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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



the maximum of decreased excitability is reached in 

 the middle of the returning phase and that the ex- 

 citability remains unchanged both at the peak and at 

 the valley of the reverberating waves. Mathematically 

 speaking, the sinusoidal curves representing the 

 reverberating waves and the excitability changes are 

 90° out of phase, the maximum of cortical excitability 

 being one quarter of a period ahead of the peak of the 

 reverberating waves (15). 



Though intimately related with the corticothalamic 

 re\-erberating wa\es, the periodic \ariation of cortical 

 excitability following an afTerent stimulation may be 

 manifested in the absence of detectai^le repetitive 

 waves. At the onset of continuous illumination of the 

 retina, for instance, the waxing and waning of the 

 cortical excitability can be demonstrated even within 

 the prolonged period of postexcitatory depression dur- 

 ing which the reverberating waves are not distinctly 

 visible (16). The significance of the periodic excita- 

 bility change of this kind is not known. Its relation 

 with the spontaneous brain waves has been discussed 

 by Gastaut (37) and Lindsley (44). The periodic 

 variation in excitability of spinal neurons has been 

 described by Bernhard (7), but the mechanism in- 

 volved in this case is believed to be diflferent. The rela- 

 tion of the reticular formation to cortical excitability 

 has been studied by King et al. (40). 



Inlnaction of Afferent Imjnilies in the Cerebral Cortex 



Information concerning the excitabilit\ change of 

 the cortex on which the present discussion is based is 

 obtained mainly from experiments in which the corti- 

 cal excitability is determined by a testing volley 

 having the same source as the conditioning one. When 

 two afferent volleys of different origin, an auditory 

 and a callosal, for instance, are sent to the same locus 

 of the cortex, the e\oked responses to the combined 

 stimuli are characteristically different. The cause of 

 the difference seems to lie in the fact that the callosal 

 and thalamic afferent volleys arri\e at diflerent strata 

 of the cortex and activate different sets of neurons, 

 among which a certain nmnber of common elements 

 are involved in the responses to both \-olleys (19). 

 Probaljly for the same reason the cortical response to 

 acoustic stimuli can ije inhibited or facilitated by 

 simultaneous or successi\e stimulation of a sxmmetri- 

 cal point on the opposite cortex (10). 



In an extensive study of the interaction at different 

 levels of the variously evoked afferent impulses, 

 Amassian (i) observed that both the cortical and the 

 thalamic responses to the second of two successive 



stimuli, delivered at certain intervals to the same ner\e 

 or to two separate branches of a peripheral nerve, 

 were always defective. The blocking of the cortical or 

 subcortical response by an antecedent volley was 

 interpreted as inhibition as distinguished from oc- 

 clusion. Occlusion, by definition, denotes the phe- 

 nomenon in which the total effect of two processes 

 when activated simultaneously is smaller than when 

 activated separately due to partial overlapping or 

 sharing of common elements. In real occlusion one 

 process should not be completely abolished, though 

 it may be greatly lessened, by another simultaneously 

 occurring process. If it is, it would merely indicate 

 that the common elements can be totally activated by 

 either of the two processes. Thus, the complete block- 

 ing of the cortical or subcortical response to stimula- 

 tion of the \olar branch of the ulnar nerve by previous 

 stimulation of the dorsal branch of the same nerve 

 described by Amassian may be regarded as a phe- 

 nomenon of inhibition, and the defect of cortical 

 response to splanchnic stimulation preceded by a 

 shock to the tibial ner\e a phenomenon of occlusion, 

 since the splanchnic nerve and the tibial nerve do have 

 separate focal areas of projection with overlapping 

 fringes in the thalamus and the cortex. 



This argument about occlusion is perhaps also 

 applicable to Marshall's investigation of the inter- 

 action between the ipsilateral and contralateral visual 

 pathways which converge with overlapping terminal 

 branches on some common neurons in the visual 

 cortex (53). It is significant that bilateral interaction 

 does not occur at the geniculate level, since the optic 

 fibers from the two retinae do not mingle but termi- 

 nate in separate laminae of the lateral geniculate 

 bodv. There are no common neurons available at that 

 level for the impulses from both sides to act on each 

 other, although the impulses reach the same nucleus. 

 However, if two stimuli of equal strength are applied 

 to the same optic nerve at proper intervals the genicu- 

 late response to the second stimulus is greater than 

 that to one stimulus alone. Apparently, the neurons 

 in the subliminal fringe are recruited into action due 

 to temporal sunnnation. 



Modifiealion of Cortical Excitability by Cointant 

 Inflow of Afferent Impulses 



It has long been known that the proper excitability 

 level in an animal when awake is maintained by an 

 incessant action of afferent impulses. It has frequently 

 been reported clinically that patients completely 

 depri\ed of sensory al)ilities fall asleep immediately 



