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3.03 Anatomy of the Hippocampus and the DeclarativeMemory SystemR. D. Burwell and K. L. Agster, Brown University, Providence, RI, USA

ª 2008 Elsevier Ltd. All rights reserved.

3.03.1 Introduction 47

3.03.1.1 A Short History of the Anatomy of Declarative Memory 47

3.03.1.2 Overview of the Hippocampal System 47

3.03.1.2.1 Nomenclature 47

3.03.1.2.2 Location of the hippocampal system structures 50

3.03.1.2.3 Cross-species comparisons: Human, monkey, and rodent 51

3.03.2 The Parahippocampal Region 52

3.03.2.1 The Postrhinal Cortex 52

3.03.2.2 The Perirhinal Cortex 53

3.03.2.3 Entorhinal Cortex 54

3.03.2.4 Presubiculum 57

3.03.2.5 The Parasubiculum 58

3.03.3 The Hippocampal Formation 58

3.03.3.1 The Dentate Gyrus 59

3.03.3.2 The Hippocampus Proper 61

3.03.3.3 The Subiculum 62

3.03.4 Conclusions 64

3.03.4.1 The Flow of Sensory Information through the Hippocampal System 64

3.03.4.2 The Comparative Anatomy of the Hippocampal System 65

References 65

3.03.1 Introduction

3.03.1.1 A Short History of the Anatomyof Declarative Memory

A half century ago, Scoville and Milner (1957)described profound memory loss following bilateralmedial temporal lobe resection in the landmarkpatient HM. In the following years, scientists study-ing memory and the brain narrowed in on thehippocampus as the critical structure for everydaymemory for facts and events. In the past two decades,however, we have come full circle: It is now apparentthat the cortical areas surrounding the hippocampalformation also play critical roles in memory. Today,it is generally accepted that the hippocampalformation and the nearby parahippocampal regiontogether are necessary for human declarative mem-ory, but many questions remain concerning thefunctional diversity of structures within the so-calleddeclarative memory system. To what extent can thefunction of hippocampal and parahippocampal sub-structures be dissociated? How discrete are such

functions? How do these structures interact topermit encoding, storage, consolidation, and retrievalof representations of facts and events? Whatadditional cognitive functions might be supported?Understanding the structure and connectivity ofthese regions is necessary for generating and testingsound hypotheses about the neurobiology ofmemory.

3.03.1.2 Overview of the HippocampalSystem

3.03.1.2.1 NomenclatureThe structures that are the topic of this chapter havevariously been termed the medial temporal lobememory system (Squire and Zola-Morgan, 1991),the hippocampal memory system (Eichenbaumet al., 1994), and the hippocampal region (Witterand Amaral, 2004). The terms hippocampal regionor hippocampal system have the advantage that theterminology translates effectively from humans toanimal models of human memory, including rodents.

47

CITATION: R. D. Burwell and K. L. Agster. Anatomy of the Hippocampus and the DeclarativeMemory System. In H. Eichenbaum (Ed.), Memory Systems. Vol. [3] of Learning and Memory:A Comprehensive Reference, 4 vols.(J.Byrne Editor), pp. [47-66] Oxford: Elsevier.


These regions are thought to support a type of mem-

ory that has been variously called episodic memory,

declarative memory, or autobiographical memory.

For research on memory using animal models, the

terms episodic or episodic-like memory may be most

appropriate.The hippocampal system comprises the hippocam-

pal formation and the parahippocampal region

(Figure 1). The hippocampal formation includes the

dentate gyrus, the hippocampus proper (fields CA1,

CA2, and CA3), and the subiculum (Figure 2). The

primary criterion for inclusion in the hippocampal

formation is the trilaminar character of the structures.

In addition, the included structures are connected by

largely unilateral pathways beginning with the dentate

gyrus granule cell input to the CA3 (Figure 3). CA3

pyramidal cells, in turn, provide a unidirectional input

to the CA1. Finally, CA1 projects to the subiculum.

