794 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



of adequate stimulus. Suppression with selective 

 facilitation appears to us to be a general principle of 

 neural organization, exemplified not only in the 

 mechanism of prehension but also in the organiza- 

 tion of spinal reflexes (extensor thrust), labyrinthine 

 reactions, autonomic function (e.g. urination), learned 

 behavior and Pavlovian conditioning. 



Although the precentral cortex offers a mosaic 

 anatomical arrangement of units with direct access 

 to small groups of spinal motor neurons, it is un- 

 likely that any single category of movement re- 

 quires restricted Betz cell activity. The survival of a 

 small precentral area containing few Betz cells, 

 (for example, 39 counted in one experiment) after 

 removal of all the remainder on one side, was found 

 by us (25) to potentiate a remarkable degree ol 

 recovery of exploratory behavior in the opposite 

 limbs. This would indicate a wide extent of facilitat- 

 ing effect on subcortical mechanisms. 



In the loss of the placing reactions the correspond- 

 ing release of neck reflexes indicates that at the cor- 

 tical level these also are incorporated into fully in- 

 tegrated movement. The exaggerated labyrinthine 

 reactions we have found to follow bilateral ablation 

 of the posterior parietal cortex, a lesion that abolishes 

 the optic righting reflex (26), indicates that in the 

 parietooccipital seginent of the extrapyramidal 

 system visual responses restrict and limit vestibular 

 reactions in the same manner we have outlined for 

 the pyramidal contactual mechanism. The release 

 of 'sham rage' by a lesion of the limbic system indi- 

 cates that differential modulation of hypothalamic 

 mechanisms has its origin in this area. Since the limbic 

 cortex also determines withdrawal and flight be- 



havior (26), it is not surprising that autonomic re- 

 sponses associated with these reactions should also 

 be modulated from this area; but we suggest that 

 they are a by-product of motor behavior rather than 

 an expression of an isolated affective 'center.' Their 

 counterpart, conditioned pleasurable reactions and 

 their associated hypothalamic autonomic effects, 

 is to be expected from the neocortex, as Bard & 

 Mountcastle (5) have hinted, for the context of this 

 area is largely tactile. Sexual behavior, being largely 

 tactile and visual exploratory in nature as well as 

 olfactory, is a prominent association with overactivity 

 of the Rolandic mechanism for exploratory move- 

 ment. The release of these in the Kliiver-Bucy bi- 

 lateral temporal lobe ablation can also be inter- 

 preted as 'transcortical release' from temporal 

 avoiding reactions (26). 



The general principle that removal of the 'control 

 of higher centers' results in exaltation of lower 

 'centers' was recognized by Claude Bernard (g) 

 and others and was further developed by Hughlings 

 Jackson (57). We would prefer to restate the proposi- 

 tion by saying that removal of one of the competing 

 factors in the control of movement at any level re- 

 sults in overaction of the others, and at different 

 levels loss of differentiation of reaction results in 

 lowered threshold for the coarser stimulus. 



This introduction to the problems of motor mecha- 

 nisms does not attempt to correlate the large mass of 

 information on detailed interaction of nuclei and 

 fiber tracts that is available, or the known specific 

 properties of the many neuron systems. Rather it 

 attempts an outline of the general nature of the 

 behavioral problem to which all types of neuronal 

 interaction must ultimately have reference. 



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