790 



Comparative Animal Physiology 



B), anterior horn cells of spinal cord,^"** and giant cells in stellate ganglion 

 of cephalopods.^^"* 



Even in nerve nets of coelenterates, where anastomoses have been de- 

 scribed many times in silver-stained network, vital stains show that the pro- 

 cesses of the neurones actually intertwine or run along parallel with one 

 another, but the libers do not fuse. These net "synapses" are not structurally 

 polarized, as are synaptic boutons; as much surface of one neurone as of 

 the other is exposed at an apposition synapse. Similarly, the junctions be- 

 tween segments of giant fibers in earthworms and crustaceans are contigu- 

 ous but the surfaces are equal; these are monosynaptic junctions."*^ In bouton 



--^^5^--^^ 



By 



\ lb 



Fig. 297. Figures of polysynaptic junctions. A, Endings of axons on dendrite of a 

 giant Mauthner cell of catfish, Ameiurus. From Bartelmez and Hoerr.** B, Branching 

 axon ending in bulbs and club endings on a cell in the oculomotor nucleus of the gold- 

 fish. From Bodian."^ 



and basket synapses (polysynaptic junctions) the branches from several axons 

 converge onto one receiving neurone, and the total active surface of the re- 

 ceiving neurone is much greater than that of any one incoming axon. 



Polarity. Synaptic transmission usually occurs in only one direction, ortho- 

 dromically; antidromic (backward) impulses stop at the receiving neurone 

 soma (cell body and dendrites). One explanation of polarized transmission 

 lies in the fact that in such centers as sympathetic ganglia acetylcholine is 

 liberated in appreciable amounts only at preganglionic axon terminations, 

 not by antidromic impulses. A second esxplanation of polarity is that a cer- 



