CENTRAL REGULATORY MECHANISMS ; INTRODUCTION 



1243 



nervated by a branch of the lateral line nerve, are 

 adapted to perception of the currents produced by 

 the periodic discharges of an electric organ of low- 

 power. This detecting apparatus is clearly one of the 

 factors making possible the acrobatic agility of these 

 fishes. In them the enormous cerebellum fills the 

 cranial cavity with its exuberant proliferation One is 

 tempted to see in this structural contrast a confirma- 

 tion of the concept that cerebellar regulation is es- 

 sentially concerned with the postural and phasic in- 

 nervation of the skeletal musculature rather than 

 with nervous functions in general. 



The mesencephalic reticular formation responsible 

 for arousal and the thalamic nuclei mediating its 

 rostral influence qualify in every way for inclu.sion 

 among the regulators of central activity. The original 

 conception of Magoun and of Moruzzi was that this 

 region produced a continuous stream of impulses 

 which ascends to and energizes the cortical neuronal 

 networks, and that the magnitude of this stream is 

 determined simply by llic intensity of aflTerent influ- 

 ences from the receptors. This concept now appears 

 to be too simple if one accepts the views of Jasper and 

 Fessard, which have been so well described by 

 Jasper himself and by French in this Handbook. Even 

 if the mesencephalothalamic mechanisms for arousal 

 become recognized as of psychophysiological sig- 

 nificance, it will not however be necessary to aijandon 

 the concept that they play a major role in the general 

 regulation of cerebral activity, a regulation which 

 cannot be other than the resultant of global func- 

 tioning. 



The reciprocal connections between the ascending 

 reticular formation and the cortex pose problems 

 which are still far from being solved. One of these is 

 the intimate mechanism of cortical arousal by reticular 

 influences. Another moot question is that of the par- 

 ticipation of active inhibition in reticulocortical inter- 

 actions. The only effects clearly revealed by experi- 

 mental study of reticulocortical and corticoreticular 

 influences are excitatory. The hypothesis based on 

 these considerations proposes that cortical arousal 

 regularly produces a corticifugal discharge back to 

 the reticular neurons which intensifies and prolongs 

 their activity, so in turn arousing them to further 

 corticipetal discharge. The free play of this exchange 

 of excitatory impulses, in the absence of an inhibi- 

 tory autoregulation, might bring the organism danger- 

 ously close to a convulsive crisis. The demonstration 

 of inhibitory mechanisms acting at the cortical or 



mesencephalic levels would confirm the view that 

 the ascending reticular formation plays a homeo- 

 static role in the oserall functioning of the brain. It 

 would further permit us to integrate into our inter- 

 pretive synthesis the observations which led Hess 

 to postulate a hypnogenic center exerting its eflfect by 

 active inhibition. '-' 



In the present state of our knowledge, interpreta- 

 tion of the functions of the rhinencephalic and non- 

 olfactory cingulate structures is very difficult, a situa- 

 tion not dissimilar from that we meet in the case of 

 the cerebellum. Experimental or pathological stimu- 

 lation of these structures, particularly of the nuclei of 

 the amygdaloid complex, results in striking changes 

 in overt behaxior and mental state both in animals 

 and man. Even bilateral lesions of these structures, 

 however, do not significantly impair the visceral or 

 somatic activities which are clearly affected by stimu- 

 lation. The rhinencephalon is thus not es.sential for 

 the integration of the functions, the centers for which 

 lie chiefly in the brain stem and hypothalamus. In 

 still other regions, such as the anterior cingulate area, 

 neither stimulation nor ablation yields clear results. 

 The recent demonstration by Penfield and Milner 

 and by A. E. Walker of the importance of the hippo- 

 campal-cingulate septum for the fixation of memory 

 in man should dictate prudence in the evaluation of 

 negative results from animal experiments. 



If we set aside this unexpected suggestion liiat the 

 hippocampus is critically related to the memory func- 

 tion, the impres.sion which emerges from the mass of 

 observations concerning the ".second .system of the 

 rhinencephalon" (as Pribram & Kriiger call it) is that 

 its various constituent structures have, in the course 

 of the phylos;enetic evolution of the vertebrates, ac- 

 cjuired the role of modulators of nervous activities 

 integrated in the more primitive subcortical regions 

 in which lie the nervous mechanisms for fundamental 

 instincts. This role of modulation of intensity would 

 account for their importance in the orientation of be- 

 havioral patterns and in the control of emotional ten- 

 sion. The description and analysis of these subtle in- 

 fluences must take full account of the nuances of 

 psychological dialectics. The writers of the pertinent 

 chapters in this Handbook have succeeded perfectly in 

 this task. 



- Since these lines were written, tlie existence of sucli an 

 inhibitory mechanism in the pontine reticular formation has 

 been demonstrated by Moruzzi and his associates. 



