Mil. mo K L E C T H 1 C M K A S U R E M E N T S 235 



The field of electrical measurements in biology can be divided 

 into two general parts; potential measurements and impedance 

 measurements. In studying the former one attempts to measure 

 the electric potentials generated by biological cells and from these 

 deduce something of the functional characteristics of its cells. In 

 the latter, one measures the way in which an externally applied cur- 

 rent flows in and around cells in an attempt to learn more about the 

 structural characteristics of the cells and tissues. Before consider- 

 ing each of these fields separately, it will be profitable to consider 

 certain features that both fields have in common. 



1. Structure of Cells and Tissues 



From an electrical point of view, a living cell consists of an 

 ionized salt solution called protoplasm, which is a good conductor of 

 electricity, surrounded by a cell membrane, which is a very poor con- 

 ductor. The cell is usually immersed in a salt solution, which is a 

 good conductor. Because of the nonconducting cell membrane and 

 the conducting solution around the cell, it is very difl&cult to make a 

 current flow through the cell. The path of least resistance is around 

 the cell. It is this short-circuiting effect of the intercellular fluid 

 that makes these measurements so difficult. 



A tissue is an aggregation of similarly specialized cells united for 

 the performance of a particular function. From our present point 

 of view each cell is a separate entity and is in no way dependent upon 

 its neighbors. The various cells of a tissue are not all the same size 

 by any means, and the shapes may vary considerably. Therefore, 

 except in special cases, we Avould not expect any special electrical 

 characteristics from a tissue that we would not also expect from a 

 miscellaneous collection of cells of similar size and shape. 



2. Equivalent Circuits 



Whenever an electric current is conducted from one point to 

 another, in any sort of medium, it should be possible to construct 

 an equivalent circuit, that is to say, represent the circuit in terms of 

 the basic electrical components. Failure to do this in the past has 

 resulted in a great deal of erroneous thinking relative to electrical 

 problems in biology, and many apparently puzzling phenomena ap- 

 pear quite simple when anah^zed in this elementary way. 



An equivalent circuit, as the name implies, is a circuit made up of 



