J. C. ECCLES 341 



monosynaptic activation of niotoncuroncs, the monosynaptic action 

 exerted by the same afferent fibres on the cells of Clarke's column has also 

 been investigated (Mclntyre, 1953). There, too, synaptic depression was 

 observed, but it was less readily demonstrated, since that synaptic relay 

 has such a high safety factor. 



It has been concluded from these investigations on monosynaptic 

 action that disuse had reduced synaptic efficacy and also rendered synapses 

 more susceptible to the adjuvant effects of activity. In particular the very 

 prolonged potentiation of disused synapses provided a response that 

 appeared to be particularly relevant to the problem of the synaptic 

 mechanism of learning (Eccles and Mclntyre, 1953 ; Eccles, 1953 ; Mclntyre 



T953)- 



Comparable results have been obtained using intracellular recording 

 of the excitatory postsynaptic potentials induced monosynaptically in 

 motoneurones (Eccles, Krnjcvic and Miledi, 1959). Total synaptic disuse 

 was effected by severing the nerve to one muscle of a synergic group, the 

 other one or more nerves serving to give control synaptic activation. 

 This procedure also introduced the complications of chromatolysis of the 

 dorsal root ganglion cells and shrinkage of the afferent fibres, though these 

 effects would be much less than when the afferent fibres were severed just 

 distal to the dorsal root ganglion cells. There was as well chromatolysis of 

 those motoneurones with severed axons. However, there was the advan- 

 tage of allowing a comparison to be made between the mtracellularly 

 recorded potentials (the excitatory postsynaptic potentials) evoked mono- 

 synaptically ill the same motoneurone by normal afferent pathways and 

 those changed by operation. No additional complication was introduced 

 by the chromatolysis of motoneurones, for the responses of chromatolysed 

 motoneurones to monosynaptic stimulation via normal and severed 

 afferent pathways exhibited the same ditiercnces as were found with 

 normal motoneurones. It was found that, relative to the control afferent 

 path, the monosynaptic excitatory action of the disused path to the same 

 motoneurone was reduced to about half after 2-4 weeks of the total 

 inactivity. Furthermore, it was increased relatively much more by post- 

 tetanic potentiation, so that the normal size was largely regained during 

 the maximum potentiation that occurred a few seconds after an intense 

 conditioning activation (Fig. 3). Thereafter, the synaptic potential 

 progressively declined, but often a small residual potentiation occurred for 

 as long as 20 minutes, while, after application of a similar conditioning 

 stimulation to a control nerve, the post-tetanic potentiation usually passed 

 over into a prolonged phase of depression (Fig. 4). 



