NERVOUS CONTROL OF INSECT MUSCLES 91 



stance might be involved, just released in smaller quantities. The smaller 

 responses often show quite marked facilitation. In a few instances this has 

 been as great as sixfold. The larger end-plate potentials show facilitation of 

 only a fraction, say 1/3, of the initial magnitude, but this, combined with 

 variation in magnitude of the secondary response, may lead to a doubling 

 of the total response, including occasionally an overshooting of the zero 

 base line. The magnitude of the "slow"-fiber end-plate potential can be 

 raised or lowered by raising or lowering the resting potential with the aid 

 of polarizing or depolarizing current. This is consistent with the view that 

 the end-plate potential is due to the release of a chemical transmitter sub- 

 stance which raises the permeability of the membrane in the end-plate re- 

 gions to some or all ions. 



The situation in the extensor tibiae of the jumping leg appears to be 

 considerably more complex. The end-plate potential type of response is 

 obtained from only about 10% of all the muscle fibers. In about 20% there 

 is a response of a different kind. The response consists of a slow depolariza- 

 tion rising to a height of not more than 1 mV in about 50 msec, and declin- 

 ing in about 800 msec. These potentials show almost no facilitation, but they 

 summate during repetitive activation to produce a depolarization plateau. 

 The two types of responses were designed S^a and S^b respectively 

 (Hoyle, 1955b). The slow depolarizing fibers (S^a) contract smoothly at 

 all frequencies. Many of the larger end-plate potential-type fibers (S^b) 

 show small twitches in response to each stimulus and only contract 

 smoothly at higher frequencies characteristic of tetanus. 



Ordinarily the process of mechanical excitation is linked to the multi- 

 terminal anatomical arrangement, but the local response evoked by 

 passing current through an intracellular electrode produces quite a marked 

 local twitch. It remains possible, therefore, that under tetanus even 

 a single end plate could elicit an appreciable mechanical response from 

 the whole fiber, especially in short fibers, and so a single end plate 

 might be the mode of innervation in some short-fibered insect muscles. 

 Morison (1928) and Tiegs (1955) have claimed that many insect muscles 

 receive only one end plate, at one end of the fiber ; Tiegs showed that they 

 may nevertheless receive two axons. Intracellular recordings from fibers 

 of this kind would be of considerable interest. 



These results also raise the question as to whether or not insect muscle 

 can be excited directly. Many earlier workers, e.g., Heidermanns (1931), 

 Solf (1931), thought that in their experiments they were exciting the 

 muscle directly in the same way that frog sartorius or gastrocnemius can 

 be excited. The effectiveness of direct stimulation in frog muscle is due to 

 the fact that an adequate depolarizing current readily sets up an active 

 membrane response which is progagated as a spike of all-or-nothing char- 



