NERVOUS SYSTEM AND BEHAVIOUR 



439 



Two-way synapses are usually of the relay type. This appears to be the 

 situation in through-conduction systems of sea pens, sea anemones, etc., 

 as well as the giant-axon synapses of polychaetes just described. Certain 

 coelenterate responses, however, depend upon interneural facilitation, 

 whereby several impulses are necessary to overcome synaptic resistance 

 (integrative synapses). This is the normal condition in the oral disc of 

 anemones, and becomes operative in through-conducting systems of other 

 coelenterates under abnormal conditions — following magnesium anaes- 



Fig. 10.16. Details of Nerve Fibres in the Sea Anemone Metridium senile 

 (a) Crossed axons in synaptic contact; (b) end-plate of axon on retractor muscle sheet. 

 (From Pantin (82).) 



thesia or after paring down the conduction tract — when relay synapses 

 revert to the integrative type (20, 21, 24, 52, 92, 1 1 1, 128). 



Invertebrate junctions just described are non-polarized and possess a 

 single synaptic interface. In polarized junctions the preganglionic fibres 

 terminate in small swellings (boutons), fine twigs or spiral windings (Figs. 

 10.14, 10.18). Efferent axons become attenuated near their ends and termi- 

 nate on muscles and other effectors as fine fibrils, swellings, plates, ribbons, 

 etc. (Figs. 10.2, 10.16). Processes of transmission between nerve and effector 

 cell may be identical with interneural transmission. From a functional 

 viewpoint, polarized synapses have been classified as : one-to-one or relay, 

 several-to-one or integrative, and one-to-several or multiplying. 



A well described relay type of synapse in invertebrates is that between 

 second- and third-order giant axons of squid. In a fresh preparation the 

 impulses in the postganglionic fibre faithfully follow the frequency of 

 preganglionic firing (Fig. 10.19). A local or synaptic potential can be dis- 

 tinguished after fatigue and is evoked by direct stimulation of the ganglion. 



