268 BIOPHYSICAL STUDIES ON NERVE AND MUSCLE 



When the membrane is like that of nerve (Figure 10-3) — partially perme- 

 able to several ions — the potential across it can be related to the permeabil- 

 ity constants, (P (Chapter 8). The deduction gives 



RT 



E = -= ln (Z ®r cr/H (?r c J 



where the summations are of the products of permeability ((P) and concen- 

 tration (c) for all the ions (z), and the superscripts refer to outside (out) and 

 inside (in) the cell. In other words the permeability constants express as a 

 number the contributions which the different ions make to the potential dif- 

 ference across the membrane. Thus a membrane which is selective can pass 

 one ion more quickly than another, so that the (P's are not equal. In the case 

 of resting nerve, (? K+ » (P Na+ or (P CI _ . The rapid potential changes which 

 occur while the impulse is passing by are now generally believed to result 

 from rapid changes in the permeabilities. The reader is invited to follow the 

 fascinating efforts of our contemporaries, Ussing, Teorell, Sollner, Schldgl, 

 and other membrane researchers. 



outside position! outside 



! of 



ACE _ ACE impulse ^ 



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protein layers. 



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1 



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inside inside 



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l oJ^r^ 



Na +| - 



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RESTING (p NQ+ low) ACTION (/J Na+ high) 



Figure 10-3. Illustration of Construction and Ionic Penetration (;u's) of the Nerve Mem- 

 brane. During rest the mobilities of Na + and K + are low, but during action they be- 

 come momentarily very high. (Refer back to Fig. 6-8.) 



Sodium deficiency in the external electrolyte slows down the speed of con- 

 duction. Further, it leads to a spike height which decreases as the impulse 

 passes down the nerve — "decremental conduction," it is called. Sodium is 

 pumped out of the axon through the membrane by a yet unknown mecha- 

 nism, and this requires energy. It has been found that, during treatment 

 with metabolic inhibitors, adenosine triphosphate (ATP), the mobile power 

 supply, disappears at about the same rate at which the sodium pump slows 

 down and stops. It is therefore inferred that reactions involving the hy- 



