4 INVERTEBRATE PHYSIOLOGY 



turn the input for another order of neurons and both have nonhnear curves. 

 Addition of output-input curves can in the extreme case readily produce a 

 step function, representing a kind of labile multiunit threshold providing 

 stability and discrimination (Fessard, 1954). 



Besides the spike threshold we must recognize a separately variable 

 subthreshold excitability. This is manifested as a nonlinear increase in the 

 membrane potential with increase in stimulating current below spike 

 threshold (Fig. 1 ). The active, graded response which does not propagate 

 does not have a threshold, but it has a very labile excitability. 



100 



Fig. 1. Stimulus-response relation of the 

 subthreshold local potential in the third-order 

 giant fiber of the squid. Stimulus applied to 

 the stellate ganglion directly ; recording from 

 the same ganglion. Two experiments are 

 shown : A. Cathodal stimuli, whose voltages 

 are shown by the upward deflected square tops, 

 elicit local responses, whose amplitudes are 

 shown by the downwards deflected triangular 

 waves, plus one spike which goes ofif scale and 

 is seen as a descending phase above the base 

 line. Plotted as per cent of threshold on ab- 

 scissa, per cent of maximum recorded local 

 potential on ordinate (spike ofif scale). B. 

 Cathodal stimuli (not shown) give responses 

 above base line, anodal stimuli of same in- 

 tensity below. Plotted as above but ordinates 

 are cathodal response minus anodal to show 

 the development of nonlinear, nonelectrotonic, 

 "active" local response. (From Bullock, 1948; 

 reprinted with permission of the Journal of 

 Neurophysiology. ) 



Subthreshold activity exhibits lability also in other ways (Bullock, 1948 ; 

 Bullock and Hagiwara, 1955). Its rate of rise and especially its rate of fall 

 vary even from moment to moment under repeated low-frequency stimu- 

 lation. It may hesitate for many milliseconds before growing up into a 

 spike or starting its fall. Its spatial decrement may vary, possibly as a con- 

 sequence of change in time course or possibly as a consequence of a labile 

 decremental propagation. In some conditions, there is a heightened ex- 

 citability after a subthreshold response, beginning without any refractory 

 period (e.g., fresh axon of squid). But in other conditions there occurs a 

 depression after such a response, with a recovery which requires many 

 milliseconds (e.g., fatigued axon or synapse of giant fibers of squid). This 

 depression may be followed by a supernormal period. These considerations 



