NERVOUS SYSTEM AND BEHAVIOUR 



455 



Spontaneous central activity in vertebrates shows much uniformity in 

 all groups investigated. Characteristic of these animals are smooth slow 

 waves (spectrum chiefly 1-30/sec), which can be recorded from deafferented 

 and isolated fragments of brain. Slow rhythmic waves of this nature have 

 been described in only a few invertebrates. The central nervous systems of 

 the latter are characterized rather by fast spike-like activity, the dominant 

 frequency range of which is some twenty times that of vertebrates. 



Most invertebrate studies have involved arthropods. Central ganglia 

 of crayfish (Cambarus) and horse-shoe crab (Limulus) reveal complex 

 asynchronous activity taking the form of fast spikes (500-1, 200/sec), 

 and slow and intermediate waves (40/sec or less) (Fig. 10.24). Fast peaks 



Fig. 10.24. Record of Rhythmic Electrical Activity in 

 Central Ganglia of Limulus 



(Top) recording from connectives between deafferented cephalothoracic and first 

 abdominal ganglia. (Second from top) similar but fast activity reduced by filtering. 

 Whole records = 5 sec. Lower two records filtered to reveal slow activity. (Third from 

 top) whole record = 2 sec. (Bottom) record = 0-5 sec. (From Bullock (18).) 



dominate the records; slow waves are more conspicuous in ganglia, and 

 spike potentials in connectives and nerve roots (Limulus). Separate neuronal 

 pulses are recognizable in some records, the several neurones discharging 

 more or less rhythmically but at different frequencies. Further data are 

 available for earthworm, slug, etc. Carefully controlled studies on other 

 groups would probably be rewarding, e.g. cephalopods, brachyurans, 

 polychaetes. 



Various interpretations have been placed upon the vertebrate encephalo- 

 gram. According to one viewpoint, the slow waves in vertebrates and 

 invertebrates are outward manifestations of slow oscillations of potential 

 in groups or masses of nerve cells. We may regard rhythmic activity as an 

 inherent property of some central neurones. Such cells have been com- 

 pared to relaxation oscillators, which are continually being charged by 

 intracellular metabolic processes, and periodically discharge whenever a 



