NERVOUS CONTROL OF INSECT MUSCLES 89 



The "Slow" System 



So far the "slow" systems have only been demonstrated in the extensor 

 tibiae muscles of the orthopterans Periplaneta, Locusta, and Schistocerca 

 (Pringle, 1939; Hoyle, 1953a and 1955b). A detailed investigation has 

 only been undertaken in the case of the Locusta metathoracic extensor 

 tibiae and the Schistocerca prothoracic and mesothoracic extensor tibiae 

 (Hoyle, unpublished). Using external recording, Pringle (1939) showed 

 that the action potentials of the "slow" system may facilitate considerably, 

 by as much as sixfold. In the locust metathoracic extensor tibiae the ex- 

 ternally recorded action potentials are extremely variable, depending on 

 the electrodes used and on their position (Hoyle, 1955b). Two types of 

 record may be observed, small spikes and slow shifts of potential. With 

 balanced input there is seldom any sign of facilitation, but with focal ex- 

 ternal recording, using a suitably small electrode, small spikes can be ob- 

 tained from many sites which clearly resemble the characteristic shape of 

 end-plate potentials. Some of these show considerable facilitation. At fre- 

 quencies above about 60 per second there is also summation. In other po- 

 sitions the potentials cannot be regarded as end-plate potentials, and show 

 neither facilitation nor summation. Under these circumstances intra- 

 cellular recording offers the only possible method of resolving the situa- 

 tion. With this technique many surprising features have become apparent 

 which could not even have been suspected from an analysis of records 

 from external electrodes. 



Since every muscle fiber is innervated by the "fast" fiber, it follows that 

 "slow" activity must involve contractions of some or all of the same fibers. 

 The first surprising feature is that there is a response to "slow"-fiber stimu- 

 lation in only a fraction of the muscle fibers. In the Locusta metathoracic 

 extensor tibiae the fraction is about 30%. Partial innervation of this kind 

 has recently been recorded in crustacean muscles (Furshpan, 1955). The 

 innervation in the Locusta and Schistocerca prothoracic extensor tibiae 

 muscles is 40-50% and in the mesothoracic muscles about 40%. The second 

 surprising feature is the extremely wide range of magnitudes of the re- 

 sponses (Fig. 6). This is the case even when fibers with similar resting 

 potentials and similar "fast" action potentials are compared. In the Schis- 

 tocerca mesothoracic extensor tibae the responses vary from 2 to over 50 

 mV in dift'erent fibers of the same preparation. The smaller responses are 

 pure end-plate potentials, the larger ones compound. The largest of the 

 compound responses are almost as big as the "fast" responses of the same 

 fibers. 



The largest responses show virtually no facilitation and do not summate ; 

 they are so similar to the "fast" responses that the same transmitter sub- 



