The Control of Impulse Frequency 45 



the peripheral nerves of the limbs of crabs, Hodgkin examined 

 the relationship between strength of a steady depolarizing current 

 and impulse frequency, and between the average impulse interval 

 and the interval required after stimulus onset to produce the first 

 spike. About half the fibers used in the experimental procedure 

 adapted rather quickly to a steady depolarization, so that inter- 

 spike interval increased continuously as a function of time. The 

 remaining fibers, however, gave prolonged and extremely regular 

 discharges in response to a steady depolarizing current. In these 

 latter cells, the interval between the onset of stimulus current and 

 the appearance of the first impulse was identical with that for 

 succeeding interspike intervals — except when the current strength 

 was very high (fig. i8). These results have been interpreted as 

 indicating the importance of sub-threshold or local response | 

 development in determining the duration of impulse interval.—' 

 There is now, in fact, wide acceptance of the contribution to 

 frequency control in many sense organs made by the rate of 

 growth of sub-threshold activity. Experimental results verifying 

 the data from Hodgkin's model sense organs have, for example, 

 been obtained from the slowly-adapting stretch-sensitive neuron 

 in the crayfish (fig. 19). With this preparation, increase in 

 stimulus intensity (applied by stretching the receptor muscle) 

 resulted in a corresponding rise in the prepotential which occurred 

 in advance of successive impulses. The time taken to attain 

 impulse threshold decreased, therefore, primarily as a result of the 1 

 rate of increase of the local response. ^ 



With stimuli of very large intensities, which produce relatively 

 high impulse frequencies, the membrane refractory period cer- 

 tainly influences interspike interval, both in the model sense- 

 organ studied by Hodgkin and in primary sensory neurons them- 

 selves. This effect is illustrated graphically in figure 20, where the 

 deviation of the initial response time from that of discharge 

 interval may be clearly seen at the higher stimulus strengths. 

 In figure 19, the influence of the refractory period in deter- 

 mining the firing frequency of the slowly-adapting stretch- 

 sensitive neuron of the crayfish is indicated by the change in firing 

 level traced by the broken line. Thus, when a slowly-adapting 

 neuron is confronted with an unvarying depolarization, the 

 impulse frequency is determined both by rate of development of 



