NERVOUS CONTROL OF INSECT MUSCLES 85 



even twice these have been found in some species. Now the resting poten- 

 tial is directly related to the potassium gradient across the muscle-fiber 

 membrane and an increase in external potassium effects a reduction in the 

 resting potential (Hoyle, 1953b; Hagiwara and Watanabe, 1954). At 

 30 mM K per liter the mean resting potential of locust muscle fibers is 

 reduced to 35 mV, half the maximum possible value. Low resting potentials 

 may therefore be due to inadequate mixing between the haemolymph and 

 the saline in certain regions of the muscle. Theoretically there should be 

 little difference between the resting potential values of healthy fibers 

 bathed in the same saline, and so higher values of the range are probably 

 more nearly the correct ones. Fibers with a low resting potential may be 

 aging, fatigued, damaged, or partly depolarized by high local external 

 potassium. Results obtained from them must be examined in the light of 

 these possibilities. When Wilson ( 1954) found that the responses obtained 

 in the low-resting-potential fibers differed in several respects from those 

 obtained in the higher ones, he claimed that there were two dift'erent sorts 

 of fiber and that these should be associated with the "slow" and "fast" sys- 

 tems. He did not attempt to stimulate the "slow" and "fast" nerve fibers 

 separately, or even let the preparation do this for him, i.e., by leaving the 

 connections with the ganglion intact and recording during spontaneous ac- 

 tivity or reflex stimulation. Although he suggested that a high local ex- 

 ternal potassium concentration might be leading to the low values for rest- 

 ing potential, he did not carry out the obvious test and raise the potassium 

 level whilst recording from the large-resting-potential ("fast") fibers. 

 Had he done so he would have seen that this treatment converted his 

 "fast" fibers into "slow" ones. In other words, all Wilson's observations 

 were probably on "fast" fiber responses recorded from both high- and low- 

 resting-potential muscle fibers. 



The "fast" responses of both the locust and cockroach muscles consist 

 of large depolarizations which often overshoot the zero potential base line. 

 Overshoots up to 20 mV may have been recorded from locust fibers having 

 large resting potentials. In many cases, however, the potential fails to over- 

 shoot or even quite reach the zero level ; this is almost always associated 

 with a low resting potential and so possibly with poor conditions. Hagiwara 

 (1953) and Hagiwara and Watanabe (1954) found similar responses in 

 the wing muscles of Oxya and the sound muscles of Graptopsaltria and 

 Platypleura, though overshoots were rarely observed except in Platy- 

 pleura. 



W'hen locust muscle is gradually cooled, the time course of the response 

 lengthens and an obvious step appears in the rising phase. This step is 

 sometimes noticeable in the rising phase at ordinary temperatures, par- 

 ticularly with a fast time base. As the temperature drops to about 12° C 



