EFFECTS ON BIOELECTRIC ACTIVITY OF NERVES 593 



Discussion 



The enhancement of activity of nerves during irradiation is not due to 

 the additional energy supplied by radiation at the time the nerve responds 

 to stimulation. The possibility of such synergistic action between the energy 

 supplied by radiation and the nerve fiber itself can be ruled out by dis- 

 continuing irradiation when the nerve is responding in an enhanced manner; 

 the nerve continues to respond in an enhanced manner even without con- 

 comitant irradiation. There is^ moreover, the phenomenon of continuation 

 of enhancement even after the cessation of irradiation (Figs. 5 and 16). 



The experiments summarized in this paper were not designed to test the 

 theory of excitation proposed by Hodgkin and Huxley, but the results can 

 be shown to be compatible with the theory. According to this theory, an 

 increase in spike amplitude is the result of an increase in the flow of sodium 

 into the cell. Since, in the present experiments, the external concentration 

 of sodium was kept constant, it appears that irradiation altered the prop- 

 erties of the biologic system in such a way that the membrane became more 

 selectively permeable to sodium ions, the result being an increase in ampli- 

 tude of the action potential. If, according to the theory, the propagation 

 of the impulse depends on sufficient amplitude of the action potential, then 

 the increase in sensitivity due to irradiation would also result from the same 

 increased permeability to sodium. The rate of rise of the action potential 

 has also been shown, according to the ionic theory of excitation, to be related 

 to the magnitude of the action potential; this relationship was also observed 

 as a result of irradiation. Since the velocity of propagation is a function of 

 the resistance of the outside and the inside media of the fiber, as well as the 

 amplitude of the action potential, the situation is somewhat more complex 

 when one deals with the effect of radiation on conduction velocity. It ap- 

 pears that radiation affects the conduction velocity chiefly by altering the 

 resistance of the internal media of the fiber. 



The above speculations on sodium permeability appear to be substantiated 

 by the work of Rothenberg (1950), who showed that x-irradiation of squid 

 axons produced increased permeability to sodium. On the other hand, 

 Mullins (1939) concluded that beta irradiation of Nitella reduced the rate 

 of sodium transport across the cell membrane. The apparent inconsistency 

 can be reconciled, perhaps, if one recognizes that Rothenberg (1950) gave 

 fairly low doses which allowed the normal excitatory processes to take place. 

 Under these circumstances, he found an increase in permeability. These 

 results would correspond to the period of enhancement observed in the 

 present experiments. Mullins (1939) recorded the rate of sodium transport 

 in cells damaged by irradiation, and his results would correspond to the fall 

 in response reported in the present experiments. 



