144 INVERTEBRATE PHYSIOLOGY 



sence of twitches and of conducted muscle action potentials. Instead, 

 only slow contractions and facilitating junctional potentials are present. 

 The two nerve-muscular systems are here very independent. In arthropods, 

 on the other hand, all indications are that the same muscle fibers are in- 

 volved in the various contraction types (see below), and that the type of 

 innervation is much the same. In other phyla fast and slow contractions 

 are sometimes definitely due to two different types of muscle fibers ; but 

 it will be again necessary to obtain more information before meaningful 

 comparisons with the better-known mechanisms can be made. 



The use of intracellular electrodes may help much for the rapid increase 

 of our knowledge about these questions. However, the results with decapod 

 crustaceans to be described may be a warning that, even with these 

 methods, the finer details may be so complex that conclusions as to types 

 may be difficult to draw. 



One factor influencing the effect produced by a nerve impulse arriving at 

 the muscle fiber is undoubtedly the state of the fiber. Not only may there 

 be previous facilitation and fatigue, but the actual state of stretch of the 

 muscle fiber may have a considerable influence on the outcome. In verte- 

 brate striated muscle it is well known that the heat produced during isomet- 

 ric and isotonic twitches is quite different. Ralston and Libet ( 1953) found 

 in addition that the mere stretching of a vertebrate motor end plate may 

 result in a conducted muscle potential when this did not occur in the relaxed 

 muscle fiber— which brings to mind what von Uexkiill (1904) long ago 

 showed, that it is often easier to make stretched muscles contract. He found 

 that brittle star arms would invariably contract upward when hanging 

 down, whether or not the stimulation was nearer to the stretched side of 

 the arm. In cases of myogenic contractions, stretch is also known to be 

 an important factor, as in vertebrate intestine and in the heart of Helix 

 (Willems, 1932). It seems likely that at least in some of these instances 

 reflex activity is not present. 



These considerations may have made it clear that a great variety of 

 conditions exist, and that the conclusions which can be drawn from the 

 study of any one preparation or even of the muscles of one group may have 

 only a limited applicability to other systems. With this reservation in mind 

 the following review of recent investigations may be undertaken. 



Investigations in Arthropods 



Among the invertebrates, the arthropods still remain by far the most 

 favorable for neuromuscular studies. Their muscle fibers have lent them- 

 selves readily to the application of intracellular electrodes. The results thus 

 obtained with insects and the intriguing studies of the relations in indirect 



