ACTION POTENTIAL CHANGE BY BETA IRRADIATION 



601 



paper where the differential sensiti\ity was not discussed. Basically the sur- 

 vival curves are sigmoid — the radiation having little or no effect in the 

 early stage, followed by a fast fall as most of the activity is suppressed, and 

 ending in a more gradual approach to zero activity. These experiments repre- 

 sent the mean results determined from all experiments. The gamma curve of 

 Fig. 1 is an exception, since the gamma fiber usually showed a sensitivity 

 lying between beta and delta fibers. The gamma fibers were the most difficult 

 of the A group to assay and showed a large variability which is e\ident in the 

 summary of all experiments (Table I). 



In the experiment on cat 545 (Fig. 1). a 0.1 mm brass foil filtered the 

 strong beta source. Although such a filter was seldom used, this experiment 

 is particularly interesting, since two components of the delta potential are 

 seen, and a second component of the beta potential is revealed after 16 

 minutes of irradiation. The ''walking out" of such a hidden component was 

 frequently seen, an effect which is difficult to explain, since each potential 

 component results from a popidation of fibers. 



A check lor C fibers is illustrated in Fig. 2 at the 16 minute recording. 

 At this time the stimulus strength was increased, and the oscilloscope time 



100 



50 



SLOW 



CAT 584 R VAGUS 

 147 MC T 26.3»C 



MIN 



20 



1000 



2000 

 MC MIN 



3000 



X'S PRESENT---- 

 16 



Fig. 2. Response of vagus nerve activity to a 147 mc Rn source. Changes are 

 illustrated as in Fig. 1. The vagal potential was di\ided into two components: "A" 

 group, which was of high amplitude and velocity, and "slow"" group, which is made 

 up of a mixture of B and delta fibers. This latter group was more radiosensitive. 

 After 16 min of irradiation (bottom of figure) the stimulus strength was increased 

 to reveal many conducting C fibers. 



