440 THE BIOLOGY OF MARINE ANIMALS 



It appears after a delay of 0-5 msec, and it is propagated with decrement 

 (reduction to \ amplitude in 0-5 mm). Also investigated have been one-to- 

 one (relay) synapses between giant axons in the c.n.s. and peripheral 

 motor fibres of decapod crustaceans. 



The excitation of most giant-fibre systems probably depends upon some 

 degree of sensory summation, implying the existence of integrative (several- 

 to-one) synapses between afferent fibres and the giant axon. Complex 

 multiple synaptic junctions have been recognized on giant nerve cells, viz. 

 on first-order giant cells of the squid and cephalic giant-cell bodies of 



Fig. 10.17. Synaptic Transmission between the Two Giant Axons 

 in the Anterior End of Pro tula 



The upper beam gives pre- and post-synaptic potentials from electrodes 3 and 6 mm 

 back of extreme anterior end ; lower beam shows ascending and descending spikes 9 and 

 12 mm from this end. Time in msec. (From Bullock (24).) 



serpulids (Fig. 10.18). The stellate ganglion of octopods is an integrative 

 centre containing synapses between small pre- and post-ganglionic fibres. 

 Electrical recording from this region shows a high development of facilita- 

 tion, and a burst of pulses at some optimal frequence is required to produce 

 maximal response. In multiplying synapses the post-fibre shows repetitive 

 discharge. Wiersma investigated an instance of the latter in the crayfish, 

 where a single impulse in a central giant fibre gives rise to repetitive firing 

 in efferent fibres within the roots of the abdominal ganglia (22, 23, 24, 124, 

 125, 127, 132). 



CHEMICAL TRANSMISSION 



Current theory postulates that when a nerve impulse reaches the end of a 

 nerve fibre it causes a chemical transmitter to be released at the nerve 

 ending. In the case of the vertebrate motor axon the transmitter is acetyl- 

 choline: on release it diffuses across the short distance separating nerve 



