ORGANIZATION OF PRIMITIVE CENTRAL NERVOUS SYSTEM 403 



sequence, so that the number of axons and the number of impulses delivered 

 by each progressively diminishes with repeated inhibitory impulses. If the 

 excitatory and the inhibitory effects of the two types of pregangUonic neurons 

 are explained by the differential sensitivities of the efferent neurons of the 

 ganghon, the number of anatomically addressed connexions can be minimal. 

 The sequences themselves can also be explained as the consequence of the 

 sensitivity of certain efferent neurons to the activity of others. It requires 

 more postulates, and to my mind less probable ones, to explain these pheno- 

 mena as arising from chains of anatomically addressed interneurons. 



In the locust, the alternation of the antagonistic dorsoventral and longi- 

 tudinal flight muscle is co-ordinated by a centrally determined sequence of 

 motor impulses, such that impulses alternate in the two sets of motoneurons 

 to the antagonistic muscles (Wilson, 1960). This central sequence, which 

 normally brings about the reciprocatory movements of the wings, goes on 

 irrespective of whether the effector muscles and all the peripheral sensory 

 field of the thorax is removed, except that a stream of impulses must be arriv- 

 ing from certain hair sensilla of the head which are normally excited by the 

 flow of air as the animal flies. However, flight is stopped by contact of the 

 legs with the ground. Again it is reasonable to suppose that relatively few 

 afferent neurons are adequate and that both the excitatory and the inhibi- 

 tory preganghonic effects are widespread in the sense used here, i.e. they run 

 to a large volume of the neuropile and to no particular neuron. 



The ventilatory rhythm of the locust is similarly controlled by centrally 

 determined sequences (Miller, 1960) and in the cricket it has been possible to 

 demonstrate that both inhibitory and excitatory premotor interneurons can 

 be stimulated at definite loci in the brain (Huber, 1960) from which they 

 descend to have an appropriate effect on the probability of motor firing from 

 each of the potentially independent pacemakers which initiate ventilatory 

 movements from the ventral cord. 



A rather clear-cut case where one preganglionic axon alone can have a 

 general inhibitory and another a general excitatory effect is found in the 

 feeding response of the fly. One of the three neurons of a single chemosensory 

 hair, for example, of the tarsus, is sufficient to start and keep in activity the 

 comphcated pattern of movements which withdraw and fold up the proboscis 

 of the fly and stimulation of another of the neurons is sufficient to reverse 

 these processes. There may be a comphcated series of central mechanisms, 

 and in this case internuncial neurons are probably involved. At present, 

 however, it is unnecessary to assume that these two types of sensory neurons 

 have hmited specific central connexions; they have opposite effects on the 

 probability of firing of many motoneurons, and excitation from different 

 hairs variously situated over the legs and labellum of the two sides sums 

 together in a way which suggests that the sensory axons all feed into a common 

 pool (Dethier, 1953). 