Because corticocortical connections in the brain are

overwhelmingly reciprocal, such a multisynaptic, uni-

directional circuit is unique. In contrast, the entorhinal

cortex projects to all portions of the hippocampal

formation. The connectivity and laminar structure of

the entorhinal cortex differentiate it from hippocampal

formation structures. The dentate gyrus, hippocampus

proper, and subiculum are therefore collectively

referred to as the hippocampal formation (Figure 3,

structures shown in yellow), and the entorhinal cortex

is considered part of the parahippocampal region.The parahippocampal region, also called the

retrohippocampal region, includes the perirhinal,

postrhinal (or parahippocampal), entorhinal, presu-

bicular, and parasubicular cortices (Figure 1). The

postrhinal cortex in the rodent brain is considered the

(a)

(d)

(b) (c)

D/S

Figure 1 Comparative views of the hippocampal system for the human (left), monkey (middle), and rat (right). The upper panel

shows the relevant structures in lateral views of the human brain (a), the monkey brain (b), and the rodent brain (c). The lower panel

shows unfolded maps of the relevant cortical structures for the human brain (d), the monkey brain (e), and the rodent brain (f).

Shown for the human andmonkey brain are unfolded layer IV maps of the perirhinal (PER) areas 35 and 36, parahippocampal (PH)areas TF and TH, and entorhinal cortex (EC). Figures adapted from Burwell RD, Witter MP, and Amaral DG (1995) The perirhinal

and postrhinal cortices of the rat: A review of the neuroanatomical literature and comparison with findings from the monkey brain.

Hippocampus 5: 390–408; Insausti R, Tunon T, Sobreviela T, Insausti AM, and Gonsalo LM (1995) The human entorhinal cortex: A

cytoarchitectonic analysis. J. Comp. Neurol. 355: 171–198. Shown for the rodent brain are unfolded surface maps of the PERareas 35 and 36, the postrhinal cortex (POR), and the lateral and medial entorhinal areas (LEA and MEA). The rodent POR is the

homolog of the primate PH (see text for details). In the monkey and the rat brain, the parasubiculum (ParaS) is interposed between

the entorhinal and POR/PH (arrows). The pre- and parasubiculum, which are components of the parahippocampal region, are notshown (but see Figure 2). Abbreviations: cs, collateral sulcus; rs, rhinal sulcus; DG, dentate gyrus; D, dorsal; L, lateral; M, medial;

ParaS, parasubiculum; PreS, presubiculum; S, septal; Sub, subiculum; T, temporal; V, ventral.

48 Anatomy of the Hippocampus and the Declarative Memory System


(a)

(c)

(b)

Figure 2 Comparative views of the hippocampal formation with the pre- and parasubiculum. (a) Coronal sections of the

human brain (a), monkey brain (b), and rat brain (c) showing the cellular layers of the hippocampal formation structures: thedentate gyrus (green), CA3 (blue), CA2 (purple), CA1 (red), and the subiculum (yellow). (d) An unfolded map of the rodent

hippocampal formation. Rodent schematics adapted from Burwell RD and Witter MP (2002) Basic anatomy of the

parahippocampal region in monkeys and rats. In: Witter MP and Wouterlood FG (eds.) The Parahippocampal Region,Organization and Role in Cognitive Functions. London: Oxford University Press. The presubiculum (light orange) and

parasubiculum (dark orange) are also shown at two rostrocaudal levels in panels (e) and (f). Also shown are perirhinal areas 36

and 35 (panels (c) and (e)), the lateral and medial entorhinal areas (LEA and MEA, panels (e) and (f)), and the postrhinal cortex

(POR, panel (f)). Abbreviations: DG, dentate gyrus; D, dorsal; L, lateral; M, medial; ParaS, parasubiculum; PreS, presubiculum;S, septal; Sub, subiculum; T, temporal; V, ventral.

