3: The Control of Impulse Frequency 



In the last chapter we have seen that special regions of sensory 

 neurons are adapted for the generation of maintained electrical 

 changes when acted on by a stimulus. It is usually assumed that 

 the intensity and dynamic parameters of a stimulus are encoded 

 in the frequency of impulses which reach the central nervous 

 system, and these latter are in fact dependent upon mechanisms 

 at the nerve-ending. Yet, in spite of its obvious importance as a 

 physiological phenomenon, the control of impulse frequency in 

 sensory neurons has not received the experimental attention it 

 deserves. This is especially surprising in view of the advantages 

 which have been gained by the development of intracellular elec- 

 trodes. Only a few preparations have been examined extensively 

 with the purpose of elucidating the relationship between the slow 

 potential changes and the frequency of the resulting impulse train 

 evoked by a particular stimulus. These investigations have, how- 

 ever, emphasized the complexity of the frequency control process 

 and the present limitations in our comprehension of its operation. 

 In primary sensory neurons the frequency of propagated action 

 potentials is critically dependent upon the magnitude of the 

 receptor potential elicited by the stimulus. Katz, working with 

 the frog muscle spindle, was the first to make quantitative measure- 

 ments of this relationship^' and he showed that a plot of impulse 

 frequency versus receptor potential magnitude was essentially 

 linear. Similar measurements have now been made for at least 

 three other sensory neurons, revealing the same relationship (see 

 fig. 22). Nevertheless, a simple explanation of this relationship 

 in terms of the known properties of the nerve membrane is not 

 immediately obvious. 

 S.O.— D 



