NERVOUS SYSTEM 163 



After a threshold shock a second stimulus was applied in the 

 relative refractory period of the first. As a consequence the 

 potential remained high and numerous small peaks were observed 

 and later another spike potential occurs. The local, non-propagat- 

 ing nature, low amplitude, frequency of repetition, and additiveness 

 of the small potentials suggest that they are synaptic, developing at 

 the junction of small nerve fibres with the giant axons. These post- 

 synaptic potentials are then responsible for re-exciting the fibre, and 

 may explain the after-discharge phenomena of reflex arcs in the 

 earthworms (CoUier, 1938, 1939 a, b) previously explained by the 

 production of humoral transmitters. 



The post-synaptic nature of these potentials rests on six points : 



(1) They are not artefacts from neighbouring fibres since the 

 spike of the lateral giants does not cause potential changes in the 

 median fibre. 



(2) They are independent of the spike potential since they occur 

 during the absolute refractory period and appear upon the falling 

 phase of the spike. 



(3) They appear at sites which do not generate spikes. 



(4) They are graded and they summate. 



(5) They arise at various sites on the axon, often far from the 

 site of stimulation. 



(6) They give rise to spikes that may be initiated at different 

 sites of the axon (Kao and Grundfest, 1957). 



The histological work of Smallwood (1926), Stough (1930) and 

 others makes no mention of synaptic endings impinging upon the 

 surface of the giant fibres, but branches of these axons are known 

 to ramify into the neuropile that comprises the rest of the ventral 

 nerve cord, and it is in this tangled mass of fibres that the connec- 

 tions between giants and sensory and motor fibres must occur, and 

 where the synaptic potentials mentioned above have their origin. 

 It is suggested that these giant fibre branches may be afferent or 

 efferent (Kao and Grundfest, 1957). 



The Septa 



As figured by Stough (1930) the giant fibres are divided into 

 discrete sections by membrane partitions that cross the axon 

 completely sloping either forward or backward, according to the 



