Variation in the Response and Threshold to Electrical Excitation of X-irradiated Isolated Frog Nerve with Dose and Dose-rate

Summary

An increased excitability (measured as a change in amplitude of monophasic sub-maximal action potentials in a humid chamber) of the isolated sciatic nerve of frog to electrical stimulation has been found after a radiation dose of 100 rads of 50 kv x-rays, although the effect probably proceeds increasingly from zero dose. The effect is dose-rate dependent in the sense that a minimum dose-rate of between 500 and 700 rads/min is required before the effect is obtained. The increased excitability is probably due to a lowering of the threshold of individual nerve fibres caused by a depolarization of their membranes, as demonstrated experimentally by demarcation potential measurements. The effect is either reduced or abolished completely by either increasing the external potassium or reducing the sodium concentration respectively, by stripping the epineural sheath from the nerve, or by reversal of the stimulating electrodes so that the anode lies between cathode and recording electrodes. Recovery times after irradiation to normal excitability or membrane polarization are much shorter for nerves irradiated in solution than for those irradiated in moist air. The results suggest that the cause of the increased excitability is a depolarization of the neural membrane caused by extracellular accumulation of potassium due to an influx of sodium and efflux of potassium resulting from an unspecific increase of membrane ionic permeability by irradiation.