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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



postcommissural fornix and mammillary body. An 

 attempt was also made to remove the hippocampus 

 to observe vvliat effect this might have on the electrical 

 activity of the neocortex. The results are shown in 

 figure 6. After acute bilateral hippocampal ablation, 

 the cortex was observed to spindle, as in the condition 

 of relaxation or sleep, and it was found extremely 

 difhcult to induce arousal of the cortex ijy physiologi- 

 cal stimuli. Howe\er, stimulation of tiie intralaminar 

 nuclei still produced desynchronization of the neo- 

 cortex but only for the duration of tiie stimulus itself 

 Since in the guinea pig stimulation of the anterior 

 thalamus and intralaminar nuclei evokes a widespread 

 recruiting response all over the cerebral cortex, and 

 since stimulation of the septum produces the saine 

 effect (50), the possibility is suggested that the an- 

 terior thalamus and intralaminar nuclei may in some- 

 way be e.xcited by the reticular acti\ating system and 

 hippocampus, or possibly septum, and thence in- 

 fluence the whole of the neocortex. Thus, stimulation 

 of the intralaminar nuclei after ablation of the hippo- 

 campus might cause the desynchronization of the 

 neocortex, but since the main afferent pathway to the 

 anterior thalamus would be interrupted, physiological 

 stimuli would fail to arouse the neocortex. Neverthe- 

 less, it inust be remembered that this type of acute 

 experiment was performed in a few instances only, 

 that it is very traumatic and that the animal may have 

 been comatose for other reasons, for example because 

 of incidental damage to lower brain-stem structures 

 or thalamus. It is interesting to note that Akimoto 

 et at. (9) have recently reproduced Hess' (55) sleep 

 reaction ijy stimulation of the anterior thalamus at 

 low frequency ijut obtained arousal at higher fre- 

 quencies. 



Records from single cells in tlie hippocainpus of the 

 cat (15, 16) or rabbit (49) indicate that single cells 

 are also responsive. Incidental obser\ations of this 

 kind were also made by Tasaki et al. (iio) in the 

 course of a study on the lateral geniculate body. In the 

 case of visual stimuli, as shown in figure 7, Green & 

 Machne (49) found that rh\lhmic imit bursts oc- 

 curred at approximately the frequency of the theta 

 rhythm although they were rarely, if ever, in exact 

 phase. They had more difficulty in seeing any svn- 

 chronization with other forms of afferent stimulation. 

 Hippocampal units seemed to respond specifically to 

 one mode of peripheral stimulation only, providing 

 the electrode was in the hippocampal pyramids. 

 About half a millimeter below the surface of the hip- 

 pocampus, where the electrode would be expected to 

 enter the dendritic layer, no spontaneous activity 



occurred. This persisted for a further 1.5 mm, after 

 which units were again seen responding either to 

 specific modalities or to a variety of modalities. At 

 this time, presumably, the electrode tip was either in 

 the granule cells of the gyrus dentatus or in area 

 hj_5. In this regjion, just within the layer of granule 

 cells in the gyrus dentatus, there are a number of 

 Golgi type II neurons which seem to link adjacent 

 groups of dentate granule cells. This seems to suggest 

 that there might be a one-for-one relationship between 

 granule cells and pyramidal cells but that the Golgi 

 type II neurons might respond to a variety of 

 modalities. 



The evoked potential following physiological 

 afferent stimulation has not been studied greatly. It is 

 readily seen in the monkey despite the difficulty of 

 obtaining the theta rhythm. Green & Adey (45) 

 noticed that it could be obser\ed readily in the cat 

 but seemed very sensitive to anoxia, and that it failed 

 quickly with repetitive stimulation, responding best 

 at intervals of aijout once every 30 sec. Adey et al. (2) 

 have noticed both the evoked potential and theta 

 rhythm in the Australian phalanger, Trichosuras 

 riilpecula, where its characteristics seem to be not un- 

 like those seen in the cat. 



At present, then, we are not in a position to assign 

 any specific functions to the hippocampus. Among 

 suggestions that have been made are: a) that it is 

 concerned in emotion (83), a view discussed by 

 Brady in Chapter LXIII of this work; h) that it is 

 concerned in visceral activity (65, 74); c) that it is 

 concerned with memory mechanisms (78); d) that it is 

 part of a general forebrain suppressor system (61); 

 (') that its activity might, in some way, he the converse 

 of that of the neocortex on the reticular activating 

 system (46). At the present time none of these sug- 

 gestions can be taken altogether seriously. Howe\'er, 

 it is possible that the rhinencephalon, placing upon 

 the rostral part of the reticular activating system, in 

 some way modulates the activity of the cerebral 

 cortex, perhaps enhancing or suppressing it, or being 

 concerned with preservation of activity within the 

 cerebral cortex. All speculations about memory, 

 emotion and visceral activity face one serious stum- 

 bling block: the susceptibility of the hippocampal 

 formation to seizure discharge, for if seizures may 

 result either from stimulation of the hippocampus or 

 from lesions in tiie hippocampus, and if these seizures 

 spread to other areas of the brain, then great re- 

 strictions must he placed upon the interpretation of 

 data depending upon stimulation or lesion-making, 

 for the seizure effects may arise not from the hippo- 



