162 



THE PHYSIOLOGY OF EARTHWORMS 



Stimulation, but there is no delay in transmission since the bridge 

 to the second lateral conducts rapidly and the unblocked fibre 

 determines conduction velocity in both fibres. 

 Normally single stimulation by external electrodes causes one 



Fig. 51. Correlation of internally and externally recorded spikes. 

 Microelectrode impaled median giant axon, between a pair of 

 external electrodes. External trace is zero reference for resting 

 potential; (a) weak stimulus, threshold for median giant axon 

 elicited diphasic responses on external trace, corresponding to 

 internally recorded spike of that fibre ; (b) second, stronger stimulus 

 excited also lateral giant axons whose spike appears as later diphasic 

 response. Activity of these fibres does not distort internally recorded 

 spike of median giant axon (from Kao and Grundfest, 1957). 



spike to occur, but occasionally repetitive firing is observed. This 

 has been noted previously by Amassian and Floyd (1946). The 

 first spike rises rapidly but the later ones are preceded by a pre- 

 potential that rises slowly. The cause and nature of this repetitive 

 discharge was studied by applying brief shocks in rapid succession. 



