VOL. 4 (1950) PERMEABILITY AND NERVE FUNCTION, I 79 



erorterten Eigenschaften der halbdurchlassigen Membranen ihre Erklarung linden 

 werden". From the discussions of Du Bois-Reymond, Hermann, Ostwald and others 

 concerning the mechanism underlying the generation of the electric currents during 

 nerve activity there finally emerged the membrane theory formulated by Bernstein 

 early in this century^. This theory forms the basis of all modern concepts of conduction 

 and has been an extremely useful working hypothesis. Essentially the theory assumes 

 that the nerve fibre in resting condition is surrounded by a polarized membrane, selec- 

 tively permeable to potassium ions. The concentration of these ions inside the nerve 

 fibre is high compared with that outside. There is, therefore, a tendency for the potassium 

 ions to move to the outside, but they are kept back by the negative ion for which the 

 membrane is impervious at rest. Thsre thus develops a positive charge on the outside 

 surface of the membrane and a negative cha-ge on the inside. When a stimulus reaches 

 the surface, a breakdown of resistance occurs ; the permeability for the negative ion is 

 increased, resulting in a depolarization. The depolarized point of the membrane is 

 negative to the adjacent region; whereby a small electric current, the "Stromchen" of 

 Hermann, is generated. This current in its turn stimulates the adjacent region, leading 

 there to a depolarization. The same process is repeated in successive parts of the nerve 

 fibre and in this way the impulse is propagated along the axon. 



Recent developments have made necessary a modification of the membrane theory 

 in its original form. It has been shown by Curtis and Cole'^ and by Hodgkin and 

 Huxley^ that during the passage of the impulse there occurs not only a depolarization 

 but an actual reverse of the charge. This result was obtained in experiments on the giant 

 axon of Squid by the introduction of an electrode into the interior of the axon and by 

 direct determination of the potential across the membrane. The spike potential was 

 found to be markedly greater than the potential difference in rest, in some cases it was 

 nearly twice as great. There are some technical difficulties which make the exactness of 

 the absolute values uncertain, but the fact that the charge is reversed during activity 

 appears to be unquestionable and well established. It follows that the assumption of 

 a simple depolarization cannot be maintained. The process responsible for the gsneration 

 of the flow of current is complex and is not merely an abolition of the resting potential. 



The availability of radioactive ions made possible the study of the movement of 

 ions across the neuronal surface membrane. Such investigations were initiated during 

 the last two years by Hodgkin and Huxley^ and Keynes^" in England and by Rothen- 

 berg in this laboratory^^. The results will be fully discussed in the following paper. 

 They show that sodium and potassium ions are being constantly exchanged, the latter 

 at least to some degree between the inside of the axon and its outer environment. The 

 ionic equilibrium is a dynamic and not a static condition. The conclusion is similar 

 to that encountered in many other fields where radioactive or stable isotopes were used 

 (Schoenheimer12) 



During activity the outflow of potassium and the influx of sodium are greatly 

 increased. The data of the two laboratories are in good agreement and supplement each 

 other. According to the Cambridge group about 2 • lo"^^ mole of potassium leaks per cm^ 

 surface per impulse; Rothenberg's experiments indicate that the influx of sodium is 

 about 4-io~i2 mole per cm^ per impulse. The question how this movement of the two 

 species of ions in opposite direction may account for the reverse of the cha-ge is still 

 open. No satisfactory hypothesis has been advanced so far. It is obvious, however, that 

 events must take place in the active membrane, the site of the electrical manifestations. 

 References p. 93195- 



