238 HOWARD J. CURTIS 



• before any measurements in this field are attempted it will be neces- 

 sary to know something of the possible sources of potentials in bio- 

 logical systems. 



There are in general two sources one could think of as the origin 

 of these potentials. The first is static or frictional electric charges 

 arising at an interphase between solid and liquid, solid and air, etc. 

 That such potentials exist in biological systems can be shoA\Ti in a 

 number of ways. For example, if a suspension of living cells is 

 placed between two electrodes and a direct current passed through 

 the suspension, it will be seen that the cells will move under the in- 

 fluence of the electric field. This means that there must be a surface 

 charge on the cell membrane. A measurement of the speed of migra- 

 tion of the cell gives a measure of the electrophoretic mobility, from 

 which it is possible to compute the surface potential under certain 

 conditions. The importance of these potentials has been discussed 

 by Abramson (1) and by Abramson, Moyer, and Gorin (£) (see also 

 Chapter IX). 



The second source of bioelectric potentials is quite distinct from 

 the surface charges and is probably ionic in origin. If an electrode 

 is placed against the cut surface of a living muscle and another placed 

 on an uncut portion of the muscle, a potential difference will be found 

 to exist between the two electrodes. Moreover, a current will flow 

 between the electrodes for long periods of time. This fact shows con- 

 clusively that the potential cannot be of static origin, since if it were 

 it would be rapidly dissipated. This potential is also probably not 

 directly due to a chemical reaction, since the electromotive force is 

 roughly proportional to the absolute temperature. Most chemical 

 reactions have temperature coefficients much larger than this. 



The dissociation of an electrolyte into positive and negative ions 

 cannot of itself give rise to a potential, since the ions are always pres- 

 ent in equal numbers. However, under certain circumstances the 

 two kinds of ions may become somewhat separated from each other, 

 giving rise to local potential differences. This is probably the origin 

 of most bioelectric potentials usually measured. It remains to be 

 considered how th's separation takes place. 



Suppose that the ions are free to diffuse away from their point 

 of origin after they are formed. In general one ion will diffuse faster 

 than the other and in this way they will become somewhat separated. 

 For example, if a tube containing a strong sodium chloride solution is 

 dipped into a beaker containing a weak sodium chloride solution in 



