37^ BRAIN MECHANISMS AND LEARNING 



Fcssard has found when I was attempting to give some picture of the neuronal 

 mechanism of learning, at least at the simplest level. 



Fessard: I remember your diagrams. 



Gerard : I hope this will be elaborated in the general discussion on Saturday. I 

 remind you that in Lloyd's experiments with the monosynpatic reflex each 

 arterent did not influence the response induced by any other afferent and Dr 

 Fessard's results seem to be in conflict with that unless you take into account the 

 existence of interneurones. 



KoNORSKi: Post-tetanic potentiation (P.T.P.) was first discovered many years 

 ago in neuromuscular preparations, hi curarized preparations the P.T.P. is very 

 strong and long lasting. We did some of these investigations with Lubinska and 

 found that the P.T.P. lasted for minutes. Do you think this is a different pheno- 

 menon from the P.T.P. found in C.N.S.? 



Fessard: I would say it is just the same. 



KoNORSKi : Dr Kozak and Dr Bruner have shown that the P.T.P. in neurosalivary 

 preparations (where the effector is a salivary gland) is even more prolonged — from 

 10 minutes to half an hour. So we see that the P.T.P. phenomena are general and 

 apply to the C.N.S. as well as to the peripheral nervous system. 



Fessard: As Dr Eccles just recalled, motoneuiones may be taken as models of 

 any others, but there may be strong quantitative diff^erences. 



Magoun: Is there any value in considering the alterations in the sensitization of 

 enervation in this question. The increment of excitability in this case occurs at the 

 receptor sites on the post-synaptic membrane rather than on the pre-synaptic 

 membrane. As long as one is looking for all possible instances in which there is an 

 enhancement of excitability persisting for a long time, I wonder if there would be 

 any concepts to be applied from the field* of sensitization of denervation. 



Eccles. There are important facts: for example, Axelsson and Thesleft have 

 shown that when you cut the nerve to a mammalian muscle and it degenerates, the 

 well-known hypersensitivity to acetyl-choline in tact is not a real hypersensitivity 

 of the cndplate region, but simply a spreading sensitivity at the same level through- 

 out the whole length of the muscle fibre which effectively becomes a long, 

 extended cndplate. This has been tested by micro-injection techniques. 



Magoun: What about the influences in the C.N.S. such as have been explored 

 by Stan? 



Gerard: Dr Luco commented on that. Would you like to add anything? 



Luco: Professor Canon did some experiments on sensitization. If you cut the 

 nerve to the motor nerve cell, the sensitization is increased. I want to add one 

 interesting thing. We cut the post-ganglionic fibres of the superior cervical gang- 

 lion which innervates the nictating membrane. We re-innervated the membrane 

 with cholinergic fibres of the hypoglossal nerve, and the sensitivity to adrenaline 

 was normalized after re-innervation with a very different nerve. In other words, the 

 normal sensitivity which had been lost was regained with a different nerve wluch 

 normally is not innervating the membrane. 



Gerard : Let us not forget the plasticity or enduring effects of activity even at a 

 simpler level than the synapse, in the nerve fibre itself After brief activity there is a 

 change in the magnitude and duration of the after-potentials which increases from 

 milliseconds to minutes, and the heat or oxygen consumption increases for hours. 



