ACTION POTENTIAL CHANGE BY BETA IRRADIATION 599 



and empty bulbs for controls were placed in contact with the nerves. Because 

 of the great irradiation hazard, it was necessary to carefully shield the 

 chamber with lead and maintain a working distance of more than 5 feet 

 during most of the experiment. At no time did a film badge worn on the 

 wrist indicate an above-tolerance exp>osure of the experimenter. 



An analysis of the action potentials was made by projecting them onto 

 squared paper and measuring the amplitude or area of the potential com- 

 ponents as being proportional to their activity. Considerable uncertainty was 

 involved in determining the beginning and end of a potential component, but 

 criteria of choice were maintained throughout an experiment, and the 

 relative changes are a valid measure of the irradiation effect. 



Results 



Potential components were identified and labeled on the basis of their 

 relative conduction velocities and amplitudes. For the sural and saphenous 

 ner\es the beta and delta components were readily seen, while the gamma 

 component was far more variable, and the C fiber potentials could usually 

 be viewed only at stimulus intensities damaging to the beta fibers. To view 

 the G fiber potentials, the stimulus strength was increased at infrequent 

 intervals during an experiment, or else a spot check was made at the con- 

 clusion of an experiment to determine whether C activity was still present. 

 In a few cases, second beta and gamma fractions could be seen; the second 

 beta peak of the compound action potential usually appeared after the beta 

 peak had been depressed somewhat by irradiation. 



The components of the vagal potentials were even more difficult to sep- 

 arate and identify. The delta and B fiber potentials in all but three cases 

 appeared together and were called the "'slow'" group, while the fastest 

 component was called "A" group, after Middleton et al. (1950). 



Control recordings under these experimental conditions showed large vari- 

 ation in survival times and order of loss of potential components. However, 

 the control potentials always persisted much longer than the experimental, 

 and the components were lost in random order in contrast to the fast and 

 systematic loss of potential components conducted through the irradiated 

 segment. In some experiments, the shorter conduction distance made it dif- 

 ficult to identify the subpotentials in the control recordings. The three types 

 of controls described earlier were consistent in giving a picture of long term 

 viability for nonirradiated nerves. One must remember, however, that the 

 irradiation effects reported here are a summation of a fast response to the 

 beta radiations, plus a much slower deterioration which occurs without 

 irradiation. The latter effect was enhanced in the higher temperature 

 experiments. 



