1242 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



nervous system of vertebrates, and doubtless of other 

 higher metazoans, regulatory processes which de- 

 serve to be considered as of a second order. A part of 

 the information which they employ does not stem 

 directly from the periplicral sense organs. It arises 

 from the very depths of tlie central gray matter and 

 gives rise to influences wiiicli in turn affect their own 

 output sequence. Clearly these closed-circuit regu- 

 latory activities can, when powerfully activated, pro- 

 duce effects which are readily apparent in modifica- 

 tions of posture and movements. However, in general 

 these effects are so intimately integrated into the 

 overall ljeha\ior of the animal that it is difficult to 

 recognize their origins. The impossibility of detecting 

 the participation of these mechanisms contrasts sharph- 

 with the seriousness of the disorders resulting from 

 suppression of their activity. Thus there arises the 

 diHiculty of defining the precise nature of this regu- 

 latory function. 



The functional connections which have been 

 demonstrated experimentally between the bulbar re- 

 spiratorv center, tlic apneustic center and the pneumo- 

 taxic center in the anterior part of the pons consti- 

 tute a striking example of ner\ous regulation operat- 

 ing by means of a closed central circuit. These three 

 groups of neuron cell bodies, all of which lie in the 

 brain-stem reticular formation, are almost certainly 

 linked together functionalK' through an interchange 

 of impulses, some transmitting information, others 

 evoking excitation or inhibition regulating the dura- 

 tion and force of inspiration. This kind of nervous 

 regulation of respiration, which may be called endog- 

 enous, is intimately associated with controlling reflexes 

 but cannot be identified with them. Electrophysio- 

 logical studies will doubtless exentually demonstrate 

 the reality of this circulation of regulatory influences 

 which is so far only a vtif dc resprit. 



The hypothesis of thalamocortical reverberating 

 circuits must be subject to similar reservations. Based 

 on indirect experimental findings, not all of which 

 are equally convincing, it still lacks definitive valida- 

 tion bv direct oscillographic studies. We also need a 

 clearer understanding of tlie functional significance of 

 the exchange of information postulated to occur 

 between the cerebral cortex and the \arious thalamic 

 nuclei with which it is connected. 



Despite the uncertainty in the interpretation of 

 these concepts, we may assume that the central regu- 

 latory activity just considered characterizes the func- 

 tion of the cerebellum, the ascending reticular forma- 

 tion and its cephalic extension in the thalamus, and 

 certain cortical and subcortical structures of the 



rhinencephalon scnsu lata. This hypothesis has led to 

 the discussion of these structures, otherwise so di\erse, 

 in the same part of the present Handbook. 



The cereljellum, associated as it is with the great 

 ascending and descending tracts of the neura.xis and 

 reciprocally connected with the reticular formation 

 and the telencephalon, is without doubt the organ 

 most legitimately to be considered a regulator of 

 central activities. It has long been known that its 

 destruction does not abolish either any single simple 

 reflex or any of the series of reflex chains by which 

 the animal maintains itself erect and in equilibrium 

 wiili respect to gravity. Cerebellar lesions on the con- 

 trary may cause a caricature-like exaggeration of these 

 reflexes. Its electrical stimulation causes changes in 

 postural tonus and in phasic contraction of large 

 groups of muscles in patterns often de\iating from 

 the principle of reciprocal inner\ation. Moreover, the 

 response to electrical stimulation of the anterior lobe 

 is determined within narrow limits by the pre- 

 existing distribution of postural tonus and phasic 

 movements of the muscles participating in the re- 

 sponse. Everything happens as if cereliellar control 

 always tends to re-establish an equilibrium. Finally it 

 may be noted that the cerebellar cortex, the function- 

 ing of which is eminently tonic, shows a surprising 

 autonomy in its spontaneous electrical activity. Thus 

 the role of the cerebellopetal afferents appears to be 

 limited to intensification and (less certainly at the 

 moment) to inhibition of the autochthonous activity 

 of this organ. 



Nevertheless, in spite of the considerable mass of 

 information which is available and which is critically 

 reviewed in Brookhart's excellent chapter in this 

 Hnndhook, the exact nature and raison d'etre of cere- 

 bellar regulation still escape us. Impressed by the 

 great number and variety of its afferent paths and by 

 its reciprocal connections with all the projection areas 

 of the cerebral cortex. Snider has suggested that the 

 cerebellum may be "the great modulator of neuro- 

 logic function." However seductive may be this view 

 of an investigator who has contributed greatly to our 

 knowledge of cerebellar connections, we hesitate to 

 embrace it. There is one comparative anatomical fact 

 which does not seem to have received attention and 

 which appears to us quite significant. Two groups of 

 teleosts, the cyprinoids and the mormyrides, both have 

 a rich cutaneous innervation in the cephalic region. 

 In the former, slowly-moving fish with mediocre 

 motor capacity, the innumerable cephalic receptors 

 are gustator\ . In this group the cerebellum is rudi- 

 mentary. In the latter, the cutaneous receptors, in- 



