152 



THE PHYSIOLOGY OF EARTHWORMS 



monitoring the impulses from muscles midway along the body. 

 Roberts (1960) found this a difficult operation though for the 

 synapse is apparently very easily fatigued, being stimulated to 

 failure during the dissection of the preparation. Even in successful 

 experiments the accommodation of the giant fibre: motor fibre 

 junction was complete as soon as the giant fibres were stimulated. 

 But in some cases muscle action potentials were seen after a 25 

 msec delay. After passage of only a few shocks (at 1 /sec) in a 1:1 

 ratio the transmission across the synapse became irregular for 

 5-20 sec and then ceased to function (Fig. 49). The size of the 



Rapid 

 accomodation 



GF 



GF branches ? 



gues rapidly 



Motor fibre 



Sensory fibres 

 (Slow adapting) 



Neuromuscular 

 junction (Slow fatigue) 



Fig. 49. The physiological properties of the giant fibre reflex of 

 Lumbricus (after Horridge and Roberts, 1960 and Roberts, 1960). 



potential recorded varies during this period, presumably indicating 

 that more than one motor fibre is present in each segmental nerve. 

 In response to the same electrical stimulation a wave of slower, 

 smaller action potentials can be seen, and the accommodation of the 

 junction to these impulses is less rapid. The conduction velocity of 

 the motor fibres in the segmental nerves is of the order of 0-4 m/ 

 sec, suggesting that the giant fibre or some other rapidly con- 

 ducting system does not operate in the segmental nerves, unlike the 

 situation in some polychaetes such as Myxicola (Horridge and 

 Roberts, 1960). Recordings of the size of potential obtained from 

 the segmental nerve in relation to giant fibre spikes, however. 



