268 HOWARD J. CURTIS 



Membrane Capacitance. This is a fundamental property of 

 all cells and can be determined quite accurately by the impedance 

 method. It has been worked out for a large number of different 

 cells and surprisingly enough it amounts to about 1 /jlL per square 

 centimeter of cell membrane for all cells so far measured, and seems 

 to be independent of the condition or state of functional activity of 

 the cell {9). 



Resistance of Interior of Cell. This can be measured quite 

 accurately for cells of known geometric shape, and is computed 

 from the extrapolated infinite frequency resistance. This is of 

 course one of the important cellular constants and, in those cases in 

 which it has been possible to check this method with other methods, 

 the agreement has been very good. 



Membrane Resistance. This is one of the most important 

 properties of the cell, since it is presumably proportional to the 

 ionic permeability; many attempts have been made, some success- 

 fully, to measure it by this method. Unfortunately, this measure- 

 ment is very difficult. The reason for this can be seen from Figure 

 11a. The membrane resistance is relatively high and is shunted by 

 the very low resistance of the solution and by the low reactance of the 

 membrane capacity. In the case of a cell suspension the fact that 

 there was an actual ionic flow of current through the membrane 

 would become evident only as a discrepancy in the computed volume 

 concentration. In order to measure the membrane resistance then, 

 the volume concentration would have to be measured by an independ- 

 ent means, and, if there is a difference between this and the volume 

 concentration as computed from the impedance data, the membrane 

 resistance could be computed. However, since the best volume con- 

 centration measurements are good to only about 1% even in the most 

 favorable circumstances, the membrane permeability would have to 

 be relatively enormous before the impedance volume concentration 

 would be appreciably smaller than that measured by other methods. 

 For this reason the membrane resistance has never been directly 

 measured by this method on a suspension of cells. 



In one instance, that of a single nerve fiber from the squid, a change 

 of impedance was observed to accompany the propagation of the 

 nerve impulse, and this was interpreted as being due to a change in 

 ionic permeability of the cell membrane (8). Even here no accurate 

 absolute measure of the membrane resistance was obtained, but 

 changes in resistance were recorded Math considerable accuracy. 



