NERVOUS CONTROL OF INSECT MUSCLES 93 



effect of interference between the third axon and the "slow" one can also 

 be studied. So far no combination of stimuli has been found which leads 

 to an inhibition of either the "slow" or the "fast" systems. The effect of 

 stimulating S- is to increase rather than to decrease the mechanical re- 

 sponse. Intracellular recording reveals that stimulation of So eft'ects a 

 small hyperpolarization in those muscle fibers which have a low resting 

 potential. The hyperpolarizations summate to raise the resting potential 

 up to but never beyond the value of 70 mV, which is probably near the 

 potassium equilibrium potential. Hyperpolarization of the resting mem- 

 brane is a property of the crustacean inhibitory nerve. The locust hyper- 

 polarizer axon is only present in the metathoracic leg, and it may perhaps 

 be an evolutionary relic of a once common inhibitor axon. Now^ its hyper- 

 polarizing function alone seems to remain ; this could be of value to insects 

 in which there is a fluctuating, often high, value of haemolymph potassium 

 as in grass-feeding locusts. The potassium tends to depolarize the muscle 

 fibers ; and any agent acting against this tendency would ensure a larger 

 end-plate potential and active membrane response, which both depend on 

 the resting potential, and in turn greater activation of the contractile 

 mechanism would follow. 



This is merely speculative, but the presence of the locust S- axon is 

 significant. It must be taken to indicate that somewhere in the class there 

 may be fibers possessing the property of peripheral inhibition. 



Locust Muscle : The Overall Picture 



The locust pro- and mesothoracic extensor tibiae nuiscles are small and 

 functionally nonspecialized. They seldom support the weight of the body, 

 so are probably not called upon to produce prolonged activity. They are 

 concerned with providing some of the thrust needed during walking and 

 running and in checking quick movements of the antagonist flexors (which 

 normally take the weight of the thorax) . Their "slow" fibers are well suited 

 to most of these tasks. The graded end-plate potentials in the various fibers 

 ensure that some muscle fibers are ready with near-maximal twitches for 

 immediate work against the inertia of the system. Other fibers producing 

 little tension at the start of a train of excitation come in later as facilita- 

 tion and summation develop, to reinforce the first-acting fibers. A smooth 

 development of tension is always assured even when excitation is at high 

 frequency. Evidently the "slow"-fiber peak tension of 2/5 maximal "fast"- 

 fiber tension is adequate for all slow movements and tonus. The tension 

 can always be reinforced by calling in the "fast" system and so utilizing 

 the other 3/5 of the fibers. Perhaps in many movements the "fast" system 

 is used alone. 



The metathoracic extensor tibiae muscles are specialized for the function 



