284 



C. T. GAFFEY 



4 8 12 16 20 24 28 



Hours after irradiation for action potential toss 



Fig. 7. The logarithm of the dose of irradiation (910 Mev alpha particles and 455 

 Mev deuterons) is plotted against the survival time of neural excitability. High energy 

 particles given in doses near 300 krad promptly inhibit the action potential of frog 

 sciatic nerve. Above 100 krad each point on the diagram is the mean of 2 experiments; 

 below 100 krad each point is the average of 3 experiments. 



the time delay between 2 spike peaks on oscillograms and the distance be- 

 tween recording electrodes. Alterations from irradiation in conduction veloc- 

 ity, latent period, and stimulus strength do not appear strongly related to 

 suppression of the spike amplitude, because when the propagated impulse 

 was 90% abolished, conduction velocity was retarded by only 25 to 30% of 

 its original value, and the stimulus strength and latency period were changed 

 by approximately 25 and 20%. From recent studies employing 2 Grass stimu- 

 lators (Model S-4), it was found that the refractory period increases (after 

 a small transient decrease) before conduction velocity reduction, action po- 

 tential depression, latency period prolongation, and stimulus strength altera- 

 tions and is, thus, the earliest index of radiation damage that the author 

 has noted. 



Dose Rate Studies 



The influence of modifying the dose rate at which alpha particles were 

 administered to isolated sciatic nerve was investigated. The cyclotron's beam 

 was adjusted to deliver high energy particles at the rate of 0.5, 1.0, 2.0, 4.0, 

 and 8.0 krad per minute in 8 experiments. The survival of excitability was 

 found to be independent of the intensity at which irradiation was absorbed 



