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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



in another orthopteran QRomalea), Ripley (61) found 

 at least six steps of contraction strength as he increased 

 the intensity of the stimulus applied to the motor 

 nerve. Of these, four represented twitches and two, 

 slow contractions. It seems likely that at least the four 

 twitch contractions involved different sets of muscle 

 fibers. 



The points of similarity between the.se two groups 

 of arthropods include the small number of motor 

 nerve fibers (35, 51, 58, 61, 62), multiterminal (15, 

 25, 36) and polyneuronal (36, 51, 58) innervation of 

 single muscle fibers, and different contraction t)pes 

 (fast and slow) evoked from the same muscle by stimu- 

 lation of different axons (36, 58, 61). The abo\e 

 references are to both histological and physiological 

 studies, and the evidence derived from the latter will 

 be considered in greater detail. For example, Pringle 

 (58) was able to distinguish twitch and slow contrac- 

 tions in the flexor tibia of the cockroach and observed 

 visually that the same muscle fibers could be involved 

 in either type of contraction. There have been differ- 

 ent explanations offered for these two contraction 

 types. Pringle, on the one hand, has suggested that 

 the distinction between twitch and slow contraction 

 would lie in the number of muscle fibers activated ijy 

 the two types of motor axons. Thus, he attributed the 

 facilitation observable in the slow system to a pro- 

 gressive recruitment of additional muscle fibers, 

 each fiber giving an all-or-nothing response. Wilson 

 (86), on the other hand, has recently studied the same 

 muscle, using intracellular electrodes, and suggested 

 that the two contraction types occur in two different 

 groups of muscle fibers, much as in the slow and 

 twitch systems of the frog. His evidence, however, was 

 indirect inasmuch as the contraction of the impaled 

 fibers was not recorded, and the motor axons were 

 not separately stimulated. There is yet a third pos- 

 sible explanation for the two contraction types in in- 

 sect muscle. According to this explanation, the fast 

 axon would give rise to a fast muscle action potential 

 (spike) which would evoke a twitch; while the slow 

 axon, innervating many of the same muscle fibers, 

 would give rise to a slower, smaller, facilitating 

 muscle potential, the mechanical response to which 

 would ije a slow smooth contraction. This is the 

 mechanism which seems largely to explain the fast 

 and slow systems in the crustaceans (see above). It is 

 known that this mechanism must be present to some 

 extent, and is probably the most important of the 

 three, although the other two may operate as well. 

 Both in crustaceans and in insects, however, there is 

 the possibility of slow potentials exokint; spikes and 



thereby twitches so that recruitment may occur in the 

 slow as well as in the fast systems, and further there 

 may be muscle fibers innervated solely by either fast 

 or slow a.xons. 



Evidence for fast and slow potentials occurring in 

 the same fibers of insect muscle has recently been pro- 

 \ided by Hoyle (36). He worked mostly with the 

 extensor of the tibia of the migratory locust and 

 showed that it received three motor axons. One of 

 them, which ran in a separate nerve, seemed to in- 

 nervate all the fibers of the muscle and evoked in 

 them an action potential consisting of a spike arising 

 from an e.p.p. The accompanying contraction was a 

 rapid twitch. This nerve fiber was designated by the 

 letter F, as an abbreviation for fast. A second axon was 

 referred to as Si (signifying a slow response), even 

 though its stimulation resulted in fast action poten- 

 tials and contractions in some of the muscle fibers. 

 That is. Si seemed to have two different types of end- 

 ings and could produce markedly different effects in 

 two classes of muscle fibers. In about two-thirds of the 

 Si-innervated fibers, stimulation of that axon evoked 

 small, remarkably slow potentials, longer than i sec. 

 in duration. They were capable of summating to 

 plateaus of depolarization of 50 mv (during repetitive 

 nerve stimulation) without giving ri.se to spikes. The 

 other one-third of the fibers supplied by Si showed 

 very much more rapid e.p.p.'s which could give rise 

 to spikes. The size and speed of these latter e.p.p.'s 

 were similar to, but somewhat less than those follow- 

 ing stimulation of F. The slow responses were desig- 

 nated as Shi and were accompanied by slow contrac- 

 tions, while the faster potentials, which gave rise to 

 twitches, were referred to as Sib. Of the total number 

 of fibers in the muscle, only about 30 per cent were 

 supplied with .Si endings of either type (20 per cent 

 Sia; 10 per cent Sib). The third axon. So, which was 

 smaller than either F or Si, produced an electrical 

 response in only a few of the fibers but a contraction 

 in apparently none of them. The muscle potential 

 consisted of a brief depolarization followed by a more 

 prolonged hyperpolarization (up to several hundred 

 msec). Although ijoth phases were small (less than i 

 mv) the hyperpolarizations could summate during 

 repetitive activity and thus raise the resting potential 

 of the muscle fiber. The S2 response was most clearly 

 seen in fibers with low resting potentials and could 

 not raise the membrane potential above the level of 

 about 70 mv. Hoyle has not been able to demonstrate 

 that S> causes any inhibition of either contraction or 

 action potentials evoked by the other two axons. In 

 fact, stimulation of S^ sometimes seemed to augment 



