332 BRAIN MECHANISMS AND LEARNING 



SUMMARY 



The purpose of this chapter is to insist upon the frequent presence, in 

 many species and centres, of interneuronal contacts that (i) are as close as 

 classically accepted synapses and (ii) exhibit the same variable submicro- 

 scopic structure. These contacts are dciidro-dcudritic, dendro-somatk, soinato- 

 sonmtic and axo-axoiiic. No valid morphological reason enables us to deny 

 that these contacts are operant and, at the same time, to accept that 

 regular axo-somatic or axo-dendritic contacts arc functional. Two 

 formulae sum up all the facts: 



I. Pre- and post-synaptic pars of a diftcrent nature arc tunctitMially 

 polarization synapses; 



II. Pre- and post-synaptic pars of a similar nature arc reversibility 

 synapses, which means functional alternance between the receptor and the 

 effector pars. 



No submicroscopic entity (chondrioma, microvesicules, etc.) is typical 

 of a functional contact. Only discontinuity and contiguity are always 

 present. 



GROUP DISCUSSION 



EccLES. I am very interested. Professor Establc, in all that wealth oi histological 

 material that you have just shown us; it gives rise to a great many problems of a 

 functional kind. There are certain physiological reactions which we have not yet 

 fully understood and I think that some of these new kinds oi synaptic contacts will 

 be important in trying to develop explanations. I refer particularly to the dorsal 

 root potential, where depolarization of the presynaptic fibres is conducted elec- 

 tronically out along the dorsal roots which may also carry the Toennies reflex 

 discharge. The nervous system does not always work in the forward running 

 direction, it can work backwards too and these synaptic contacts which )ou have 

 mentioned give us possibilities of making explanations of such phenomena. But 

 the point I v^ant to make in a general way is that the clectron-microscopists are 

 showing us what a highly specific and very intimate structural relationship there is 

 across the synaptic junction; yet I still feel that the essential criterion of a synapse is: 

 does it work functionally? I believe that even a contact with 200 A separation over 

 a large area is insufficient to give any functional meaning or pertormance. There 

 must be a process of specific chemical secretion on the one hand and specific 

 chemical receptivity on the other, linking across that junction to give a functional 

 performance to a s\-napse. I wonder whether we should define a synapse as a 

 structure with a close histological relationship ; or whether we should try to restrict 

 the word to close relationships that do work functionally as far as the generation or 

 inhibition of impulses is concerned. 



EsTABLE. Morphologically one must accept contiguity plus discontinuity as the 

 basic criteria. Thereafter, wc must be carcKi! in discarding as non-functional 