Anatomy of the Hippocampus and the Declarative Memory System 49


homolog of the parahippocampal cortex in the primate

brain. The perirhinal and postrhinal cortices are themajor recipients of cortical afferents, and they project

heavily to entorhinal cortex. The entorhinal cortexand the pre- and parasubiculum also receive direct

cortical inputs. The entorhinal cortex projects directlyto all components of the hippocampal formation and

all other components of the parahippocampal region(Figure 3). The entorhinal connections with CA1, the

subiculum, and all other parahippocampal structures

are reciprocal.One practical problem in the comparative anat-

omy of these structures is the confusing use of the

term parahippocampal. In the rodent brain, the termhas only one use (i.e., in the phrase ‘parahippocampal

region’). In the human and monkey brains, the term is

used in two additional ways. First is the parahippo-campal cortex, a cortical region in the medial

temporal lobe that is a component of the parahippo-campal region (and is the homolog of the rodent

postrhinal cortex). Second, the parahippocampalgyrus is the fold or gyrus that contains a large portion

of the entorhinal, perirhinal, and parahippocampalcortices.

There are also discrepancies in the terminologyfor the perirhinal cortex. In Brodmann’s (1909)nomenclature, which includes verbal and numericterms, the verbal term for area 35 was perirhinal,and the verbal term for area 36 was ectorhinal.Although Brodmann defined area 36 as a very narrowstrip of cortex that did not include the temporal pole,other classic studies, which reported more detailedcytoarchitectonic analyses of these regions, includedthe temporal pole in area 36 (von Economo, 1929;Von Bonin and Bailey, 1947). There was no designa-tion in Brodmann’s nomenclature for the caudallylocated region we now call the parahippocampalcortex (reviewed in Suzuki and Amaral, 2003b).Using a different nomenclature, von Economo andKoskinsas named the rostral perirhinal/ectorhinalregion areas TG and TGa and the caudal (parahip-pocampal) region areas TF and TH. In modernterminology, the term perirhinal cortex was used todesignate the combined areas 35 and 36 (Amaralet al., 1987) or 35a and 35b (Van Hoesen andPandya, 1975). In the latter nomenclature, area 36was termed TL.

Currently, the most commonly used nomencla-ture for memory research in the primate brain isperirhinal cortex comprising areas 35 and 36.Burwell and colleagues (Burwell et al., 1995;Burwell, 2001) adapted that nomenclature for use inthe rodent brain. The term ectorhinal is no longer inuse except in rodent brain atlases. Thus, within acomparative framework for experimental neu-roscience, it seems reasonable to adhere to thenomenclature of perirhinal cortex as designating thecombined areas 35 and 36 for both the rodent andprimate brains.

3.03.1.2.2 Location of the hippocampal

system structures

The focus of this chapter is the rat hippocampalsystem about which we have the most detailed ana-tomical information, but it is worth noting that thereare surprising similarities and interesting differencesbetween these structures in the rodent and the pri-mate brains. The upper panel of Figure 1 shows thatthe hippocampus is C-shaped and relatively larger inthe rodent brain (Figure 1(c)). The dorsal or septalportion of the region is associated with the fimbria-fornix and the septal nuclei. The ventral or temporalportion of the structure is associated with the tem-poral cortices. The hippocampus is relatively smallerin the primate brain (Figure 1(b)). The structure isstill shaped like a C, though shallower and rotated

Figure 3 Simplified schematic of the hippocampal

system. The schematic includes the hippocampal formation(structures in yellow) and the parahippocampal region

(structures in red, blue, green, and orange). The

hippocampal formation comprises three-layered structurescharacterized by largely unidirectional connections,

whereas the parahippocampal region comprises six-

layered cortices characterized by reciprocal connections.

Note that the perirhinal and postrhinal cortices (PER andPOR) have reciprocal connections with CA1 and the

subiculum. Abbreviations: DG, dentate gyrus; EC,

entorhinal cortex; LEA, lateral entorhinal area; MEA, medial

entorhinal area; ParaS, parasubiculum; PER, perirhinalcortex; PH, parahippocampal cortex in the primate brain;

POR, postrhinal cortex in the rodent brain; PreS,

presubiculum; subiculum (SUB).



about 90� clockwise, such that the opening is pointingupward. In the primate brain, the rostral hippocam-pus is associated with the temporal cortices, and thecaudal hippocampus is associated with the septalnuclei. Accordingly, for cross-species comparisons,the best terminology for the long axis of the hippo-campus is the term septotemporal.

