NERVOUS CONTROL OF INSECT MUSCLES 



87 



+10 r 



a,, h. c. 



-10 - 



-fcO 



Fig. 5. The time course of typical insect muscle action po- 

 tentials recorded with an intracellular electrode at 20° C during 

 "fast"-fiber stimulation. Traces taken from fibers with 60 mV 

 resting potential from: a, Calliphora vomitoria; b, Periplaneta 

 americana; c, Schistocerca grcgaria. The probable time courses 

 of the pure end-plate potentials are shown with dotted lines to 

 emphasize the magnitude of the secondary responses and their 

 effect on recovery. 



with a duration of 1-6 msec. Only the latter are comparable to those evoked 

 by the natural stimuli, the end-plate potentials. They may be evoked by 

 depolarization of about 20 mV. The surprising thing is that the spikes are 

 never larger than about 25 mV as measured from the depolarization 

 plateau, which is usually about 14 mV depolarization (45 mV membrane 

 potential level). Also they are very variable in both magnitude and dura- 

 tion even when recorded from the same site. Theoretically the largest ones 

 might just be capable of exciting the resting membrane and so setting 

 up propagation, but electric pulses of similar magnitude and duration to 

 the spikes rarely give rise to equally large responses. This means that the 

 insect secondary responses are only local events ; there is no evidence of 

 their being propagated, as Fatt and Katz ( 1953) have demonstrated propa- 



