CONDUCTION OF THE NERVE IMPULSE 



"9 



E^,,; this explains the reversal of the membrane poten- 

 tial during activity. If ^^a is increased to some extent 

 b\ a stimulating current pulse, a further increase in 

 gif., can be brought about by a regenerati\e process; 

 an increase in ^^a causes a rise in the memijrane 

 potential which in turn gives rise to a further increase 

 in gf^i,. The theory is self-consistent. The readers who 

 are interested in this beautiful scheme are referred to 

 the original article (59). 



It may be worth pointing out that there are in the 

 sodium theory a number of assumptions that are not 

 directly proved by experiments. They assume in the 

 first place that the axon membrane under voltage 

 clamp is spatially uniform; this may not be a safe 

 assumption. They assume also that the capacit\- of 

 the membrane is connected in parallel to the cmf of 

 the membrane (p. 85). They did not exclude the 

 possibility that the sodium ions bound in the sub- 

 stance of the membrane (instead of the free sodium 

 ions in the medium) exert direct influence upon the 

 amplitude of the action potential. There are several 

 more assumptions in the theory. Although most of 

 these assumptions appear to be reasonable, it is also 

 true that one can make a set of entirely difTerent 

 assumptions and explain almost the same amount of 

 experimental data. 



There is at present a large volume of work dealing 

 with the movement of sodium or potassium ions 

 across the excitable membrane. The principal findings 



pertinent to the discussion in this chapter are a) a 

 steady outward current through the axon membrane 

 is carried almost exclusively by potassium ions (60), 

 and b) there is an exchange of intracellular potassium 

 with extracellular sodium associated with repetitive 

 excitation of the axon (74). It is generally agreed that 

 the amount of the Na-K exchange associated with 

 repetitive excitation observed in invertebrate axons 

 is close to the \alue expected from the sodium theory. 

 It should Ije kept in mind in this connection that 

 there are excitable tissues which do not require any 

 sodium ion in the medium to produce action potentials. 

 Crustacean muscles studied by Fatt & Katz (31) 

 are a well-known example, and the plant cell, Nilella, 

 investigated by Osterhout and his associate (93, 94) 

 is another. This fact suggests that the role of the 

 sodium ion in the medium inight be only indirectly 

 connected with the process of action potential pro- 

 duction. The alternatis'e explanation of this fact is 

 that the mechanism of action potential production is 

 verv different in difTerent tissues. 



The author wishes to express his gratitude to the following 

 colleagues who have kindly read the manuscript of this chapter 

 and have given many important suggestions; Dr. M. Fuortes, 

 Dr. S. Hagiwara, Prof. A. L. Hodgkin and Dr. C. S. Spyro- 

 poulos. The manuscript was prepared with the valuable help 

 of Mrs. Mary Allen, Mrs. Claire Mayer and Mrs. Lydia N. 

 Tasaki, to whom the author also wants to express his apprecia- 

 tion. 



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