The Control of Impulse Frequency 49 



which are initiated after shorter intervals of time than those 

 separating the first, second, etc., impulses. Successive impulse 

 intervals, moreover, tend to increase as a function of time. This 

 behavior is especially obvious at low stimulus intensities (fig. 21), 

 where there can be little possibility of the recovery processes 

 (relative refractoriness) which follow each action potential inter- 

 fering with succeeding spikes. Thus, since it appears that 

 refractoriness is not involved, the loss in frequency response must 

 result from accommodative changes in the electrically-excitable 

 membrane. 



Impulse trains may also be evoked in the eccentric cell by 

 injecting brief repetitive pulses of current across the membrane 

 at different frequencies. When this was done, it was found that 

 smaller total quantities of electric charge were needed to maintain 

 comparable spike frequencies than when a steady current was 

 used. None the less, a gradual increase in impulse interval was 

 also observed with this mode of stimulation. This was manifested \ 

 by the increasing failure of the stimuli in the later parts of a train / 

 to evoke successive impulses. Thus, even in the absence of 

 accommodative changes induced by prolonged constant stimulating 

 currents, accumulative changes in membrane excitability tend to 

 extend the effective duration of the refractory period and to 

 prolong the interval of reduced sensitivity following each impulse. 

 As FuoRTES and Mantegazzini point out, little is understood 

 either of refractoriness or accommodation other than * . . . that 

 the first is an unknown process brought about by firing and the 

 second is an equally unknown process due to the stimulus '. To 

 date, it does not appear that either process can be adequately 

 explained on the basis of known parameters in the mathematical 

 model of the axonal membrane proposed by Hodgkin and 

 Huxley. ^2 



Although the initial decline in impulse frequency characterizes 

 the response of many nerve cells to stable levels of stimulating 

 current, this process is not always continuous in all preparations; 

 for a steady impiilsp; frequency is often approached and maintained 

 following an initial-frequency decline. During such a steady, 

 phase "Uf firing the impulse frequency tends to b e, line arly relate^d 

 to niembrane depolarization, as was first observed by Katz^' 

 an3 later confirffied for other primary and secondary sensory 



