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L. J. MULLINS 



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442 422 402 



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Mean Interspace Size (A) 



Fig. 3. The lower curves show the changes in the partition coefficient (number of 

 interspaces that fit) of various ions as the mean interspace size of the membrane is 

 varied. The curve at the top is the membrane potential computed on the basis that 

 ion permeability and partition coefficient will vary proportionally. 



anomalies of K + movement in axons can be explained if the ions move in single 

 file through channels in the membrane, and that movement in one direction 

 tends to inhibit movement of the same ion in the opposite direction. That this 

 effect does not appear important for other ions might seem to suggest that the 

 number of K + channels is small when compared with those for other ions, while 

 permeability data indicate the reverse. An explanation of this coupling as well 

 as that between membrane potential and electrical resistance, as noted by 

 Jenerick (1953) for muscle can be obtained by considering the mechanism in- 

 volved in the diffusion of K+ through membrane channels. Assuming that an 

 ion has been partitioned to a channel of proper size, its progress along the chan- 

 nel will be influenced by the radius at each point along the channel. Because 

 the macromolecules that form the channel must be undergoing thermal motion, 

 the channel must vary in size from one point to another and an ion at one end 

 will only progress when it obtains energy from a collision at the same time 

 that the channel size immediately ahead is within permitted limits. If, on the 



