148 INVERTEBRATE PHYSIOLOGY 



the joint. It was found that, of the two motor fibers innervating these 

 muscles, the "fast" axon causes rotation in one direction, the "slow" in the 

 opposite. 



Comparing muscles as a whole, differences in the process of their neuro- 

 muscular transmission can be demonstrated, not only between species and 

 between muscles with different location, but also between homologous 

 muscles. In Homarus it was observed (Wiersma, 1951) that the closer 

 muscles of its three types of claws show a remarkable difference in their 

 mechanical reaction to single and double nerve impulses in the "fast" axon. 

 In a more detailed study (Wiersma, 1955) of these effects, it was found 

 that in this species there is a strong correlation between the appearance of 

 diphasic action potentials and twitches. In general many crustacean muscles 

 show monophasic potentials with outside leads, even when twitches are 

 present (Wiersma and Van Harreveld, 1938a; Wiersma and Wright, 

 1947) . But in Homarus it was found that single impulses in the "fast" axon 

 of the small claws of the second and third leg result in a clearly diphasic 

 deflection, accompanied by a twitch. In the cutter claw of the first thoracic 

 legs, which shows at best a slight movement of the tip on a single impulse, 

 the deflection is largely monophasic, while in its partner, the crusher claw, 

 which does not move at all, it is completely so. When two impulses at a 

 short interval are given, the mechanical effect in the cutter claw is dramatic, 

 as it closes completely and with considerable force. A nearly maximal 

 diphasic deflection precedes this contraction. In the crusher claw no me- 

 chanical effect is obtained, and a summation of the monophasic deflections 

 takes place. In the small claws the second deflection is only somewhat larger 

 than the first, and the summated contraction is still not strong enough to 

 close the claw. For this animal it seems certain that conducted spikes are 

 responsible for the diphasic deflection. The very strong faciliation effect 

 which a single impulse in the "fast" axon of the cutter claw exhibits, in con- 

 trast to the much weaker one of the other muscles, is a good demonstration 

 how slight differences in properties can make crustacean muscles especially 

 well adapted for certain special functions. 



In general, the relations between junctional potentials, spikes, con- 

 ducted spikes, and the resulting contractions remain uncertain. The great- 

 est problem in this regard remains the one which has been named the 

 paradox (Wiersma and Van Harreveld, 1938b). There can be little doubt 

 that under given circumstances, especially on low-frequency stimulation, 

 stimulation of the "fast" axon will result in large junctional potentials, 

 which do not elicit a contraction, while similar stimulation of the "slow" 

 axon will result in small junctional potentials, accompanied by a slow con- 

 traction. How this is possible when both potentials occur in the same mem- 

 brane is completely unknown. This quandary is, of course, caused by our 



