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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



California rock lobster), recording from the muscle 

 fibers with intracellular electrodes while stimulating 

 the four motor axons separately. In this way 20 or 30 

 fibers in each preparation were sampled and if, upon 

 stimulation of a particular axon, a monophasic 

 muscle potential in the direction of depolarization 

 was recorded at the microelectrode, it was concluded 

 that the impaled fiber was innervated by the axon in 

 question. While reasons were presented for believing 

 that the sampling was not entirely random, the fol- 

 lowing results were recognized. Some of the muscle 

 fibers did receive all four of the motor axons (and pre- 

 sumably the inhibitor as well); these were, however, a 

 small minority comprising about 5 to 10 per cent of 

 those tested. The remaining fibers were approximately 

 equally distributed among tho.se innervated by one, 

 two, or three of the motor axons, and different com- 

 binations of two and three axons were found. Further, 

 the size of the muscle potential which stimulation of 

 a nerve fiber evoked could vary considerably from 

 muscle fiber to muscle fiber. Thus this muscle shows 

 remarkable heterogeneity with respect to the number, 

 combination and effectiveness of the motor axons 

 which supply its individual fibers. This is a factor 

 which it is important to consider in attempting to 

 correlate muscle potentials and contraction and em- 

 phasizes the desirability of performing such experi- 

 ments on single muscle fibers. 



It is not known to what extent this analysis applies 

 to the simpler case of a muscle receiving only two 

 motor axons, but from experiments cited above, and 

 from some incomplete work with intracellular elec- 

 trodes (unpublished observations), it seems as if most 

 fibers receive both axons. Here, too, there is hetero- 

 geneity with respect to the size of the muscle poten- 

 tials evoked and some fibers have been found in which 

 the 'slow' axon elicited much larger responses than 

 did the 'fast' ; more work is needed, however. 



MULTiTERMiN.^L INNERVATION. In a histological Study 

 d'Ancona (14) encountered considerable variation 

 among different crustaceans, but in extreme cases he 

 observed numerous nerve terminals on single muscle 

 fibers, at times to the extent of an ending for each 

 sarcomere (cf. 41). van Harreveld (73) has also de- 

 scribed numerous axon terminations on single fibers. 

 In one particularly good preparation (from the 

 opener-muscle of crayfish) he observed 28 nerve end- 

 ings on one muscle fiber, the latter being 3.5 mm 

 long; it is not known whether this density of endings 

 is tvpical. Because of the numerous terminations on 



each muscle fiber and the fact that even small nerve 

 branches may run for considerable distances on the 

 fiber surfaces, a dense tree of intramuscular nerve 

 elements was observed, van Harreveld (72) described 

 this as a 'feltwork'. Holmes (30), however, contested 

 these observations and showed that connective tissue 

 around the mu.scle fibers could be made to stain to 

 give the appearance of a 'feltwork'. While this is un- 

 doubtedly so, it is very unlikely that the structures 

 which van Harreveld observed were not terminals 

 since he was able to follow them back to larger axon 

 branches, and since in the opener-muscle with its 

 double innervation, he saw two fibers terminating to- 

 gether at each ending. 



Another controversial point which became asso- 

 ciated with the question of multiterminal innervation 

 concerned the presence of conducted muscle action 

 potentials of the type found in the vertebrates. Because 

 the potentials which were usually recorded with ex- 

 ternal electrodes summated and were monophasic, 

 Wiersma & van Harreveld generally propounded the 

 view that such propagated potentials would not be 

 found in crustacean muscle and that the spread of 

 excitation would be effected solely by the numerous 

 nerve endings (79, 82). In 1946, Katz & Kuffler (42) 

 were able, however, to record in crayfish and crabs 

 diphasic muscle action potentials which were propa- 

 gated at a velocity of about 20 cm per sec. The con- 

 ducted spikes were seen to arise from summating po- 

 tentials resembling the vertebrate endplate potential. 

 The summating potentials were recorded from only 

 circum.scribed regions of the muscle fibers, thus pro- 

 viding physiological evidence against multiterminal 

 innervation. Twitches were seen to accompany spikes, 

 while with the local-type potentials slower smoother 

 contractions were observed. Thus they viewed crus- 

 tacean mu.scle as essentially similar to that of the 

 vertebrates, but with some quantitative differences: 

 namelv, that in the crustaceans the safety factor for 

 transmission would be lower so that facilitation and 

 recruitment would be more important and the eflicacy 

 of local endplate potentials in evoking contraction 

 would be enhanced. 



These two views were subsequently reconciled by 

 Fatt & Katz who clearly showed, using intracellular 

 recording electrodes, the presence of conducted 

 muscle spikes (20) but, on the other hand, also gave 

 a physiological demonstration of multiterminal in- 

 nervation (22). In the latter paper they reported that, 

 wherever a microelectrode entered a muscle fiber, an 



