VIII. BIOELECTRIC MEASUREMENTS 



241 



aritlim of the K+ concentration outside the cell as a function of the 

 membrane potential, a straight line should be obtained. Figure 2 

 shows the results of a set of measurements taken on the giant nerve 

 cell of the squid. Here the membrane potential is plotted against 

 the logarithm of the K+ concentration outside the cell. When the 

 K+ concentrations inside and outside are equal, there is no membrane 

 potential. However, as the K+ concentration outside the cell is 

 reduced, the negative potential of the cytoplasm of the cell increases 

 in a straight line until cjuite a low value of K+ concentration is 

 reached, and then this relationship breaks down. Why the Nernst 



-20 



Fig. 2. Membrane potential 

 from the squid giant axon, meas- 

 ured between one electrode in- 

 side the cell and another out- 

 side. The potential is measured 

 as a function of the relative K "*" 

 concentration outside the cell 

 compared to the normal K"*" 

 concentration of sea water (12). 



+ 60. 



0.1 

 RELATIVE 



1.0 

 POTASSIUM 



10 100 



CONCENTRATION 



relation is not followed beyond this point is not known, but presum- 

 ably some of the postulates are no longer valid. Experiments such 

 as these give us considerable confidence that the above explanation 

 for cell potentials is correct in a general way. 



2. Electrodes 



In making potential measurements in biological systems it is al- 

 ways necessary to make an electrical connection between the meas- 

 uring device and the electrolytic solution of the biological system. 

 This in\()lves a metal-to-liquid junction. Ideally one could think 

 of finding a metal that would show no potential difference between it- 

 self and an aqueous solution. This appears to be impossible and the 

 pi'oblem has resolved itself into one of finding an electrode that will 

 give constant and repi()ducil)le potentials when placed in contact 

 with any salt- solution. 



When a metal is placed in water there is a tendency for some of 

 the metal to go into soltition. This is known as the solution pressure 



