NEUROMUSCULAR MECHANISMS 151 



that, by stimulation of certain parts of the nerve bands going to the muscle, 

 relaxation was considerably speeded up — indicating the existence of in- 

 hibitory axons. 



In the adductor muscle of Mytilus there is no obvious difference between 

 fast and slow muscle fibers. In contrast to Anodonta and Pcctcn, in which 

 the fast fibers are striated, here all fibers seem to be smooth (Lowy, 1955). 

 Using the animal's naturally occurring contractions and relaxations, Lowy 

 (1953) found action potentials present in this muscle, even during the 

 long-lasting tonic states, in which they were infrequent. Higher-frequency 

 bursts occurred at the onset of contraction, but also at the onset of relaxa- 

 tion. The latter are rather unexpected, but might come about when both 

 excitatory and inhibitory impulses were reaching the muscle, when the 

 inhibitory ones would inhibit the contraction but not the concurrent muscle 

 action potentials. Lowy (1955) now reports that muscle action potentials 

 can also be obtained from muscles, isolated from the ganglia. He proposes 

 that these may be due either to the presence of nerve impulses in the 

 peripheral nerve-ganglion system or be of myogenic nature ; he favors the 

 former view. With Barnes one may still have doubt whether only one type 

 of muscle fiber is present in this muscle ; for, though all fibers may be 

 smooth, this would not be a certain demonstration that some of them 

 cannot contract much faster than others. -^ 



Another instance in which a molluscan muscle consists of two types of 

 muscle fibers, but this time in series with each other, has been reported by 

 ten Gate and Verleur (1952) — the retractor muscle of the main tentacle 

 of the snail Helix pomatia. The distal part is dark in color and hollow and 

 gives much more phasic contractions than the tonic light-colored proximal 

 part. 



From these observations one may be inclined to believe that in the 

 molluscs as in the amphibians two separate neuromuscular systems are 

 present, of which the slower one may well be mainly under local nervous 

 control in contrast to the situation in amphibians. Under the circumstances 

 it would be futile to speculate about the types of innervation of the muscle 

 fibers as such. Inhibition, for instance, may well be an effect wholly located 

 in the peripheral nervous system, and therefore not comparable to that of 

 the arthropods. One would like to have at hand more data about the high- 

 est forms in this phylum, such as the squid. But here the investigations of 

 Prosser and Young (1937), which showed that a single impulse in the 

 giant motor fibers would lead to a maximal contraction of the whole part 

 of the mantle, whereas smaller axons seemed to give similar but more re- 

 stricted twitches, are still the only indication that polyneuronal motor 

 innervation may be a possibility in molluscs. 



In the long-fibered anterior byssus retractor of Mytilus, whose fibers 