In the human brain, the rhinal sulcus is relativelysmall and is associated with only the most rostralportion of the perirhinal cortex. The collateral sulcusforms the lateral border of the parahippocampalgyrus (Figure 1(d)). As in the monkey brain, theentorhinal, perirhinal, and parahippocampal corticesoccupy the parahippocampal gyrus and the temporalpole. The perirhinal cortex occupies the temporalpole and continues caudally. The entorhinal cortexlies in the medial portion of the anterior parahippo-campal gyrus and is bordered rostrally and laterallyby the perirhinal cortex. The parahippocampal cor-tex forms the caudal border of the perirhinal cortex.

In the monkey brain, which is less gyrencephalic(smoother) than the human brain and more gyrence-phalic than the rat brain, the rhinal sulcus isassociated with the full extent of the perirhinal cor-tex. Area 35 is a narrow band of agranular cortex thatoccupies the fundus and the lateral bank of the rhinalsulcus. Area 36 is a larger strip of dysgranular cortexlocated lateral to area 35 and including the temporalpole (Figure 1(b) and 1(e)). All but the most rostralpart of the lateral border of area 36 is shared with areaTE of inferotemporal cortex. The rostrolateral bor-der is formed by the superior temporal gyrus. Suzukiand Amaral (2003a) extended the border of area 36rostrally and septally to include the medial half of thetemporal pole on cytoarchitectonic and connectionalgrounds. The monkey parahippocampal cortex, com-prising areas TH and TF, is located caudal to theperirhinal and entorhinal cortices (Figure 1(b) and1(e)). Area TF is larger than area TH and is laterallyadjacent to area TH. Area TH is a thin strip of largelyagranular cortex medially adjacent to area TF. Theparahippocampal cortex is bordered rostrally by theentorhinal and perirhinal cortices, laterally by TE,medially by the para- and presubiculum, and caud-ally by visual area V4.

In the rodent brain, the rhinal sulcus, or fissure asit is sometimes called, is the only prominent sulcus(Figure 1(c) and 1(f)). It extends along the entirelateral surface of the brain, though it is quite shallowin its caudal extent. The region is bordered rostrallyby the insular cortex. Insular cortex is classicallydefined as the region overlying the claustrum. The

transition from insular cortex to the perirhinal cortexoccurs when claustral cells underlying layer VI of thecortex are no longer visible. The perirhinal cortexcomprises two cytoarchitectonically distinct strips ofcortex, areas 35 and 36. Area 36 lies dorsally adjacentto area 35. The entorhinal cortex provides the ventralborder of area 35. The dorsal border of area 36 isformed by secondary somatosensory cortex, rostrally,secondary auditory cortex at midrostrocaudal levels,and ventral temporal association cortex at caudallevels. The postrhinal cortex is located caudal toperirhinal cortex and provides the caudal border. Itlies ventral to the ventral temporal area and dorsal tothe medial entorhinal cortex (Figure 2(f )).

3.03.1.2.3 Cross-species comparisons:

Human, monkey, and rodent

A comparative analysis of the unfolded maps of thehuman, monkey, and rat brains shows that the spatialrelationships of the perirhinal, parahippocampal/postrhinal, and entorhinal cortices are similar(Figure 1). Aside from the obvious differences inscale, the relative size differences are also interesting.Studies in rats, monkeys, and humans suggest that theperirhinal cortex accounts for roughly 3% of thecortical surface area, suggesting that the region scaleslinearly with cortical surface area. Also, in all threespecies, the surface area of the perirhinal cortex isroughly twice that of the postrhinal/parahippocam-pal cortex. Therefore, postrhinal/parahippocampalcortex also appears to scale linearly with brain size.The relative size of the entorhinal cortex, however,differs dramatically across species. In the rat brain, itssurface area is more than three times that of theperirhinal cortex, but in the primate brain, entorhinalcortex is considerably smaller than the perirhinalcortex.

