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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



the large number of motor units. Where conducted 

 muscle potentials are absent, however, gradation of 

 contraction can be effected by variations in the 

 degree of excitation within each muscle fiber. In some 

 of these cases, therefore, one need not be surprised if 

 only a few motor axons are found to innervate whole 

 muscles. The general scheme of presentation will be 

 to consider together studies on the same group of 

 animals. 



ARTHROPODS 



Crustaceans 



Some of the elementary features of crustacean 

 nerve-mu.scle systems were known in the nineteenth 

 century, chiefly from the work of Biedermann. In a 

 histological study (7) he observed that the nerve 

 fibers ramifying on the surface of the opener-muscle 

 (abductor of the dactylopodite) of the crayfish claw 

 were apparently all branches of only two axons. 

 These two fibers ran \ery close to one another and 

 both divided at each branch point in approximately 

 the same place. The a.xons remained in juxtaposition 

 even into the very fine ramifications on the surface 

 of single muscle fibers. Thus both axons seemed to in- 

 nervate the same muscle fibers. Mangold (51) corrob- 

 orated these findings and introduced the term 'diplo- 

 tomic' branching for this type of concomitant division 

 of two nerve fibers. He also corroborated the double 

 innervation of single muscle fibers. Mangold saw that 

 as a fine nerve twig, consisting of two efferent axons 

 within a single sheath, approached the surface of the 

 mu.scle fiber the nerve sheath joined and became 

 continuous with the sarcolemma. Then the axons 

 would continue to ramify beneath the sarcolemma and 

 thus come into close contact with the striated ele- 

 ments. He also saw that more than two axons might 

 divide together in a manner analogous with diplo- 

 tomic splitting, and he figures three axons doing so 

 on the surface of the closer-muscle (adductor of the 

 dactylopodite) of the crayfish claw. Hoffman (29) 

 examined the gross anatomy of the innervation of the 

 four most distal muscles of the claw. One of his un- 

 usual observations was that a single nerve fiber in- 

 nervated two muscles: an axon going to the extensor 

 of the carpopodite (stretcher-muscle) continued into 

 the next segment to end in the opener-inuscle. He 

 showed diplotomic branching for the opener-, 

 stretcher-, and bender- (flexor of the carpopodite) 

 muscles, and found three or more axons going to the 

 closer-muscle. Hoffman also studied the distribution 



of these various axons within the two nerves which 

 run through the length of the claw . 



Knowledge of the differences in function of the 

 two or several axons innervating a single muscle also 

 goes back to Biedermann. Richet (60) had observed 

 that with weak stimulation of the crayfish limb nerve 

 the claw opened, and with stronger shocks it closed. 

 Biedermann (8) extended these obser\ations. In ex- 

 periments in which he eliminated one or the other of 

 the two muscles (opener- or closer-muscle) operating 

 the dactyl, he was still able to obtain opening or clos- 

 ing depending on the strength of nerse stimulation; 

 and if electrodes placed at the muscle evoked con- 

 traction, excitation of the nerve could inhibit this 

 contraction. Biedermann correctly concluded that 

 there must have been separate nerve fibers to mediate 

 the inhibitory process; and since the opener-muscle, 

 for example, receixed only two axons, one of these 

 was designated the excitor and the other the in- 

 hibitor. 



Lucas provided information on the function of the 

 additional axon in a muscle with a triple innersation. 

 He observed (49) both slow and twitch contractions 

 in the closer-muscle of the loijster claw; and further 

 found that the strength-duration curve for indirect 

 stimulation (by way of the nerve) showed a sharp 

 discontinuity at the point at which one type of con- 

 traction was replaced by the other. From these, and 

 additional experiments on the crayfish claw (50), 

 Lucas concluded that the two contraction tvpes were 

 e\oked by stimulation of different 'substances' (types 

 of nerve element.s). 



This conclusion has been amph confirmed and it is 

 now known that more than one t\ pe of motor axon 

 can inner\ate a single crustacean muscle and that 

 the electrical and mechanical responses e\'oked by each 

 can differ with respect to amplitude, time course, 

 facilitation and fatigue. The most convincing demon- 

 stration of this was pro\ided by van Harre\eld & 

 VViersma (74) who succeeded in isolating and stimulat- 

 ing single functioning motor axons from crayfish limb 

 nerves. In particular they studied ner\e fibers exoking 

 contraction of the closer-muscle. In normal prepara- 

 tions (but not in regenerated claws) two such axons 

 were always found and stimulation of the remainder 

 of the nerve did not result in contraction of this 

 muscle. A single impulse in the thicker of the two 

 axons evoked a rapid twitch, while a train of impulses 

 was needed in the thinner fiber to produce even a 

 small contraction. About 30 sec. were required to at- 

 tain maximal tension during stimulation of the thin 

 axon at 40 shocks per sec. This time for the thick 



