INPflBITORY NEURONS 3 



tion, had the great attraction that it offered a simple solution, in which only 

 one type of transmission was necessary. In more or less altered forms it 

 became the basis of a considerable number of inhibitory theories, most of 

 which were of electrical nature, and represented the majority viewpoint for 

 several decades from 1900 on. These will be reviewed in this volume by Lloyd. 



It is of considerable interest to note that the known examples of specific 

 inhibitory axons came under close scrutiny at this time. Wedensky (1891, 

 1892) himself was obviously convinced that the vagus inhibition of the heart 

 was similar in nature to that in peripheral muscle, and expressed the conviction 

 that he would be able to show this. I have not found that he really published 

 such experiments. But for the inhibitor of the crayfish, Frolich (1907), who 

 was one of the first to explain central inhibition in these terms, wrote a long 

 paper, claiming to show that it was all done by excitatory impulses. Inhibition 

 would be due to fatigue, weak stimuli fatiguing the closer muscle and strong 

 ones the opener. This was a rather feeble attempt all around, though in certain 

 aspects it resembles modern viewpoints. Sherrington (1906) on his part, 

 discussed the possibility that the motor axons of the one muscle might inhibit 

 the other by specific inhibitory endings. 



The fact that Hoffmann (1914) later showed rather convincingly that the 

 peripheral inhibition in the crayfish did depend on specific nerve fibers had 

 little general influence, since so many were already otherwise committed. It 

 also took more than the later proof (Marmont and Wiersma, 1938) that 

 stimulation of these fibers gives, under all physiological circumstances, 

 nothing but inhibition, to revive to the slightest extent the interest in central 

 inhibitory fibers. This did not come about until the discovery of Renshaw 

 (1941) of what are now the cells named after him and especially of course the 

 investigations of Eccles (1957) and his school, which go a long way to show 

 that these structures are purely inhibitory. These developments bring us to 

 present times, in which additional facts have come to light which support the 

 existence of purely inhibitory neurons, as well as a number of other findings, 

 which show that certain inhibitory phenomena are based on different processes. 



Bullock, in an as yet unpubhshed work, has enumerated four processes by 

 which inhibition of a neuron can occur, based on the results of experiments 

 with intracellular electrodes. These he enumerates as follows: "(a) by some 

 degree of refractoriness following a suprathreshold excitatory process; (b) 

 following a subthreshold excitatory postsynaptic potential; (c) inliibition 

 from weak electrical fields acting to reduce ongoing activity already present 

 or the likelihood of its beginning; (d) from specific influx which causes an 

 active synaptic response in the hyperpolarizing direction, the socalled direct 

 inhibition, regarded by some authors as due to the specificity of the trans- 

 mitter agent, by others as due to specificity of the responding patch of synaptic 

 membrane." 



Of these possibilities which do not necessarily exhaust all, the first one is 



