INHIBITORY NEURONS: 



A SURVEY OF THE HISTORY OF THEIR DISCOVERY 

 AND OF THEIR OCCURRENCE 



C. A. G. WiERSMA 



California Institute of Technology, Division of Biology 



The presence of inhibitory piienomena in nervous activity has long been 

 recognized, and many hypotheses have been advanced regarding its nature. 

 Among these, the simplest, namely that central inhibition is the result of the 

 activity of specific inhibitory fibers, has received less attention than many 

 others. Tt is the purpose of this paper to consider the reasons why this came 

 about and also to ponder the feasibility of its revival. 



Although inhibition of one type or another appears as part and parcel of 

 many if not most coordinated actions of the nervous system, even of rather 

 primitive forms (see Horridge's paper, this volunie, p. 395), its treatment has 

 hardly been in accord with its importance. If this may seem an overstatement, 

 I may refer to the first two volumes on neurophysiology of the Handhook of 

 Physiology, in which mention of inhibition is made on a surprisingly small 

 number of pages in less than half of the chapters of the first volume, and does 

 not appear at all in the second. 



A major reason for this is undoubtedly the difficulty of the necessary 

 investigations, which always have to refer to excitation. In addition, there 

 are so many diff'erent ways in which inhibitory phenomena do or could come 

 about, that anybody asked to explain a given instance may well hesitate 

 even to start speculation on a solution. 



From the beginning the number of "inhibitory theories'' has been large, 

 involving all kinds of processes that on the basis of analogy could be opposed 

 to excitation. To mention a few: inhibition by wave interference and inhibi- 

 tion by anabolic processes. It is quite possible by some (smart) interpretation 

 to see in these the prototypes of modern ideas. But httle purpose is served by 

 such a procedure as often the meaning given to the terms used has been so 

 changed, that no comparison is warranted. The main conclusion one reaches 

 after studying these hypotheses is that they present very clever reasoning 

 indeed with the few 'Tacts" at hand; and the main lesson, that without 

 sufficient facts no real progress is possible. 



The history of inhibition by specific nerve fibers started when the brothers 



2 ' 1 



