ACTION POTENTIAL CHANGE BY BETA IRRADIATION 603 



made between the C and fast A fibers. 5 showed the C's were less sensitive, 

 and 1 showed the con\erse. For the vagus, it was conchided that the C fibers 

 are less sensitive to beta particles than the A and B fibers. 



Higher temperatures did not affect the quality of the responses, but 

 simply speeded the changes in the potentials. At 24-29'-C'.. the mean 37% 

 survival dose for irradiation of beta A fibers with sources ranging in 

 strength from 48-136 mc was 255.000 rep. while at 3 1-38' C the survival 

 dose for sources ranging from 84-173 mc was 60.000 rep. 



Discussion 



The current convention of considering alpha A fibers as being confined 

 to nerves of muscles has been adopted in this work. Consequently, the fastest 

 component of the sural and saphenous compound action potentials has been 

 labeled ""beta", since these nerves are cutaneous in distribution. This absence 

 of alpha fibers in our preparations, unfortunately, permits only a partial com- 

 parison of our results to those of Gerstner ( 1956 i. Our findings that gamma 

 fibers are more sensitixe than betas is in agreement with his report for gam- 

 mas and betas of the frog sciatic nerve irradiated with x-rays. Our observa- 

 tions extend to the delta fibers and suggest a continuum of increasing 

 sensitivity for A fibers as fiber diameter decreases. Gerstner's surprising find- 

 ing that alpha A fibers are more sensitive than betas mitigates against this 

 generalization. In \ lew of these results, an explanation of the relative sensi- 

 tivities must indeed be complex. 



The sigmoid shape of the sur\i\al curve can frequently be interpreted as 

 resulting from an all-or-none destruction of a nonhomogeneous population. 

 There is no doubt that the fibers in the nerve trunks represent a non- 

 homogeneous population, but at the same time, they are not destroyed here 

 in an all-or-none manner, llie reduction of the potential components 

 results from blocking of individual fibers following progressive depolarization 

 of these fibers. That such is the case is attested to by the reduction of the 

 conduction velocity which has often been reported (Audiat ct ol., 1934; 

 Bachofer and Gautereaux, 1960; Gerstner, 1956) and by the report (Audiat, 

 1932) of irradiation injury currents in nerve. Progressive reduction of the 

 action potential in the beta irradiation experiments on toad single fiber by 

 Yamashita and Miyasaka (^1952) and similar responses in the single fiber 

 experiments of Bachofer and Gautereaux 1959) also substantiate this 

 interpretation. 



Since the time of block is a difficult end point to determine for a nerve 

 trunk, the 37^r survival dose was used as a superior measure (Lea. 1947). 

 This method makes comparison of our results with those of other investi- 

 gators somewhat difficult, but it permits more precise comparison of fiber 