The homology of the rodent postrhinal cortexwith the primate parahippocampal cortex is basedon the structural and connectional similarities. Inrodents and primates, the region receives substantialinput from visual associational, retrosplenial, andposterior parietal cortices. Subcortical connectionsare also similar. For example, the rat postrhinal cor-tex is strongly and reciprocally connected with thelateral posterior nucleus of the thalamus (LPO).Likewise, the monkey parahippocampal cortex isconnected with the pulvinar, the homolog of thelateral preoptic area (LPO) in the rodent.

In human, monkey, and rat, the entorhinal cortexis a six-layered cortex characterized by a cell sparselayer (lamina dissecans) separating the deep and



superficial layers. The medial part of the entorhinalarea is, structurally, more highly differentiated ascompared to the lateral part, and the lamina dissecansis more evident. It should be noted that in the rat, themedial entorhinal area is more caudal and ventral,whereas the lateral entorhinal area is more rostraland dorsal (Figure 1(c) and 1(f)). In both rat andmonkey, the intrinsic connectivity of the entorhinalcortex appears to be organized into intrinsic bands ofinterconnectivity that form discrete associationalnetworks. In the rat, these bands of intrinsic connec-tivity project to different levels of the dentate gyrus,suggesting a functional topography. There is evi-dence that a similar topography exists for themonkey.

All hippocampal formation structures observed inthe human brain are also present in the monkey andrat brains (Figure 2). The absolute size of the hippo-campal formation is largest in the human brain andsmallest in the rodent brain, though the structure isrelatively larger in the rodent brain. As previouslymentioned, the hippocampus is situated differently indifferent species. In the human and monkey brains, itis as if the hippocampal formation has swung downand forward, such that rostral hippocampus in theprimate brain is comparable to ventral hippocampusin the rodent brain. Similarly, caudal hippocampus in

the primate brain is comparable to dorsal hippocam-pus in the rodent brain. For ease of comparativeanalysis, it is most efficient to use the terms septaland temporal to describe the long axis because theseterms can be applied similarly across all species. Theseptotemporal axis in the rodent hippocampus isequivalent to dorsoventral axis, and the septotem-poral axis in the primate hippocampus is equivalentto the caudorostral axis.

3.03.2 The Parahippocampal Region

3.03.2.1 The Postrhinal Cortex

The postrhinal cortex is located near the caudal poleof the rat brain, caudal to the perirhinal cortex, dorsalto the rhinal sulcus and to the medial entorhinal area(Figure 1(c)). Usually the postrhinal cortex arises atthe caudal limit of the angular bundle when subicularcells are no longer present in coronal sections(Figure 4(a)). At this level, postrhinal cortex ischaracterized by the presence of ectopic layer IIcells at the perirhinal–postrhinal border near theventral border with the medial entorhinal cortex(Figure 4(b), arrow). Moving caudally, the postrhinalcortex rises dorsally above the caudal extension ofthe rhinal fissure and wraps obliquely around the

(a) (b)

Figure 4 Location and photomicrograph of the postrhinal cortex (POR). (a) Drawing of a coronal section of the rat brain at

the level of the rostral limit of the postrhinal cortex. (b) Nissl-stained coronal section showing the septal and temporalsubregions of the POR (PORd and PORv, respectively). Layers are labeled I–VI. The septal subregion has a more

differentiated laminar pattern. The ventral subregion is characterized by ectopic layer II cells (arrow) that appear near the

rostral border with area 36. Abbreviations: ab, angular bundle.



caudal pole of the brain. Visual association cortex,which forms the dorsal border of postrhinal cortex,has a more differentiated laminar pattern and abroader layer IV. The precise location of the dorsalborder is difficult to distinguish cytoarchitectoni-cally. A convenient landmark, however, is providedby the parasubiculum. The dorsal border of the post-rhinal cortex on the lateral surface tends to be at thesame dorsoventral level as the parasubiculum on themedial surface. The medial entorhinal cortex bordersthe postrhinal cortex ventrally and is easily distin-guished by the large layer II cells and distinct laminarlook of the cortex.

