NEUROMUSCULAR TRANSMISSION IN INVERTEBRATES 



-a' 



quenc\' for evoking fast contractions, slow ones persist 

 with shock intervals up to about 15 sec. Apparently, 

 frequencies above one or two stimuli per second do not 

 succeed, and there is an optimal frequency which may 

 be considerably lower than this. For example, in one 

 experiment (67) a small slow contraction superim- 

 posed on the quick one followed five shocks separated 

 by intervals of 1.2 sec. The maximal slow contraction, 

 however, in response to this number of stimuli was 

 not obtained until the intervals between them were 

 increased to 13.6 sec. b) Whereas the fast contraction 

 ensues within less than o. i sec. after the first effective 

 stimulus (usually the second shock), the slow contrac- 

 tion may not begin until 30 to 1 50 sec. after the 

 beginning of a train of stimuli, c) Five or six stimuli, 

 rather than two, are often the fewest that will evoke 

 a visible .slow response. The size of the contraction 

 increases with additional shocks up to a maximum. 

 (f) The rising phase of a summated fast contraction 

 has the appearance of an incomplete tetanus. The 

 rise time for each step usually occupies less than i sec. 

 The slow contraction, however, is entirely smooth, 

 and the rising phase may extend over 0.5 to i min. 

 The initial rate of rise is extremely slow. 



The fast and slow contractions ha\e been shown to 

 occur in some of the same muscles, such as the 

 sphincters of Calliactis and Alelridium, the longitudinal 

 retractors of Metridium, etc. Can, then, a single muscle 

 fiber contract in both ways? There is some indirect 

 evidence that this can happen. When the slow 

 contraction of muscles, capable of also giving strong 

 fast responses, is observed under the microscope, all 

 regions of the muscle can be seen to be shortening and 

 no local buckling occurs (6). If the recosery of tension 

 following a quick release is compared during the two 

 types of response in the same muscle, it is found that 

 the time course is rapid in both cases and similar to the 

 original rate of tension development for the fast 

 contraction (67). Thus, there is some reason to be- 

 lieve that the same contractile material gives rise to 

 both contractions, and that the rate and extent of 

 activation of the contractile substance is the distin- 

 guishing feature. One is then faced with the problem 

 of how the two different types of activation are 

 brought about. The fact that the slow shortening 

 exhibits a longer refractory period (i.e. has a higher 

 minimum effective stimulation frequency) than the 

 fast contraction suggests that different excitable ele- 

 ments are involved. But if the same muscle fibers give 

 both types of shortening, these elements must be the 

 nerve fibers, and thus one might expect to find more 

 than one nerve net innervating such muscles. W'hile 



there is good histological evidence in the scyphozoans 

 (see below) for distinct nerve nets mediating different 

 contractions, no such observations have been made in 

 the actinians, and there is, as yet, too little information 

 to resolve this question. 



Scyphozoans 



Associated with their more free-living existence, the 

 behavior patterns of the medusae may differ con- 

 siderably from those of the anemones. Their most 

 conspicuous activity is a comparatively rapid, more 

 or less rhythmical, contraction of the circular muscu- 

 lature of the bell. These contractions provide the 

 basic swimming movement. Bullock (13) has studied 

 them using strip preparations (63) from three species 

 of scyphozoans and has compared them with the 

 contractions of anemone muscles. They differ from the 

 quick contractions of the mesenteric retractors of 

 Metridium (see above) in several respects, a) A single 

 threshold shock usually evokes some contraction of 

 the bell, b) The duration of the facilitation interval in 

 the bell is longer than in the retractor. In the 

 former muscle, a contraction following a previous 

 one by about seven .seconds is usually still augmented. 

 c) The duration of a single contraction is a fraction 

 of that in the medusa preparation. (/) The absolute 

 refractory period of the bell musculature is several 

 times longer (about 700 msec, in the medusae as 

 compared with probably less than 200 msec, in the 

 anemones). 



Because of the long refractory period and the 

 relatively short duration of the mechanical event, 

 there can be very little summation of tension during a 

 series of contractions of the bell; and facilitation 

 appears as an increase in the strength of separate 

 successive 'twitches.' In the anemone mesenteric 

 retractor, on the other hand, the facilitation interval 

 is shorter than the duration of the contraction and 

 facilitation and summation are always seen together. 

 These differences can be related to the differences in 

 function of the two types of muscles. The anemone 

 retractors are involved in withdrawal respon.ses and, 

 with summation of successive contractions, can bring 

 about a striking decrease in the height of the animal. 

 These muscles can shorten to less than 20 per cent of 

 their extended length. The musculature of the 

 medusan bell, by contrast, pro\ides a pumping action 

 and resembles the vertebrate heart in having a 

 relatively long refractory period. 



The site of the facilitation is probably, by analogy 

 with the actinians, the neuromuscular jimction. It 



