VIII. BIOELECTRIC MEASUREMENTS 261 



where Rg is the resistance along the outside of the nerve between the 

 two electrodes and Ri the internal resistance, which includes the 

 resistance along the inside of the cell between the two electrodes as 

 well as the membrane resistance. E^ is the true membrane potential. 



It will be seen that Vo = E^ only in cases Ro ^ Rp This condi- 

 tion is very difficult to fulfill. Ri is the internal resistance of the cyto- 

 plasm and cannot be altered. The outside resistance, Ro, is normally 

 very low due to the large bulk of electrolytic solution outside the cell. 

 This resistance can be increased some by decreasing the bulk of fluid, 

 but it is difficult to make it even as high as Ri. For this reason, in 

 nerve bundles it is practically impossible to measure an injury poten- 

 tial that is more than a fraction of the true membrane potential. In 

 single nerve fibers it has been found possible to dry out the nerve 

 enough so that Ro is apparently considerably higher than Ri. Under 

 these very special conditions the injury potential probably approxi- 

 mated the resting potential (17). 



In measuring these potentials it is especially important to make 

 sure that the measuring device is not affecting the potential being 

 measured. It will be seen from the equivalent circuit of Figure 10 

 that, if the effective input resistance of the measuring instrument is 

 too low, Ro will effectively be made still smaller, thus increasing the 

 inherent error of this method. It is necessary to check this very 

 carefully, especiall}' when working with single nerve fibers. 



The action potential of nerve or muscle is the electrical indication 

 of the passage of an impulse down the nerve or muscle. It can be 

 measured by the same method as is used for measuring the mem- 

 brane potential, i.e., either directly by means of an internal electrode 

 or by the injured-end technique. In addition, the "diphasic" action 

 potential may be measured between two electrodes placed at different 

 points along the uninjured nerve fiber. 



A measurement of the magnitude of the action potential is sub- 

 ject to the same limitations and errors as have already been discussed 

 for the resting potential. In addition, the potential changes take 

 place very rapidly and it is necessary to make siu-e that the recording 

 instrument responds rapidly enough. In careful work, it cannot be 

 assumed even that the electronic amplifiers are responding rapidly 

 enough, and the response time must be carefully checked in this case. 



