44 JOHN SZENTAGOTHAI 



6. CONCLUSIONS 



On the whole, histological evidence tends to show: (i) that there are no direct 

 inhibitory terminations of sensory neurons reaching motoneurons; (ii) that 

 at least the presence of one additional neuron in the inhibitory pathway has 

 to be assumed in simple reflex arcs;* (iii) that there exists a distinct type of 

 specific inhibitory neurons in the vicinity of the motoneurons, with specific 

 metabolic properties and most probably chemical transmitter activity; and 

 finally (iv) that at least in motoneurons inhibition is not mediated by synapses 

 localized on the axon hiUock or the initial segment of the axon. 



Unfortunately it has not been possible to identify the inhibitory synapses 

 histologically with certainty. Whenever a pathway yielding monosynaptic 

 excitatory activation is transected, some histologically known type of synapse 

 (in motoneurons the well known boutons terminaux) is found to degenerate 

 after a time interval necessary for secondary degeneration. This is not so in 

 the case of inhibitory pathways. Their destruction does not produce degenera- 

 tion on the inhibited neuron itself, but generally brings about abundant signs 

 of degeneration of well defined synapses on a group of neurons in the neigh- 

 bourhood of the cells upon which inhibition is exerted. Thus the histological 

 problem boils down to the detection of synapses between the short specific 

 inhibitory and the motoneurons (or other neurons upon which inhibition is 

 exerted). One should think that this is easy with the aid of Golgi methods. 

 Unfortunately neither in the literature nor in own preparations, especially of 

 the oculomotor system, have such connections been visuahzed. One must, 

 however, realize that neurites of these smaller types of neurons are not easily 

 stained with Golgi methods and since the problem has arisen only recently, 

 occasionally stained connexions of this type may have passed unnoticed by 

 earher authors. 



However hard we tried to identify these connexions with the aid of experi- 

 mental histological methods, we found nothing, but extremely fine fibres 

 forming a sparse meshwork between the inhibited neurons. These fibres may 

 be in fairly close contact with the cell surfaces, and their branches must be 

 surely beyond the power of the light microscope. Since almost in any electron 

 microscopical preparation of central nervous organs one can see a wealth of 

 extremely fine elements, which according to size are submicroscopic and from 

 structural properties can be recognized as preterminal and terminal nerve 

 branches, it would be easy to imagine the inhibitory synapses to be submicro- 

 scopic. It is a question for physiologists to decide whether, considering the 



* We have been pointing out repeatedly (Szentagothai, 1952, 1958) the theoretical diffi- 

 culties which would arise if we were to assume reflex connexions with the same afferent 

 or intermediate neuron giving off excitatory to one and inhibitory synapses to the other 

 (antagonist) muscle group. In this case it would be impossible to let muscles act in 

 different combinations, e.g. muscles A, B act against C D in one and A, D against B, C in 

 another combination, etc., as it is very common in labyrinthine eye reflexes. 



