302 N. I. GRASHCHENKOV 



tion may not occur. Minaev (1957), for example, observed no phase during 

 which the excitability of the brain increased when he exposed the head of a 

 dog to a dose of 7,000 r. 



Of great practical interest is the research which has been done on the 

 influence of small doses of ionizing radiation on the conditioned reflex 

 activity of the brain. More and more evidence is accumulating to indicate 

 that small doses of 5 to 10 to 20 r lead to changes in the higher nervous 

 activity, mainly to disturbance of internal inhibition, disinhibition of the 

 capacity to differentiate, and disturbance of the inhibiting conditioned 

 reflexes. Gorsheleva (1958), Airapetyants (1958), Malyukova (1958), and 

 Rokotova and Gorbunova (1958) noted a change in the conditioned reflex 

 activity after the administration of an even smaller dose (2 r). 



Even such small doses of radiation are capable of having a cumulative 

 effect. Malyukova (1958) and Meizerov (1958) have shown that systematic 

 exposure to x-rays in doses of 3-15 r led to serious disturbances of the higher 

 nervous activity when the total dose reached 130-190 r. Tests performed on 

 human subjects by working with radiation doses approaching the permissible 

 limit and using the speech-motor method have also revealed definite disturb- 

 ances of the higher nervous activity : delay in forming the conditioned reflex 

 connections, prolongation of the latent period of the conditioned reflex, and 

 a number of other signs indicating loss of mobility and equilibrium in the 

 nervous processes (Morozov, Drogichina et al., 1957). 



Voevodina (as quoted by Cherkasov, 1960) has shown that even after 

 rats have been exposed to x-radiation in a 10 r dose, the conditioned reflex 

 activity is disturbed. After the rats had been exposed to a triple dose of 10 r, 

 the conditioned reflexes could not be elicited for 42 days. Prolongation of the 

 latent period and disturbance of the capacity to differentiate were still ob- 

 served for the next 6 months. Even many months after the brain had been 

 exposed to this dose, functional weakening of the nervous system caused by 

 the injection of morphine again led to disturbance of the conditioned reflex 

 activity. 



Work done in Kupalov's laboratory established the fact that after animals 

 were subjected to small doses of radiation, aminazine (equivalent to chlor- 

 promazine) in a dose of 0.5 mg per kg caused acute excitation in the animal, 

 with serious disturbance of the conditioned reflex activity; whereas before 

 irradiation, similar or even larger doses of aminazine caused only some de- 

 pression of the cerebral activity. This led Kupalov to the conclusion that 

 the nervous system goes on acquiring other properties for a long time after 

 irradiation and possibly even permanently. 



Electrophysiologic methods have yielded further evidence confirming 

 what has been discovered about the radiosensitivity of the nervous system 

 by the conditioned reflex method. 



In a series of papers published during the last few years, Livanov and 