The cell layers of the postrhinal cortex have ahomogeneous look because the packing density ofcells is similar across layers (Figure 4(b)). In coronalsections, there is a broadening of the deep layers,which is due to the conformation of the region atthe caudal pole of the brain (Figure 4). In sagittalsections, however, layers II–III, V, and VI eachoccupy about one-third of the cortical depth. Theregion can be subdivided into dorsal and ventralsubdivisions based on cytoarchitectonic features. Ingeneral, the dorsal subregion has a more organizedand radial appearance. The primary differencebetween the two subdivisions is that the dorsal por-tion has a distinguishable granule cell layer IV.Another difference is that layer V of the dorsal sub-region is slightly narrower than in the ventralsubdivision.

Retrograde tract tracing studies show that three-quarters of postrhinal afferentation arise in neocor-tex. The remainder is roughly evenly dividedbetween subcortical and hippocampal afferents. Theneocortical connections of the postrhinal cortex dis-tinguish it from the nearby perirhinal cortex, in thatcortical input to the postrhinal cortex is stronglydominated by visual and visuospatial inputs. Interms of sensory input, the postrhinal cortex receivesalmost a third of its total input from visual associa-tional regions. The strongest associational inputarises in the posterior parietal cortex. Dorsal retro-splenial cortex also provides a strong projection. Theinput from frontal associational regions largely arisesin ventrolateral orbital frontal cortex. A strong inputarises in the caudal and ventral temporal area, whichis itself strongly interconnected with visual associa-tion cortices. For the most part, all corticalconnections are equally reciprocated. The exceptionis that the postrhinal cortex projects strongly to theperirhinal cortex, but the return projection is sub-stantially weaker.

The subcortical afferents are dominated by thethalamic inputs, which arise predominantly in thelateral posterior nucleus of the thalamus. That pro-jection is reciprocal. There is also input from theanteromedial dorsal thalamic group and the intrala-minar nucleus of the thalamus. The input from theamygdala is very small and is mainly from the lateraland basolateral nuclei. The postrhinal cortex alsoprojects back to the lateral and basolateral amygdalanuclei. The inputs from the septum are also relativelysmall and are dominated by the medial septum.

The postrhinal cortex projects strongly to themedial entorhinal cortex, particularly to the lateralband. The entorhinal projection is weakly reciprocal.Postrhinal cortex has strong reciprocal connectionswith the septal presubiculum and the parasubiculum.In addition to these parahippocampal connections,there are strong direct connections with the hippo-campus. The postrhinal cortex projects directly tothe septal CA1 and subiculum, and both projectionsare returned. Connections with the temporal hippo-campus are modest.

3.03.2.2 The Perirhinal Cortex

The perirhinal cortex arises at the caudal limit of theinsular cortex and can be distinguished from insularcortex by the absence of the underlying claustrum. Itis bordered dorsally by temporal association regions,ventrally by piriform and entorhinal cortex, andcaudally by the postrhinal cortex. For most of itsrostrocaudal extent, the perirhinal cortex includesthe fundus and both banks of the rhinal sulcus(Figure 1(f)). At its caudal limit, the region risesdorsal to the fundus. A signature feature of the peri-rhinal cortex in the rodent and monkey brains is thepresence of large heart-shaped cells in deep layer Vthat appear in both area 36 and area 35 (Figure 5).

Perirhinal area 36 is located dorsal to the rhinalsulcus. Although the region has a more prominentlaminar structure dorsally than ventrally, area 36 isgenerally described as dysgranular cortex. The dorsalborder of area 36 is best discerned by characteristicsof the granular cell layer, layer IV. Area 36 has afairly rudimentary layer IV as compared to the dis-crete granular layer of the dorsally adjacentneocortical areas. Another feature of the region isthe patchy layer II in which medium-sized cells areorganized in clumps or patches. The organization oflayer V cells into lines gives the region a radial look,especially dorsally. Layer VI has a bilaminar appear-ance in that the cells in the deep portion of the layer



are smaller, darker, and more densely packed thanthe cells in the superficial portion of the layer.

Area 35 is generally characterized by a broad layerI. Layers II and III tend to blend together (Figure 5).

