996 RADIATION BIOLOGY 



spicuous and death intervenes within a day or two after exposure to 

 12,000 r. These effects have been attributed to irradiation of the brain, 

 since they do not occur when the head is shielded. The application of 

 50,000 r to the entire body of guinea pigs, rabbits, and mice may lead to 

 death under the beam or within a few hours; hyperthermia, hyperesthesia, 

 intermittent seizures, and cyanosis may be noted in these animals 

 (Henshaw, 1944). Early death after massive irradiation occurs even 

 though the head is shielded (Quastler et al., 1951). 



Peripheral nerve appears to be more resistant than brain and spinal 

 cord. The sciatic nerve of the rat is apparently unaffected by X-ray 

 dosages of 4000 to 10,000 r (Janzen and Warren, 1942). To accomplish 

 complete degeneration of the nerve, approximately 75,000 r of y radiation 

 is required. There is some evidence that daily exposures to 80 r may 

 interfere with regeneration of the hemisected sciatic, although there is 

 no appreciable effect on the Schwann cells with such" treatment (Gastaldi, 



1949). 



Synthesis of acetylcholine by brain is enhanced after sublethal X 

 irradiation (Torda and Wolff, 1950). This may be a result, in part, of 

 thymic involution and a consequent decrease in concentration of the 

 choline acetylase inhibitors ordinarily present in this tissue. The 

 capacity of peripheral nerve of hypophysectomized rats to maintain the 

 action potential of muscle during repetitive stimulation is partially 

 restored by low-dose irradiation (Torda and Wolff, 1950). Increased 

 synthesis of acetylcholine may bear some relation to the parasympatho- 

 mimetic effects that are evident during the initial phases of radiation 

 sickness. It is noteworthy that atropine minimizes certain early radia- 

 tion effects, including the hypotension in rabbits (Painter et al, 1947) and 

 the hypertonicity of intestinal loops (Conard, 1951); treatment with 

 atropine has also been shown to improve slightly the survival of irradiated 

 mice (Larkin, 1949). 



The radioresistance of the adult nervous system stands in sharp con- 

 trast to the sensitivity of developing nervous tissue. The sensitivity of 

 developing neuroblasts in mouse and rat embryos during the latter two- 

 thirds of pregnancy has been reported by Hicks (1950). Irradiation of 

 the pregnant animal with 150 to 200 r results in extensive destruction of 

 the embryonic neuroblasts and severe malformations of the brain. 

 Extraneural lesions do not appear with this dosage. Susceptibility of the 

 embryonic nervous system to radiation injury has also been demon- 

 strated following irradiation of selected implantation sites in pregnant 

 rats without exposure of the mother (Wilson and Karr, 1950). The fac- 

 tors that act to influence responsiveness of tissue under various conditions 

 of growth and differentiation are unknown. It is not possible at present 

 to explain the relative sensitivities of developing and adult nerve cells 

 other than to implicate metabolic differences that exist between the two 

 (see also Chap. 13). 



