264 HOWARD J. CURTIS 



resistance is independent of the frequency and equal to the high fre- 

 quency value. He interpreted these findings in terms of the Bern- 

 stein model of cell structure, and these arguments had a good deal to 

 do with the acceptance of Bernstein's hypothesis as we know it today. 

 Bernstein postulated that cells consist of an ionized solution sur- 

 rounded by a thin impervious membrane. Hober reasoned that at 

 low frequencies the cells offer a very high resistance to the flow of 

 current because of the high resistance membrane, and all current flow 

 is through the electrolytic solution surrounding the cells. However, 

 the cell membrane acts as a capacitance, which allows high frequency 

 current to flow through it, so at high fi'equencies the current flows 

 both through the inter- and intracellular fluids. This supposition 

 also accounted for the capacitance known to be associated with 

 cells. Since this work, a great many refinements of both measure- 

 ment and interpretation have been added, but the fundamental con- 

 cepts have not changed markedly. 



2. Equivalent Circuit 



The term impedance is used to designate the generalized form of 

 resistance. The term resistance applies to direct currents and by 

 Ohm's law is defined as the ratio between voltage and current. Like- 

 wise for alternating currents the impedance is defined as the ratio 

 between the voltage and current, and may change as a function of the 

 frequency. If the system contains no inductive or capacitative ele- 

 ments, the impedance is independent of the frequency and equal to 

 the resistance. Thus the resistance is merely the zero frequency im- 

 pedance of any system. 



A living cell may be regarded as an impermeable membrane 

 surrounding a conducting fluid, with the cell itself immersed in a con- 

 ducting fluid. Thus two conductors are separated by an insubtor, 

 and this constitutes an electrical condenser. It has a certain capaci- 

 itance, known as the membrane capacitance. However, the mem- 

 brane is not a perfect insulator and some ions can pass through under 

 the influence of an electric field. Thus there is a certain ionic con- 

 duction across the membrane. Since ionic conduction is independent 

 of frequency, it is customary to speak of the ionic conduction phase of 

 the membrane impedance as the membrane resistance. 



Thus in general there are three paths that an alternating current 

 can follow in flowing through a suspension of cells: (1) It can flow 



