2^0 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



the column (54). A single electric shock evoked no 

 visible contraction. Two shocks, however, above a 

 certain low threshold did elicit a response. The size 

 of the latter was dependent upon the interval ijetween 

 the two stimuli, provided that this was greater than 

 about 50 msec, (refractory period) but not larger than 

 about 3 sees, (facilitation interval). 



These phenomena were interpreted ijy Pantin as 

 follows. The first of the two stimuli applied to the side 

 of the column would result in a nerve impulse ar- 

 riving at each 'endplate' (57) of the sphincter but 

 would be unable to activate the muscle. It would, 

 however, facilitate the neuromuscular transmission 

 process so that a second impulse arriving soon after 

 might affect activation. The increase of contraction 

 size with shorter intervals would then be clue to the 

 success of the nerve impulse at a greater number of 

 junctions. This idea implies an all-or-nothing con- 

 traction of the muscle fibers. The advantage of such 

 an hypothesis is that the introduction of a threshold 

 for successful neuromuscular transmission helps to 

 explain the sharp difTerences between the effects of 

 one and two stimuli. 



The evidence that the facilitation is a property of 

 the neuromuscular junction is that it did not seem to 

 reside in either the nerve net or the muscle \i\ itself. 

 For example, a single impulse traversed the whole 

 column nerve net, for a contraction could be evoked 

 by applying a second shock with another pair of 

 electrodes at some other point on the column, and a 

 single shock could similarly be shown to make the 

 entire column nerve net refractory. That the muscle 

 did not recjuire facilitation was shown by applying 

 the stimuli directly to the sphincter. Then a single 

 shock of intensity several times the two-shock threshold 

 gave rise to a contraction localized in the region of the 

 electrodes (56). It seems likely that stimulation of the 

 muscle was direct, since the shortening which fol- 

 lowed two weaker shocks, and which was mediated 

 by the nerve net, involved the whole muscle. 



The above hypothesis suggests an analogy with the 

 partially curarized nerve-muscle preparation of the 

 frog in which the endplate potentials (e.p.p.'s) 

 following the first nerve impulse would all be sub- 

 threshold. Tacilitation' would then represent the 

 summation of successive e.p.p.'s to a supraliminal 

 level. This model must be modified, however, to 

 account for some later observations made by Ross & 

 Pantin (68). They found that if the interval between 

 two stimuli was adjusted so that the second shock 

 just did not give rise to a contraction, then a third 

 shock separated from the second by this same interval 



also just failed to cause a contraction. On the basis of 

 the abo\e scheme, however, one would have expected 

 that the second shock would have brought the local 

 excitatory state to a le\el immediately below thresh- 

 old. The excitation following the third stimulus 

 would then have added to that level and threshold 

 would have been exceeded in some fibers. To explain 

 these oi)ser\ations the authors invoked an extra 

 process of sensitization of the neuromuscular junction 

 which would be necessary, in addition to the excita- 

 tion process. Alternatively, it seems possible to 

 account for these observations in terms of a known 

 phenomenon. This is the facilitation, as distinct from 

 the summation of e.p.p.'s, which occurs at frog 

 neuromuscular junctions and which is a property of 

 the ner\c endings (16). That is, an e.p.p. is not only 

 added to what remains from a preceding one but can 

 lie, by itself, larger. Thus in the experiment described 

 above, with the long intervals used, summation of 

 local responses might have no longer been present, 

 and only the facilitation process would have been 

 operating. It must be remembered, however, thai 

 any electrical concomitants of transmission which 

 may Idc present have not yet been recorded, and such 

 attempted explanations are only speculative. 



The responses of the sphincter of CaUiaclis described 

 above are very similar to those recorded from the 

 longitudinal retractors of the mesenteries of Mi-tridmrn 

 senile (28). In both species these muscles bring about 

 withdrawal responses and provide the quickest con- 

 tractions of which the animal is capable. They may 

 be likened to the escape reactions of some of the 

 higher invertebrates (earthworm, squid and crayfish) 

 which are mediated by giant nerve fibers. It is 

 apparent, however, that all the other activities of the 

 anemone, such as locomotion and feeding, are built 

 up from \ery slow contractions (5, 28, 55). The latter 

 include the slowest contraction known, and are so 

 leisurely that usually no movement can be seen on 

 casual oijservation, despite the fact that very consider- 

 able changes in the shape of the animal are almost 

 continuously taking place (as shown by time-lapse 

 photography, etc.). 



Although the fundamental difference between the 

 fast and slow contractions is not known, an operational 

 definition of the two is supplied by the following 

 criteria assembled from Batham & Pantin (6) and 

 Ross (67). It seems particularh useful to distinguish 

 two separate contraction types since both occur in the 

 same muscles, a) The slow contractions are evoked by 

 lower frequencies of stimulation. Whereas one shock 

 everv three seconds is usually about the lowest fre- 



