NERVOUS ACTION 



The concej)! of a cluMiiical mediator released ii die nerve ending 

 and acting directly on the seeond neuron thus appeared in many re- 

 spects to be unsatisfactory. 



General Approach 



Two features of nervous action are essential to an understanding 

 of the problems and difficulties involved, viz., the high speed of the 

 propagation of the impulse and the infinitely small energy required. 

 In meduUated mammalian nerve, the impulse travels at a rate of one 

 himdred meters per second and the energy required per impulse per 

 gram nerve is less than one-tenth of a millionth of a small calorie. 

 The recording of such an event offered many difficulties, even with the 

 use of physical methods. A really adequate electrical recording instru- 

 ment became available only with the introduction of the cathode-ray 

 oscillograph by Gasser and Erlanger. Still more difficult was the 

 detection of the energy involved. It is not surprising that Helmholtz, 

 who first demonstrated heat production in muscle, failed to demon- 

 strate it in nerves. Even A. V. Hill was unable to detect any heat 

 production in nerve for a long time, and only when he and his associates 

 developed thermoelectric methods of an amazingly high degree of 

 perfection did it become possible to measure amounts of heat of as 

 small an order of magnitude as are produced by nerve activity. 



If even physical methods encountered so many obstacles, it is 

 obvious that the study of the chemical reactions connected with an 

 event of this kind must ofTer serious difficulties. No adequate methods 

 are available for determining directly chemical compounds appearing 

 in such infinitely small amounts and for such short periods. But the 

 development of biochemistry, especially during the last twenty years 

 has shown that, in such cases, much information may be obtained by 

 the study of biocatalysts. Nearly all chemical reactions in the living 

 cell are catalyzed by enzymes. Since Buchner's demonstration, in 

 1897, that fermentation may occur in cell-free extracts, a great number 

 of enzymes has been isolated, and the chemical reactions of living 

 cells have been studied in vitro. While many cell constituents and 

 intermediates, especially those closely connected with cell activities, 

 are extremely unstable and occur in concentrations too low for chemical 

 analysis, a great number of enzymes are relatively stable and, under 



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