The region lacks a layer IV and is thus considered

agranular cortex. Layer V of area 35 has a disorganizedlook as compared to the radial appearance in area 36.

As in area 36, layer VI has a bilaminate appearance. A

general characteristic of area 35 is the organization ofits cells into an arcing formation that spans all layers.

This feature is most evident below the rhinal sulcus.

The entorhinal cortex forms most of the ventral bor-der and can be distinguished from ventral area 35 by

the medium to large darkly staining stellate cells of

layer II and by the appearance of the lamina dissecans,

a cell-sparse area between layers III and V.The input to the perirhinal cortex is roughly

evenly divided between cortical and subcortical

structures. The perirhinal cortex receives input fromnearly all unimodal and polymodal associationalregions of neocortex, but there are subregional differ-ences. For example, area 36 receives roughly equalinput from olfactory, auditory, visual and visuospatial,and sensorimotor regions, whereas area 35 is domi-nated by olfactory input from piriform cortex. Thereare also subregional similarities and differences inpolymodal association input. Area 36 receives thelargest cortical input from temporal associationregions followed by insular and frontal regions. Incontrast, area 35 receives the larger input from insularcortex followed by temporal association and frontalregions. Of course, there is also a heavy intrinsic inputfrom area 36. Areas 36 and 35 each receive only smallinputs from posterior associational regions. As wouldbe expected, these associational connections are lar-gely reciprocal.

Perirhinal areas 36 and 35 are also differentiatedby subcortical connections. The strongest subcorticalconnections of area 36 are with the amygdala. Theafferent input arises largely in the lateral nucleus, butthe basolateral and basomedial nuclei also providesubstantial inputs. Substantial thalamic input ariseslargely in the dorsolateral group and in the reticularthalamic nucleus. In contrast, area 35 receives itsstrongest subcortical afferents from olfactory struc-tures, primarily from the endopiriform nucleus, butalso from the piriform transition area. Other substan-tial inputs arise in the amygdala, the midline andlateral thalamic groups, and the medial geniculatenucleus of the thalamus.

Like the postrhinal cortex, the perirhinal cortexprojects strongly to the lateral entorhinal cortex. Theprojection arises in area 35 and terminates mostheavily in the so-called lateral band of the entorhinalcortex (see following). The entorhinal projection isweakly reciprocated. Area 36 is weakly connectedwith hippocampal and subicular structures, althoughthese connections may be functionally important.Area 35 receives input back from the septalpresubiculum. The strongest projection back to area35 arises in temporal CA1, but it also receives inputfrom temporal subiculum and presubiculum.Other smaller inputs arise in septal CA1 and theparasubiculum.

3.03.2.3 Entorhinal Cortex

The entorhinal cortex is of considerable interest inmemory research. Not only does it provide the majorconduit for sensory information to the hippocampal

Figure 5 Photomicrographs of the perirhinal (PER),postrhinal (POR) and lateral and medial entorhinal cortices

(LEA and MEA). (a) Nissl-stained coronal section showing

PER areas 36 and 35 and the LEA. (b) Adjacent section

stained for parvalbumin. Heavy staining of layer II in the LEAprovides a useful marker for the PER-LEA border. (c) Nissl-

stained coronal section showing MEA and POR. (d)

Adjacent section stained for parvalbumin. Layers arelabeled I–VI. Parvalbumin staining also differentiates the

MEA-POR border. Other abbreviations: AM, amygdala; CP,

caudate putamen.



formation, but a number of recent discoveries also

suggest that the region may make unique contribu-

tions to the processing of spatial information.

