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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



olfactory origin. Afferems probably reach the hippo- 

 campus from the reticular activating system of 

 Moruzzi & Magoun (81) via the tegmentum and 

 hypothalamus (45, 46). Morin (79) and Guillery (52) 

 have described afferents from the mammillary 

 penduncle into the medial forebrain bundle of mam- 

 millary nuclei. 



More complex hippocampal pathways have also 

 been proposed. In particular, the Papez (83) circuit 

 requires mention. Papez conceived that a pathway 

 from the anterior thalamus, to the cingulate gyrus, to 

 the cingulum, to the hippocampus, to the fimbria, to 

 the fornix, to the mammillary body and back to the 

 anterior thalamus was concerned in emotion. That 

 this pathway exists is true, and certainly stimulation 

 of any point in this circuitous route evokes a response 

 in any other point (45). However, for emotional 

 processes it is probably not necessary that this circuit 

 should be intact, although of course this does not 

 exclude the possibility that some parts of the brain 

 involved in the circuit are also involved in the per- 

 ception, integration or expression of emotional 

 changes. Connections with the amygdala have been 

 demonstrated electrophysiologically (32, 41, 44, 45) 

 and elaborate brain-stem connections of rhinen- 

 cephalic structures have been proposed by Gloor (41). 



Cell Terminations 



Axons of the granule cells of the gyrus dentatus 

 collect into a bundle of fibers which passes between 

 the cells of fields hs and 114 and comes to lie chiefly 

 below the cells of field hs. Here, they form a fairly 

 distinct bundle from which branches spread further 

 medially to fields ho and hi. The terminals end around 

 the basal parts of the apical dendrite and the soma of 

 the hippocampal pyramidal cells, and occasional col- 

 laterals from these axons may be traced alongside the 

 apical dendrite centripetally. Thus, afferent impulses 

 reaching the granule cells are clearly relayed to the 

 pyramidal cells of the hippocampus. Examples of these 

 terminations appear in figure 4. The temporoam- 

 monic tract afferents seem to terminate axoden- 

 dritically for the inost part, both with respect to the 

 granule cells of the gyrus dentatus and the pyramidal 

 cells of the hippocampus. Since the actual terminals 

 them.selves are extremely hard to resolve with a mi- 

 croscope and are very small indeed, it is difficult in 

 normal preparations to be certain whether the maxi- 

 mal termination is at this point or whether it is along 

 the course of the apical dendrite toward the soma and 

 around the soma, for branches may be seen from the 



temporoammonic tract which radiate centrifugally 

 toward the somata of the pyramidal cells. These were 

 described by Ramon y Cajal (90) and Lorente de No 

 (73). In the hippocampus the temporoammonic tract 

 fibers are most obvious in fields hi and hj. In h^ they 

 are less prominent and are to some extent blended 

 with the fibers from the axons of the granule cells of 

 the gyrus dentatus. Lorente de No (72, 73) descrilied 

 afferents to both hippocampal pyramids and granule 

 cells from the fimbria. These seem to consist of rather 

 delicate fibers which divide as they approach the cells 

 and terminate for the most part around the i)asal 

 dendrites or the cell bodies. However, some fii)ers 

 may be traced through the cell layer toward the apical 

 dendrites where they travel parallel to these dendrites. 

 Thus, the three types of afferents all have fairly general 

 distribution with respect to the pyramidal cells; but 

 the alvear afferents are chiefly distributed around the 

 basal dendrites and soma, the fibers from the gyrus 

 dentatus are distrii;uted around the soma and the 

 basal part of the apical dendrite, and the fillers from 

 the temporoammonic tract are distributed around the 

 apical dendrite with collaterals extending toward the 

 soma and iiasal part of the apical dendrite. 



FUNCTIONS OF THE HIPPOCAMPUS 



The hippocampus has been studied from two some- 

 what different points of view. It has been of interest, 

 first, because no clear role has been attributed to so 

 large a part of the brain and, second, because, compared 

 with the neocortex, its structure appears simple and 

 therefore offers special experimental opportunities. It 

 is peculiarlv suitable for the study of field potentials 

 and their significance. 



Theories of Hip/iotam/inl Finulion 



It was recognized by early anatomists that the hip- 

 pocampus was large in animals in which the dominant 

 special sense was smell. Since the hippocampus ap- 

 peared relatively large in these macrosmatic animals, 

 it was considered to form part of the rhinencephalon, 

 the implication being that it was in some way con- 

 cerned with olfactory processes. Neither Ramon y 

 Cajal (90) nor Elliot Smith (103) considered that the 

 hippocampus was exclusively olfactory in function al- 

 though Elliot Smith (103) suggested that the dentate 

 gyrus was, believing it to be absent in anosmatic 

 animals. This misconception was clarified by Breath- 

 nach & Goldby (20) who showed that the porpoise 



