NERVOUS SYSTEM 153 



seems to show that the giant fibres may extend a short way into 

 the central end of the segmental nerves and there make synaptic 

 contact with other motor fibres (Laverack, unpublished observa- 

 tions). Histological studies by Smallwood (1926) and Stough 

 (1930) suggest that giant fibre branches extend into the neuropile 

 and it is conceivable that these may enter the central ends of the 

 segmental nerve. 



The input side of the reflex arc is also subject to rapid accommo- 

 dation. Stimulation of a segmental nerve through the body wall 

 gives rise to impulses in the giant fibre in a 1 : 1 ratio. The sensory 

 fibre : giant fibre synapse accommodates after about one-third of a 

 second. This means that at a stimulation frequency of 3 /sec only 

 one or two impulses can be monitored from the giant, at 22 /sec 

 some eight impulses are obtained (Fig. 49). Laverack (1960, 

 unpublished) has found that whilst this is generally true for the 

 median giant in response also to chemical stimulation, it is not 

 true for the lateral giants since impulses can be recorded for some 

 seconds after application of chemicals, and indeed the initial rapid 

 volley of giant potentials also lasts longer than one-third of a 

 second. It is possible that a somewhat different set of conditions is 

 encountered here, for flooding the body wall with chemical 

 solutions leads to stimulation of many sense organs situated in the 

 body wall, and as such organs do not adapt anywhere near as 

 rapidly as the giant fibres, and recovery of the sensory : giant 

 fibre synapse is very quick (Roberts, 1960) it is likely that continual 

 external stimulation maintains impulses in the sensory nerves for 

 long enough to re-excite the giant fibre at different sites after a 

 recovery period. The rapid exhaustion of the giant fibre reflex 

 therefore seems to be a function of the giant to motor fibre synapse, 

 since recovery periods are not followed by renewed conduction of 

 the impulse. The sensory : giant synapse also fatigues quickly, but 

 recovers quickly as well, the neuromuscular junction conducts 

 impulses for long periods. Different types of stimulation, however, 

 can lead to differential exhaustion of the giant fibre reflex according 

 to Kuenzer (1958). If an earthworm twitch response is com- 

 pletely exhausted by mechanical means such that even the heaviest 

 tactile stimulation leads to no giant fibre activity, it is still possible 

 to obtain twitching by electrical stimulation. Perhaps this indicates 

 that the sensory: giant synapse is after all the more important 



