ACUTE EFFECTS OF RADIATION ON SPINAL NEURONS 569 



From our preliminar)- experiments, it is difficult to discuss the ionizing 

 efTect of irradiation on the medium across the membrane. However, it can 

 be inferred that in mammals the myelinated nerve fibers or cell soma have 

 a great resistance to acute ionizing irradiation up to 14 kr. 



On the other hand, the fact that the extracellular response from the 

 afferent fibers and its intramedullary spike showed a slight decrease in ampli- 

 tude, while most intracellular responses from afferent fibers did not show any 

 significant change in amplitude, suggests that there might be some sensory 

 fibers which are more sensitive to irradiation and are damaged by doses as 

 low as 200 r. However, most myelinated fibers did not show an acute effect 

 (within 2 hours) from irradiation up to 10 kr. 



The increase in amplitude of the monosynaptic reflex immediately after 

 the initial 500 r irradiation, while both extracellular and intracellular re- 

 sponses from the motor neuron did not show a significant increase in ampli- 

 tude within 2 hours after initial irradiation, is difficult to explain. It suggests 

 that some interneuronal inhibitory passway might be primarily or secondarily 

 aflfected by irradiation and, accordingly, the number of motor neurons ex- 

 cited by an afferent volley would increase. 



Although the amplitude of the extracellular response from the motor 

 neurons showed an increase 2 hours after the initial irradiation, it is difficult 

 to interpret this as a primary effect of irradiation on the spinal neural ele- 

 ments. This finding might be due to secondary effects of irradiation, such as 

 damage to capillary blood vessels causing hypoxia or asph\-xia. This effect 

 might secondarily cause such a change in excitability of the neuronal net- 

 work around the motor neurons. 



Bakin (1946) and Lifshits (1956) concluded from different experiments 

 on the frog's spinal cord that the sensory nerve is much more easily affected 

 by irradiation than the motor nerve. Our results seem to support their con- 

 clusion, since ovir doses were of a similar range to theirs. 



As for the exceptional neural elements which might be relatively sensitive 

 to acute small doses of irradiation, and for the effect of higher doses of 

 irradiation, we must await further experiments. 



Summary 



Using decerebrated cats, the acute effect of ionizing radiation on the 

 neural elements of the spinal cord was investigated preliminarily. 



Monosynaptic reflex showed a considerable increase in amplitude immedi- 

 ately after the initial irradiation with 500 r, while the intramedullar\^ spike 

 activity of sensor)- fibers ascending dorsal column decreased gradually in 

 amplitude with successive irradiation. Njb wave (interneuronal activity) did 

 not show a sienificant change in amplitude. 



