HIGH ENERGY IRRADIATION: BIOELECTRIC EFFECTS 



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3 2 3.4 3.6 3.8 4D 4.2 4.4 4.6 



Ionic radius ( A) 



Fig. 13. The relative permeability of an ion (ordinate) is considered a physiological 

 term interchangeable with density of channel size. The ordinate on these diagrams 

 could just as conveniently read "number of available channels per unit area of mem- 

 brane." It is a reasonable assumption that channel size (abscissa) is distributed ac- 

 cording to a Gaussian curve. For neural membranes it is assumed the mode of the 

 distribution curve is (a) in the resting state that of a potassium ion, (b) in the con- 

 ducting state that of a sodium ion, and (c) in the irradiation state somewhere between 

 (a) and (b). 



sodium ions across the axonal membrane is 25 to 1. During excitation these 

 relations are reversed, i.e., the permeability of potassium to sodium is 1 to 

 25. Hence, the mode of the distribution curve during activity is assigned to 

 sodium (Fig. 13, conduction state) . 



The concept that the neural membrane behaves as though it were a 

 molecular sieve is not practical. Such a proposal does not offer an explana- 

 tion of how the cell discriminates between potassium and sodium ions as 

 indicated by p>ermeability studies. A molecular sieve model for the membrane 

 permits a small ion to pass through any channel of greater size than itself. 

 On this basis, sodium ions 3.67 A in radius should have free permit through 

 channels that sterically just pass potassium ions 4.05 A in radius. Hence, a 

 molecular sieve model fails to explain selective ion permeability. This diffi- 

 culty is removed if solvation (interaction with membrane components) is 

 "quantized." That is, an ion on entering a channel has all of its hydration 

 shells beyond the primary hydration level solvated by the wall of the 



