The Control of Impulse Frequency 47 



the local response, and — especially at high stimulus intensities — }■ 

 by the relative refractory period of the impulse-initiating regions ' 

 of the cell membrane. There appear to be great differences 

 between neurons with respect to the relative stability of both of 

 these processes during activity. In many nerve cells, including 



10 

 mV 



too msec 



Fig. 19. Intracellular records from a slowly-adapting crustacean stretch 

 receptor sensory neviron showing changes in the slope of the graded 

 responses with an increase in amplitude of the receptor potential (A-D),' 

 and indicating an increase in the impulse threshold (height of dotted line 

 in D) at high frequencies. (From Eyzaguirre and Kuffler,*' Fig. 6.) 



some primary sensory neurons, the properties of the active 

 membrane change during prolonged periods of excitation. This 

 results in accommodation and accumulated refractoriness, two 

 factors which effect neuron output; the former slows down the 

 rate of rise of successive local responses, even though the level of 

 depolarization generating the impulses may remain constant, so 

 that the threshold for triggering will be attained after successively 

 greater increments of time following a spike; and the latter 

 increases the firing threshold from the lower values obtaining at 

 the onset of the stimulus. Both these processes occur in electric- 

 ally-excitable membrane regions. Fuortes and Mantegazzini^* 



