PART TWO 



VI 



THE DYNAMICS OF THE SINGLE SYNAPSE: 

 TWO NEURONS 



Thus far we have been concerned with the formal development 

 of methods for determining the activity of structures composed of 

 neurons. We shall now attempt to make application of the theory 

 and method to experimental problems. Two paths are open to us. We 

 could, on the one hand, examine specific neural structures, seeking' 

 to determine for each the response which it mediates as a function of 

 the stimulus, or we might start with this function and attempt to 

 construct a suitable mechanism. In this and in succeeding chapters 

 we follow the first course. In the final chapters of this Part II we 

 follow the second course. The immediate problem is considered solved 

 if, from the theoretical structure, quantitative relations are derived 

 which agree with the experimental data within a suitable margin of 

 error for some range of the variables in question, and if the number 

 of parameters is not too large. Now many of the parameters may be 

 explicit functions of certain other variables which have been kept 

 constant throughout the experiment. Thus in many cases, the struc- 

 ture will have different properties when the constants of the experi- 

 mental situation are changed. In these cases we may say that the 

 structure studied makes predictions regarding activity outside the 

 domain of activity intended to be covered. Such predictions may sug- 

 gest an experimental approach not otherwise evident. If the predic- 

 tions are borne out, the theory is immediately extended in its applica- 

 tion. If not, the structure must be extended or revised in such a man- 

 ner as to include the old as well as the new properties. 



Thus on whichever course we set out, whether working from 

 mechanism to behavior, or from behavior to mechanism, we are led 

 finally into the second course when extensions are required. In this 

 we are guided by a consideration of the elements without which there 

 could be no correspondence between the activity of the structure and 

 the activity observed. If certain of the observed elements interact, 

 then the elements of the structure must be inter-connected. If the 

 action is unilateral in the experimental situation, a unilateral con- 

 nection may suffice. If the observed activity depends on the order of 

 the events of the past, the structure must contain elements which ex- 

 hibit this property of hysteresis. Thus one is limited to a consider- 

 able extent in the choice of mechanisms to be studied. 



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