REACTIONS OF NERVES AND MUSCLES To RADIATION 289 



relatively quiet irradiation can indnce rhythmical contractions, which 

 cease after a certahi time (Fig. '2). The average tonus rises as in Fig. 2 

 or falls as in Fig. 1(a) and (b). In analogy with reflex-like reactions of 

 lower animals and of rabbit intestine as described above, there is a 

 strils:ing de])endence of the effects on the dose-rate and furthermore on 

 the total dose. Until now, the lowest effective dose-rate was found to 

 be about 100 r/min in the nerve-muscle preparation of the earth- 

 worm; 15 sec of irradiation proved to be sufficient to eHcit the effects 

 and therefore the minimum necessary dose was some 25 r. These 

 threshold values, however, are dependent on the sensitivity of the 

 instrumental arrangement and should not be considered as true thres- 

 hold values in a toxicological sense. The latent period between the start 

 of irradiation and the beginning of the reaction lies in the range of a 

 minute at the lowest dose-rates and shortens with increasing dose and 

 dose-rate to non-measurable values. 



Muscle preparatio7is 



In order to differentiate the radiation effects on nervous elements 

 from those on muscle fibres, we investigated strips of the ventral body 

 wall of the leech which were completely freed from the nerve cord. 

 This isolated muscle reacts within seconds to irradiation too. However 

 the doses and dose-rates necessary to produce these effects were con- 

 siderably higher than in the case of intact never-muscle preparations. 

 Depending on the experimental conditions this radiation -induced con- 

 traction may be either partially or completely reversible or it may be 

 irreversible. Normal muscles, suspended in a Ringer-solution between 

 pH 7 and 8 saturated with air or oxygen, show an almost complete 

 relaxation following a radiation-induced contraction. Only after high 

 doses can a residual contraction be observed, and this increases with 

 the total accumulated dose. 



Relaxation of muscles following the contraction can be prevented 

 completely by suspending the specimen in a more acid solution (about 

 ])H 6), or by adding 5 ])er cent CO2 to the aerating gas mixture, or by 

 certain metabolic inhibitors e.g. dinitrophenol (9 mg/1). Under these 

 experimental conditions, irradiation will produce a permanent state of 

 contraction, persisting usually for hours, without any relaxation. If, 

 however, after irradiation the medium is exchanged for a more physio- 

 logical one the ability to relax is restored. 



Irradiation-induced contraction in a muscle unable to relax. 



Under continuous irradiation with a constant dose-rate the con- 

 traction of such a muscle follows first an S-shaped curve and then a 



