REACTIONS OF NERVES AND MUSCLES TO RADIATION 293 



length as may be seen in the second part of Fig. 3(b). The curves in Fig. 

 3 demonstrate also the dependence of the reaction on the medium. 

 During recording of the curve (a) and the first part of (b) the muscle 

 was kept in a medium preventing relaxation. Shortly before the second 

 irradiation in Fig. 3(b) was started the medium had been changed i.e. 

 the muscle was then rinsed with a solution aerated with pure O2. There- 

 fore the muscle could relax after the second irradiation. 



A single exposure to a submaximal dose produces a partial shortening 

 of the muscle. Again the latency time and the slope of the contraction 

 curve are determined by the dose -rate, whereas the final degree of 

 shortening is determined by the dose -rate as well as by the duration of 

 exposure. The process of shortening stops very soon after irradiation 

 and the muscle persists in this state of partial contraction. Under re- 

 peated exposures the muscle contracts in a step-wise fashion (Fig. 4). 



Fig. 4. Repeated irradiation of leech muscle in Ringer solution, aerated with 95 per 



cent O2 and 5 per cent CO2. 

 Irradiation pulses (from left to right) of 10, 15, 20, 25, 30, 10, 35, 40 and 50 sec. 



If the final degree of the contractions after each radiation pulse is 

 plotted against the accumulated dose sigmoid lil<;e dose-effect-curves 

 result. If one increases the dose-rate at which the single impulses are 

 delivered, the slope of the dose-effect curve becomes steeper and the 

 degree of final contraction increases as well (Fig. 5). The importance of 

 dose-rate becomes especially evident if one irradiates the same speci- 

 men repeatedly with identical doses but at different dose-rates (Fig. 6). 



Irradiation-induced contraction in a muscle that can relax 



In a medium where the metabolic processes are practically undis- 

 turbed the muscle is able to relax after the radiation-induced contraction. 

 Figure 7 shows the effect of repeated irradiation with the same dose- 

 rate but with increasing doses. The latency time in all cases is prac- 

 tically the same and the slope of the contraction curve too. The maxi- 

 mum of contraction after each irradiation depends on the irradiation 



