NERVOUS ACTION 



theory," surrounded by a polarized membrane. The polarized state 

 of the membrane is due to a selective permeability to potassium ions 

 which are many times more concentrated inside the axon than outside. 

 During the passage of the impulse the resistance of the membrane is 

 decreased. From experiments on the giant axon of the squid. Cole 

 and Curtis calculated that the resistance falls from 1000 ohms to about 

 25 ohms per sq. cm. (3). The permeability of the membrane to all 

 ions is increased and a depolarization occurs. This change in per- 

 meability may well be produced by the rapid appearance and re- 

 moval of acetylcholine (11). The depolarized point becomes negative 

 to the adjacent region and a flow of current results which stimulates 

 the next following point. There again acetylcholine is released and 

 the whole process repeated. The impulse is thus propagated along the 

 axon. At the nerve ending the surface is increased and the resistance 

 therefore decreased. This leads to a greater flow of current which 

 enables the impulse to cross the gap. Whereas, in earlier theories, 

 acetylcholine was considered as a "neurohumoral" or "synaptic" 

 transmitter, i. e., a substance released from the nerve ending and 

 acting directly on a second neuron, in the new concept the transmitting 

 agent is always the electric current, the action potential, but the cur- 

 rent is generated by acetylcholine. 



The picture is consistent with the idea of the propagation of the 

 nerve impulse as developed by Keith Lucas and Adrian. It becomes 

 unnecessary to assume that the transmission along the axon diff"ers 

 fundamentally from that across the synapse. The assumption of a 

 special mechanism at the synapse diflferent from that m the axon was 

 the chief difficulty which had to be overcome for conciliating the 

 original theory with the conclusions of the electrophysiologists. This 

 appeared necessary for any satisfactory answer to the problem. For, if 

 it is true that physical methods alone are unable to explain the mecha- 

 nism in a living cell, it is equally true that conclusions based on 

 chemical methods should not be in contradiction to those obtained 

 with physical methods, in view of the much higher sensitivity of the 

 latter. 



The facts on which the new concept is based have been recently 

 reviewed and discussed (11). Some essential features are based on 

 studies of choline esterase. A few examples may be given in order to 

 illustrate the new approach. 



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