92 INVERTEBRATE PHYSIOLOGY 



acter along the whole length of the muscle fiber. Since no insect muscle 

 has yet been shown to be capable of producing a propagated spike action 

 potential, the claims for direct excitation must be viewed with caution. 

 The presence of nerve branches throughout an insect muscle makes it 

 likely that external electrodes will excite these nerve fibers at a lower 

 threshold than that at which the muscle could be excited, and the presence 

 of simple "steps" in the response records of Solf (1931) and Kraemer 

 (1932) suggest that they were only stimulating the nerve. Roeder and 

 Weiant (1950) found that a muscle of Periplaneta became completely in- 

 excitable after the motor nerve had been cut and allowed to degenerate, 

 although the muscle fibers appeared to be still in good condition. Several 

 insect muscles cannot be made to contract unless a deliberate attempt is 

 made to stimulate them via the nerve (e.g., Tiegs, 1955). 



Inhibition 



Friedrich (1933) claimed to have demonstrated the presence of an 

 inhibitory nerve in Dixippiis leg. Actually all he obtained was a slight 

 "relaxation" of the resting tibia during stimulation below the threshold of 

 the exciting nerve. Ripley and Ewer (1951) argued that his effect could 

 have been obtained with a loosely held preparation by contraction of the 

 coxal muscles. No doubt other explanations are possible ; the phenomena 

 have certainly never been confirmed. Pringle was unable to find any evi- 

 dence for peripheral inhibition during his studies on cockroach prepara- 

 tions. Ripley and Ewer ( 1951 ) described a relaxation of the levator tarsus 

 muscle of Locust a when they raised the stimulus strength (applied to the 

 whole nerve trunk in the thorax) by threefold. I have studied the same 

 preparation in detail, monitoring nerve impulses and using intracellular 

 electrodes to record from the muscle. All the fibers in the nerve which 

 supply the tibia are excited between the limits threshold to threshold plus 

 25%. At around three times threshold an apparent inhibitory effect 

 appears, but this is due to failure to excite the nerve (Hoyle, 1955b), 

 probably because of polarization at the stimulating electrodes. These ob- 

 servations emphasize the need to monitor nerve impulses in this kind of 

 experiment. It is unlikely that observations on inhibition will be accepted 

 at the present time unless they are supported by experiments along the 

 lines indicated earlier in this paper. 



There remains, however, the problem of the function of axons like the 

 third axon (So) supplying the locust jumping muscle (Hoyle, 1955a). 

 Since this is only about half as thick as the "slow" axon which travels in 

 the same trunk, difi^erences either in stimulus strength or in conduction 

 velocity can be utilized to stimulate it separately. At the same time the 

 "fast" axon can be separately stimulated and to an incomplete extent the 



