NERVOUS SYSTEM 161 



not, however, appear correlated v^ith the degree of stretching of the 

 body. In such an extensible animal as the earthw^orm the nervous 

 system must necessarily undergo a considerable amount of 

 stretching and Bullock (1945) believed that this stretching must 

 alter the rdtes of conduction. Bullock, Cohen and Faulstick (1950) 

 later modified this opinion and state that the conduction velocity 

 remains constant regardless of the degree of stretching to which 

 the nerve cord is subjected. 



Supernormal rates of conduction within the giant fibres of L. 

 terrestris are noted when the giant fibres are conditioned by a 

 few (2-5) previous impulses. This is termed facilitation of con- 

 duction rate by Bullock (1951). The phenomenon reaches a 

 maximum after a few impulses, being independent of stimulus 

 frequency. It begins within a few msec of the absolute refractory 

 period, can rise to 10-20% above normal conduction rates and 

 declines slowly, being still appreciable after 100-200 msec. 

 Fatigue occurs with continuing stimulation. This facilitation does 

 not seem to be due simply to a phase of supernormal excitability 

 after absolute refractoriness since high rates can be obtained when 

 excitability is low and vice versa. 



The most recent work on the electrical properties of the earth- 

 worm giant fibres is that of Kao( 1956) and Kao and Grundfest( 1957) 

 in which the application of microelectrode implants have produced 

 very interesting results. 



The resting potential of these axons is about —70 mV. The size of 

 the action potentials varies from 80-100 mV, reversing the sign of the 

 potential by up to 30 mV in some cases. The spike lasts for only 

 1 msec at 20 °C, there being no hyperpolarization phase at the 

 start, and only a brief depolarization at the end of the action 

 potential. The separation of median and lateral giant fibres shown 

 by Bullock (1945) and Rushton (1945) is also confirmed since 

 completely different sets of spikes can be obtained from the two 

 systems (Fig. 51). 



Intracellular recordings using microelectrodes have also been 

 carried out on the lateral giant fibres by Wilson (1961). The results 

 confirm Rushton (1945) in the presence of electrical connections 

 between the lateral giant fibres on either side of the body. There 

 is considerable attenuation of spike size in the passage from one 

 side to the other. Impulses in one fibre may be blocked by anodal 



