X-ray effects on single nerve fibers.

Electrophysiological responses of median and lateral giant nerve fibers of the earthworm were recorded during exposure, in vitro, to x-rays. In response to external stimulation, during x-irradiation, these nerve fibers showed initially an increase in conduction velocity, a rise in spike amplitude, a decrease in relative refractory period, and an increase in sensitivity. These responses represent an enhancement of activity, attributable to bombardment with x-rays, rarely found in other biological systems. They were followed by deterioration of activity and eventual block, representative of the lethal action of x-rays, commonly observed in other biological systems. Several properties of the enhancement of activity were noted: The time at which maximum activity of one factor occurred did not coincide with the time at which the maxima of other activities occurred; spike amplitude, for example, was observed to increase while conduction velocity had already reached its maximum and was rapidly declining. The energy supplied by bombardment with x-rays did not act synergistically during the time of the enhanced response; that is, concomitant irradiation was not necessary in order to produce the enhanced response, once the nerve had been altered by x-irradiation. Once the nerve was responding in an enhanced manner, cessation of irradiation for short periods of time had no effect on the response. The phenomenon was therefore due to an irreversible alteration of the nerve fiber itself.