NERVE ACTIVITY 



knowledge derived from physical measurements, especially with 

 that of ion movements. Our knowledge is, however, still very 

 limited. The existence of the receptor has been experimentally 

 demonstrated, but the receptor has not yet been isolated. The 

 difference of its reactions towards certain tertiary and quater- 

 nary nitrogen derivatives is one very special feature and has 

 been known from pharmacological experiments for a long time. 

 On the basis of protein studies in vitro an interpretation has been 

 proposed and by a fortunate coincidence of several factors ex- 

 perimental support has been obtained. Otherwise, however, 

 our knowledge of the receptor is extremely scanty: the evidence 

 appears strong that the active site is different from that of ace- 

 tylcholinesterase. But is it a different protein? There may be 

 even two different receptor proteins as in actomyosin. The 

 number of actual sites is unknown, as is the reactivity with all 

 kinds of compounds and physicochemical factors on which the 

 reaction of a protein depends. In the case of actomyosin in- 

 tensive investigations over more than a decade have revealed a 

 great number of important properties, and yet even there the 

 interaction between ATP and actomvosin is still full of unsolved 

 problems (32). 



In the case of the acetylcholine system the difficulties are 

 incomparably greater. We do not have something comparable 

 to the model, i.e., the glycerinated fiber, in which all the com- 

 pounds of low molecular weight have been removed and many 

 other complicating elements have been eliminated. In the case 

 of nerve impulse conduction we do not yet even know where 

 the so-called active membrane is located. We know today that 

 there is a barrier which suddenly changes its properties as to ion 

 permeability and that this change is responsible for the genera- 

 tion of the electric potentials. But the greatest obstacle to an 

 understanding of the reactions of the acetylcholine system is the 

 complete lack of knowledge of the structural peculiarities of this 

 barrier. The great complexity of cellular structures has long 

 been known (10). Electron microscope studies have thrown 

 new light upon the intricacies of ultrastructure, which appears 



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