C. A. G. WIERSMA 



Wedensky-like in type. It can involve different mechanisms as pointed out by 

 ThesleflF (1959) who has recently shown for the phenomena in striated muscle 

 that the endplate becomes desensitized to acetylcholine under these circum- 

 stances. Whereas this type of process may never resuh in true inhibition, such 

 actions might well stop spontaneously active cells. Included in the second case 

 would be instances where the location of the synaptic connection automatic- 

 ally provides for inhibition. Such endings might still be considered to belong 

 to an inhibitory unit, provided all those of such a unit were equal. The third 

 possibility represents field effects. About these I am strongly biased in disfavor, 

 since in my opinion they would tend to produce besides any useful signals, a 

 lot of noise. Furthermore I have seen no evidence yet that proves their natural 

 mode of action. For instance, the electroretinagram appears to be no hindrance 

 to very selective stimulation of different neurons. It may be claimed that this 

 is due to the suppression of unwanted stimulation by the inhibitory collaterals, 

 but if such be the case, the same may work at all places where fields of physio- 

 logical origin might upset orderly processes, and be one of its main functions. 

 As to the fourth possibihty, I want to discuss certain aspects of it later, but 

 point out here that the presence of polarizing potentials is not necessary. In 

 the crayfish muscle fibers inhibition by depolarizing potentials is just as 

 effective as by polarizing ones. Perhaps it would be better to state that the 

 membrane potential is maintained at a definite level at which firing is less 

 likely. It is also questionable if one can distinguish between synapses reacting 

 to a specific transmitter agent and ones with a specific reacting membrane, 

 because both possibihties may be of equal importance. Since acetylchohne 

 can both excite and inhibit as a transmitter agent, the membrane must play a 

 part, but another inhibitory transmitter may never have excitatory properties, 

 and thus be specific. 



So far the inhibitory peripheral fibers of the Crustacea have given the best 

 evidence and most complete picture of inhibitory fibers. From these fibers 

 and not from sensory or motor axons, a substance I can be extracted which 

 causes inhibition (Florey and Biederman, 1960). This means that now they 

 completely conform to what purely inhibitory elements should be. However, 

 in the arthropods as a whole, there are still some problems concerning these 

 fibers. Becht (1959) has obtained rather good evidence of the presence of 

 inhibitory fibers in the cockroach, but this only enhances the likelihood that 

 Hoyle's (1955) grasshopper fiber is of inhibitory origin. However, this latter 

 fiber does not inhibit, but conditions the muscle to greater contraction. From 

 the fact that substance I inhibits the stretch receptors, one may tentatively 

 conclude that the inhibitory fibers of these sense cells produce the same 

 inhibitory substance. In this connection I would like to mention that Mr. R. O. 

 Eckert (1960) believes that the functional significance of this sensory inhibition 

 may be hke that in Limulus eye. He observed that discharge rate of one stretch 

 receptor diminished when neighboring ones are activated. 



