262 HOWARD J. CURTIS 



If one is interested only in the action potential, it is of course unneces- 

 sary to use a direct-coupled amplifier. 



2. Electrical Potentials around Tissues 



Electrical potential differences exist around single cells and tissues 

 under special conditions, and they have been rather extensively 

 studied by a number of investigators. No interpretation of these 

 potentials has been agreed upon by the various investigators, but it 

 seems clear that they must be interpreted on some basis different 

 from that used in the case of the membrane potentials in nerve and 

 muscle. 



In general these potentials exist between different parts of a tis- 

 sue, which is composed of a group of small cells. The potential must 

 then normally exist between different sides of the single cells of the 

 tissue. One can think of this situation in the case of skin, a gland 

 cell, or a growing plant, where there is a definite polarity to the cells 

 and where one would expect the two sides to be in contact with dif- 

 ferent solutions. It is well known that the membrane potential is a 

 function of the concentration and composition of the fluid in contact 

 with the cell membrane, so that, if two sides of a cell are in contact 

 with different solutions, in general one would expect a potential to 

 exist between the two sides. This is a possible explanation of these 

 potentials in terms of quantities that are reasonably well understood. 

 It is by no means the only possible explanation (see Lund, 18). 



Here again it would be necessary to draw an equivalent circuit of 

 the particular system under investigation and assign at least approxi- 

 mate values to the circuit elements before an amplifier could be con- 

 structed intelligently for measuring these potentials. However, in 

 general it could be said that, the higher the input resistance of the 

 amplifier or measuring device, the less would be the possibility of in- 

 troducing errors due to measurement. 



E. IMPEDANCE MEASUREMENTS 



1. Introduction 



Not long after Ohm formulated the law that has come to be 

 known by his name, it was recognized that the electrical resistance of 

 any substance is one of its fundamental properties, and consequently 

 this measurement was made on all manner of compounds and solu- 



