Muscle and Electric Organs 597 



much slower development of tension. Finally, each muscle receives one or 

 more motor (exciting) axons and usually receives one inhibiting axon, the 

 nature of the inhibition differing according to whether the contraction is 

 "fast" or "slow." Thus, in contrast to the vertebrates, which grade movement 

 mostly in the spinal cord, the arthropods show much control of gradation 

 at neuromuscular junctions. A crab "thinks in its claws." 



Pantin^^^' ^^'^ analyzed the effects of frequency, duration, and intensity 

 of stimuli to the nerve on responses of walking leg muscles (largely flexors 

 of the dactylopodite) in the crabs Maja and Carcinus. At low frequencies 

 there was little or no response to a single shock, a grealci iv-oponse after 

 several shocks (Fig. 226). With intensities just above threshold most units 

 were brought into action at 60-100 shocks per second, and a maximum re- 

 sponse occurred at 300 per second. If the frequency or intensity were in- 

 creased abruptly by 10 to 20 times threshold, or if single strong shocks were 

 inserted in a series of weak ones, the rate and height of contraction increased; 

 i.e., a quick response occurred and the muscle continued to respond at the 

 new height even though the stimuli returned to the original rate.-^" Pantin 

 contended that such a quick response was due to repetitive nerve impulses 

 started by the interposed stronger stimulus. This would make for economy, 

 as a few extra impulses from the nervous system on a low frequency back- 

 ground could change the response from the slow to the fast type. The order 

 of threshold excitability is as follows: excitor of extensor, inhibitor of ex- 

 tensor and excitor of flexor, and finally inhibitor of flexor. 



The functions of the different nerve fibers going to various muscles in 

 many species have been analyzed in detail by Wiersma and his collaborators, 

 particularly van Harreveld and Marmont (summary by Wiersma--*^). It 

 had been shown earlier^^*"'' ^^-^ that the claw nerve of several crustaceans 

 could be split into two bundles: one containing the large motor fiber caused 

 contraction of the adductor and inhibition of the abductor, the other bundle 

 containing the smaller fibers had the opposite effect. Wiersma and his as- 

 sociates traced the course of individual fibers by methylene blue staining 

 and identified their action by stimulating the separate nerve fibers and re- 

 cording contractions and muscle action potentials. 



The variation in pattern of nerve fiber distribution among different spe- 

 cies and in diflrerent muscles of one species is remarkable. Some muscles 

 receive one excitatory and one inhibitory fiber; the walking leg muscles 

 studied by Pantin are examples. Other muscles receive two excitor fibers 

 and one inhibitor.^^"^ The larger motor fiber causes a contraction of short 

 latency, rapid shortening, and high tension; this response includes an action 

 potential which appears directly with stimulation, and shows little facilita- 

 tion. Stimulation of the second or intermediate size fibers causes a contrac- 

 tion which builds up slowly so that there is an apparent long latency, marked 

 facilitation, and low tension; muscle action potentials recorded through 

 holes in the shell are small, may not appear until after several stimuli in a 

 series, and . facilitate greatly. Fatigue of the slow system does not at the 

 same time fatigue the fast system. Stimulation of the third, usually smallest, 

 fiber inhibits the response to the motor fibers and is more effective against 

 the slow than against the fast contraction (Fig. 230). During complete 



