576 C. S. BACHOFER 



able enhancement of activity without undue prolongation of the experi- 

 ments. For UV irradiation, low pressure mercury-vapor bulbs, emitting UV 

 chiefly at 2,537 A, were utilized at various dose rates. 



Spike amplitudes were measured from both monophasic and diphasic 

 recordings, with care being taken to see that stimuli were maximal or 

 supramaximal, depending on the experiment. All conduction velocities were 

 computed from the time interval between spike peaks recorded from two 

 pairs of electrodes. This obviated any possible errors due to variable latent 

 period of responses. Sensitivity to stimulation — to be distinguished from 

 sensitivity to x-rays, which was determined by the dose of x-rays required 

 to produce a given effect in the nerve — was arbitrarily defined as the 

 reciprocal of the voltage just necessary to elicit a response. This threshold 

 stimulus applied to the nerve was a single square-wave pulse of 10"^ sec 

 duration. The rate of spike rise — the rate of voltage change of the action 

 potential in volts per sec — was determined for the linear portion of the 

 rise of the spike. The somewhat slower initial voltage change, corresponding 

 to the period of electrotonic invasion preceding the impulse proper, was 

 ignored in these measurements. 



The determination of the refractory period should be explained in some 

 detail. When a nerve fiber produces an action potential in response to a 

 stimulus, there is a period within which the nerve will not respond to a 

 second stimulus. This refractory period actually has two parts: the first, in 

 which the nerve will not respond regardless of the magnitude of the stim- 

 ulus (the absolute refractory period), and the second, in which the nerve 

 will respond only if the stimulus is greater than that of normal threshold 

 strength (the relative refractory period). The relative refractory period 

 varies greatly in duration, the duration showing an inverse relationship to 

 the magnitude of the stimulus. As the magnitude of the stimulus increases 

 above threshold, the relative refractory period decreases until the nerve be- 

 comes absolutely refractory. In all cases the refractory period corresponds 

 to the time within which the nerve recovers from the events associated with 

 the production of the action potential. Since the duration of the refractory 

 period is inversely dependent on the magnitude of the stimulating voltage, 

 it was necessary to use a stimulus of a definite magnitude. For uniformity 

 of results, the stimulating voltage was set 50% above the threshold at zero 

 time. Pairs of square-wave pulses of 10"^ sec duration delivered at 1 sec 

 intervals, were used to stimulate the nerve. Six pairs of stimuli, on the 

 average, were required to determine the refractory period at each point; 

 the nerve was not otherwise stimulated. The interval between the two 

 members of the pair of stimuli was adjusted to the point where the nerve 

 just failed to respond to the second stimulus. It was thus possible to deter- 

 mine with precision the refractory period of the giant fiber for the particular 



