STRUCTURE OF NERVE CELL MEMBRANES 



2 Std. Deviation 



127 



t t 



Na T 

 3.67 



0.036 



0.04 



4.05 



1.00 



1.00 



CI" 



4.53 



0.44 

 0.45 



Radius of Ion 

 A, (I H 2 Shell) 



Area 



( Hodgkin ) 



Fig. 2. The number of membrane interspaces is plotted against their size as meas- 

 ured by the radius of a circle that can be inscribed in the interspace. Values for ion 

 sizes are from Table I, and the mean interspace size is set by assuming that the perme- 

 ability of the various ions is proportional to the number of interspaces of a size of 

 proper fit. 



cate that, as might be expected, the ratios of ion partition coefficients vary widely 

 when the interspace size distribution is shifted, but the calculated potential is 

 constant as long as the values for Na + are low. 



Some revision of the model thus far developed is required in order to accom- 

 modate the following. If Ca++ is in competition with Na + for interspaces, we 

 may expect that in spite of its low concentration in Ringer solution or sea water, 

 it actually occupies a significant fraction of Na + interspaces and that further, 

 the mobility of Ca ++ in a membrane pore is much lower than that of Na + . In 

 addition, the assumption of equal mobility for all singly charged ions must be 

 examined, as well as the objections raised by Grundfest (1955) to the notion 

 that the resting potential is specified by the ratio of K+ or Cl _ across the mem- 

 brane. The difficulties stem from a demonstration that the alteration of internal 

 K+ or CI" concentrations does not alter the potential, that injected cations block 

 conduction in the order Ca ++ > > Na+ > K+, and that injected Ca++ depo- 

 larizes. Some of these objections can be met by assuming that the rate at which 

 K+ can be introduced into the membrane is subject to saturation at certain 

 levels of concentration, that Ca++ blocks Na+ channels, and that increases in 

 internal Na+ concentration are likely to result in much increased Na+ effluxes 

 during activity. 



In a recent paper, Hodgkin and Keynes (1955) have shown that some of the 



