340 



BRAIN MECHANISMS AND LEARNING 



Further conditiouiiig tctani led to additional, but smaller, residual poten- 

 tiations, showing that a cumulative process was involved (Fig. 2). The 

 gradual changes which unavoidably occur in the reflex excitability of a 

 preparation (attributable for example to changes in anaesthesia and in 



20 6|30/ 



Jl^ 





4.0 



.ilg 



M 



■f. 



>i50 



Fu;. 2 

 Plotting of post-tetanic potentiation and residual potential as in Fig. iB, but on a much reduced 

 time-scale, note time-scale in hours and minutes on abscissae. Ordinates give sizes of mono- 

 synaptic reflex spike in millivolts. L- and Sj dorsal roots severed extraganglionically 38 days 

 previously. The open circles about 5.50 p.m. give reflex spikes in biceps-semitendinosus nerve 

 in response to single volleys in the combined L- and Sj dorsal roots, and the filled circle the 

 mean control spike in gastrocnemius nerve, these same conventions being used throughout. 

 The first tetanic conditioning (7500 volleys at 500 per second) is shown by vertical line at 

 5.55 p.m., and the five other tetanic conditionings (of similar severity) are likewise indicated. 

 The post-tetanic potentiation curves are shown for gastrocnemius and biceps-semitendinosus 

 after first and second conditioning respectively in each of the test series. As indicated by 

 abscissae the vertical shaded columns mark lapses of 2 hours, 20 minutes and 55 minutes 

 respectively, and during the second an injection of nembutal was given (Eccles and Mclntyrc, 

 1953, with permission o{ x\\z Journal of Physiology). 



circulation) prevented any precise determination ot the time constant of 

 decay of residual potentiation, but probably half decay takes at least 

 3 hours, which would make it about i6o times slower than the decay rate 

 for the normal post-tetanic potentiation after a similar conditioning 

 tetanus. 



In addition to this investigation on the effect of dorsal root section on the 



