NERVOUS SYSTEM 151 



First that longitudinal contraction is initiated by the tactile sense 

 organs on the ventral surface of the body and w^hich functions 

 only when in contact with the substratum, or secondly, that 

 transition from circular to longitudinal contraction is affected by a 

 central nervous mechanism which is inactive in the absence of an 

 adequate level of peripheral excitation, as from stretch receptors. 

 In the light of their neurophysiological findings the problem was 

 unsettled, and remains so at present. (Once movement has started 

 methods of passing the stimulation from segment to segment are 

 as explained above, but how these movements begin has not yet 

 been elucidated.) 



Properties of the Reflex Arc 



The properties of the synaptic junctions lying on the reflex arc, 

 (body wall, sense organs, ventral nerve cord, body wall muscles), 

 have recently been outlined by Roberts (1960). Using electrical 

 methods of stimulation he examined the synapse between the 

 sensory and the giant fibres, between the giant and the motor 

 fibres, and between the motor and muscle fibres (the neuro- 

 muscular junction). 



By repetitive stimulation of the central end of a segmental nerve 

 he recorded potentials from the muscles of the stimulated segment. 

 A potential was obtained after the first shock, growing in size 

 during the first few shocks in the frequency range of 1-10/sec. 

 After this the size of the potential remains constant for 

 about eighty shocks, and then gradually decays until after 700 

 shocks it is but a third of its original size. If stimulation is con- 

 tinued even longer the potential may disappear altogether, but the 

 synapse recovers quickly and within 2 minutes the muscle potential 

 is full size again, though fatigue is more rapid when the stimulation 

 is repeated over a long perid. The neuromuscular junction then is 

 capable of fairly long periods of firing without interference, but 

 eventually fatigues. The procedure, however, is not short enough 

 to account for the rapid exhaustion of the shortening ''startle" 

 reflex of the earthw^orm which is co-ordinated by the giant fibres 

 (Kuenzer, 1958; Roberts, 1960). 



The central synapse between the giant fibre and the motor nerve 

 can be studied by stimulating the giant fibre anteriorly, and 



