24 : 4/ The Molecular Basis of Nerve Conduction 451 



difference between the membrane potential V and the Donnan potential 

 F Na , computed using Equation 3 for Na + ions only. In symbolic form, 

 this is written 



where i? Na is the membrane areal resistance for a sodium-ion current 

 density, J Na . The resistance was shown to be a function of V only and 

 to be independent of F Na by varying the external Na + concentration. 

 Actually, Hodgkin and Huxley computed and used the areal con- 

 ductance, 



The conductance is slightly easier to discuss because it varies directly 

 as the permeability of the axon membrane. The conductance g Na is a 

 function both of the membrane potential V and of time; hence, the 

 permeability to sodium must depend on these variables also. 



Having found the part of the current associated with the Na con- 

 ductance, one may subtract to find the K conductance. Qualitatively, 

 it is clear that the K conductance increases with decreasing polarization, 

 but the change occurs very slowly at first as compared with the Na 

 conductance. 



These experiments may be made more complex by changing the 

 polarization first to one value V 1} and then after a few milliseconds t to a 

 second value F 2 . By observing the effects of different values of V\, V 2 , 

 and t, it was possible to confirm that the potassium-ion currents obeyed 

 the relationship 



J K = gK(V-V K ) 



where V K was at about 10 mv above the resting potential. In addition, 

 a small leakage current also existed given by 



The terms g L and F L were treated as constant, whereas V K depended 

 (in theory) on the external potassium concentration, and g K on both V 

 and time. 



Thus, the electronic arrangement diagrammed in Figure 6 proved 

 useful in studying the magnitude and time dependence of the currents 

 associated with Na + and K + in a nerve membrane held clamped at a 

 fixed potential. Qualitatively, these may be summarized by three 

 events which follow a decrease in the membrane polarization. First, 

 the membrane permeability to Na + increases markedly, although after 

 a finite time delay. Second, the membrane permeability to K + 

 increases. And finally, the membrane permeability to Na + decreases, 



