11:3/ The Absorption of Electromagnetic and Ultrasonic Energy 



209 



realized by allowing the cell wall to be a leaky capacitor; that is, to be 

 equivalent to a high resistance in parallel with a capacitor. Using the 

 actual cell shape instead of spheres also improves the application of the 

 theory to the experiment. This last is very hard except for simple 

 symmetric shapes. 



The properties of the cell wall are expressed as an areal capacity in 

 microfarads per square centimeter, and an areal resistance in ohm • cm 2 . 

 The impedance of the cytoplasm can be represented as a resistivity, 

 expressed in ohm cm. At one time, it was believed that these cell 

 constants could be interpreted to find the effective thickness of the cell 



Suspending 

 Medium 



Cell Wall 



a . /b 



c ■•:■■■! 



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 (a) 



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Cell 



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 (b) 



Figure I. (a) Diagrammatic representation of single cell in an 

 electric field. The resistance of the suspending medium is 

 lower in regions a and c than b because the cross section 

 occupied by the suspending medium is greater at a and c than 

 at b. (b) Equivalent lumped electrical parameters for the 

 preceding diagram. The cell wall is represented as a 

 capacitor. A better approximation would include a leakage 

 resistance in parallel with the capacitor. Resistors a, b, and c 

 represent the suspending medium in regions a, b, and c 

 respectively. 



membrane, but this hope has been unrealized as yet. Perhaps the most 

 impressive aspect of these data is their similarity from one cell type to 

 another. Plant cells, animal cells, nerve axons, and egg cells all overlap 

 in their electrical constants. 



The following are the orders of magnitude obtained for most cells. 

 The internal or protoplasmic resistivity varies from 10 to 30,000 ohm • cm, 

 with 300 being common for most mammalian cells. For cat nerve, a 

 value of 720 ohm • cm was measured. However, for other nerves, values 

 as low as 10 ohm -cm have been found. The areal capacitance varies 



