ANATOMICAL ASPECTS OF INHIBITORY 

 PATHWAYS AND SYNAPSES 



John Szentagothai 



Anatomy Department, University Medical School of Pecs, Hungary 



Anatomical information on inhibitory pathways and synapses in vertebrates 

 is scanty. Certain histological types of neurons (Golgi cells), axon collaterals, 

 and synapses on the axon hillock or around the initial segment of the axon 

 have been assumed to be concerned with inhibition. None of these assump- 

 tions, however, is based on direct experimental evidence. The development of 

 intracellular recording techniques has recently brought forth evidence that 

 inhibition is exerted at least on motor and some other types of spinal neurons 

 by specific and generally short inhibitory neurons. This concept offers us a 

 chance to investigate some of the better known inliibitory mechanisms in the 

 spinal cord and the brain stem with the methods of experimental neuro- 

 histology. 



This approach to the problem is based primarily on the secondary degenera- 

 tion of synapses, a method which can be exploited in two directions. Histologi- 

 cal signs of synaptic degeneration can be directly observed with the aid of 

 impregnation after destruction of nuclei or transection of pathways. Another 

 possibihty is to look for the synapses which remain intact after certain experi- 

 mental interference and sufficient time for complete disintegration and resorp- 

 tion of the nervous elements involved. Tliis procedure is especially rewarding 

 in the pursuit of short neuronal connexions, as assumed to be involved in 

 inhibitory mechanisms. For this purpose we have to destroy the majority of 

 the longer connexions or to isolate smaller parts of the CNS neuronally, 

 leaving the vascular supply intact. Any intact synapse found in such isolated 

 fragments about one or two months after isolation must arise from neurons 

 situated in the isolated part itself. We shall refer to this method briefly as 

 "method of persisting elements". 



Since the hypothesis of inhibitory action of synapses located on the axon 

 hillock (Gesell, 1940; Gesdl et ai, 1954; Retzlaff, 1954, 1957) could neither 

 be proved nor disproved, we resorted to some methods of experimental 

 embryology suited to produce by artificial recombination of tissues simplified 

 "nervous system models" completely lacking any organoid structure or 

 organized pattern of neuronal connexions. If inhibition were based on such 

 minute spatial and geometric relations between neurons, as required by this 



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