EFFECTOR MECHANISMS 387 



pulsations, are served by a diffuse nerve-net, and both together form a 

 single excitable system. A single stimulus, applied anywhere to the nerve- 

 net, elicits a contraction, and the height of the response is augmented by 

 repeated stimulation. As in the retractile response of anemones, each im- 

 pulse spreads to all parts of the muscle and repeated stimulation produces 

 stronger contractions by recruitment (25). 



In addition to the quick contractions described above, muscles of sea 

 anemones give slow smooth contractions when stimulated at frequencies 

 lower than those which elicit fast responses. These slow contractions occur 

 only when sufficient stimuli are delivered (six or more); the optimal fre- 

 quency is rather low (interval of from 6 to 8 sec), and latency is typically 

 prolonged (up to 2 min in Calliactis sphincter muscle). Now, some 



Fig. 9.11. Responses of Calliactis Sphincter to Electrical Stimulation 

 (a) Series of shocks at intervals of 2, 1, and 0-5 sec; (b) same after 28 min in 0-4 m 

 MgCl 2 + sea water (1:1); (c) same after 55 min. (From Ross and Pantin (108).) 



muscles show only slow responses (parietals of Metridium) ; others, both 

 slow and quick contractions, according to the stimulation-frequency 

 (mesenteric retractors of Metridium, sphincter of Calliactis). In a sense the 

 slow responses are also facilitated in that several stimuli are required to 

 evoke them, but once contractions are initiated, further increases occur 

 simply by summation. In coelenterates, therefore, some muscles appear to 

 be capable of producing two kinds of contractions, mediated perhaps by 

 two distinct categories of nerve fibres (16, 1076). 



Crustacea. Peripheral facilitation is developed to a high degree in the 

 limb muscles of higher crustaceans. These muscles are generally supplied 

 by only a few axons, each of which subdivides and supplies all the fibres of 

 a given muscle (Fig. 9.13). The muscle fibres thus possess multiple 

 innervation from different axons: each axon also makes contact with a 

 muscle fibre at many end-plates. 



Of the several motor axons supplying the crustacean muscle at least 

 one is inhibitory, whereas the others are excitatory but differ quantitatively 

 in the speed and strength of the contractile effects which they produce. 

 Motor and inhibitor impulses converge on the muscle fibres and a balance 



