of the existence of a possible polarized film of molecules at the surface 

 of the fiber. A simple experiment involving the measurement of what 

 is called the demarcation current proves the existence of a polarized 

 surface possessing a difference of potential across its membrane-like 

 covering. 



Demarcation Current or Current of Injury 



If a nerve fiber is removed from the body, and two non-polarizable 

 electrodes, in series with a sensitive electrometer, are applied to the 



+ + + + + + + + + + + + + + 



«-s 1 



+ + + + + + ^p* +~+ + + -i- + 



[ : > /bW 



<£> 



Fig. VI-4. The polarized state of a diagrammatic nerve fiber. Injury at 

 the right end. V2 — V\ about 50 millivolts. 



nerve in such a way that one is in contact with the surface of the nerve 

 and the other is in contact with an injured end of the fiber (Fig. VI-4), 

 a constant deflection of the electrometer will result as long as the tissue 

 is alive. Such a deflection indicates that a difference of potential 

 exists between the outside cell membrane and the exposed axis cylinder. 

 The direction in which the electrometer deflects will indicate that the 

 uninjured surface is at a higher potential than the injured end. This 

 difference in potential is very small, usually about 50 millivolts. Until 

 recently these small differences of potential have been measured by 

 means of an Einthoven string galvanometer or a capillary electrometer. 

 Millivolt potentials can now be measured with precision with modern 