The entorhinal cortex is a relatively large and

complicated structure, and its connections are topo-

graphically organized. Thus, understanding the areal

differences in entorhinal structure could provide

insight into its role in memory.In rats and other animals, the entorhinal cortex has

been divided into two subdivisions roughly equivalent

to modern definitions of the lateral and medial ento-

rhinal areas (LEA and MEA, Figure 1(f )) (Brodmann,

1909; Krieg, 1946). The LEA (Figure 5, top) is perhaps

most easily distinguished from the MEA (Figure 5,

bottom) by differences in layer II. LEA has a very

clumpy layer II as compared to themore homogeneous

layer II of the MEA. The sparsely populated layer IV,

also called the lamina dissecans, is considered a land-

mark feature of the entorhinal cortex, but there are

subregional differences. In general, the LEA exhibits a

less prominent lamina dissecans as compared to the

MEA (compare Figure 5).Some time ago, the monkey entorhinal cortex was

further subdivided on the basis of structural and

connectional criteria (Van Hoesen and Pandya,

1975; Amaral et al., 1987). The rat entorhinal cortex

has now been subdivided into six fields according to

similar criteria (Figure 6(a)) (Insausti et al., 1997).

The LEA comprises four fields: the dorsal lateral

entorhinal field (DLE), the dorsal intermediate

entorhinal field (DIE), the amygdalo-entorhinal tran-

sitional field (AE), and the ventral intermediate

entorhinal field (VIE). Each field has unique connec-

tional and/or structural characteristics. The medial

entorhinal area (MEA) is subdivided into a caudal

field (CE) and a medial field (ME). Medially, the

MEA is bordered by the parasubiculum. The MEA

border with the parasubiculum is marked by a layer

II that thickens into a characteristic club-shaped

formation.The intrinsic connections of the entorhinal cortex

are organized in a rostrocaudal manner, such that the

cells located in each of three bands of the entorhinal

area are highly interconnected but do not project

outside the band of origin (Figure 6(b)) (Dolorfo

and Amaral, 1998). Interestingly, each band of intrin-

sic connectivity spans the MEA and LEA. An

important recent discovery about these regions has

to do with the relationship of these bands of intrinsic

connectivity with the perforant pathway, the ento-

rhinal projection to the dentate gyrus. Briefly, the

lateral band projects to the septal half of the dentate

gyrus, whereas the intermediate and medial bands

project to the third and fourth septotemporal quar-

ters, respectively (Figure 6(c)). This connectional

topography suggests that functional diversity within

the entorhinal cortex may be in register with func-

tional diversity in the hippocampus.The entorhinal cortex is strongly connected with

other parahippocampal region structures. Perirhinal

(a) (b) (c)

Figure 6 Unfolded maps of the entorhinal cortex and the target of the perforant pathway, the dentate gyrus. (a) Unfolded

map of the rodent entorhinal cortex showing the LEA in light green and the MEA in dark green. Further parcellation of eachsubregion is noted by black lines (Insausti et al., 1997). (b) Unfolded map of the rodent entorhinal cortex showing the lateral

(LB) in dark green, the intermediate band (IB) in medium green, and medial band (MB) in pale green. (c) The unfolded dentate

gyrus, color coded to denote the terminations of the perforant pathway. The entorhinal LB projects to the septal half of thedentate gyrus, the IB projects to the third quarter, and the MB projects to the temporal quarter. Abbreviations: AE, amygdalo-

entorhinal transitional field; CE, caudal entorhinal field; D, dorsal; DLE, dorsal lateral entorhinal field; DIE, dorsal intermediate

entorhinal field; L, lateral; M, medial; ME, medial entorhinal field; S, septal; T, temporal; V, ventral.



input arises largely in area 35 and terminates prefer-entially to the lateral band of the LEA. Thepostrhinal input arises in all portions of the regionand terminates primarily in the lateral band of theMEA. There is a heavy return projection to peri-rhinal cortex that arises in all layers and all portionsof the entorhinal cortex, though the strongest projec-tion arises in the lateral band. Strong inputs originatefrom the pre- and parasubiculum. The parasubicu-lum targets the entire entorhinal cortex, septalpresubiculum projects more heavily to the MEA,and temporal presubiculum projects more heavilyto the LEA. The entorhinal cortex provides modestreciprocal connections with the pre- and parasubicu-lum (Witter and Amaral, 2004).

The entorhinal cortex has neocortical connec-tions, through weaker than perirhinal and postrhinalcortices. The LEA receives very strong input fromthe piriform and agranular insular cortices. Medialand orbital frontal regions provide a strong projec-tion. Input from the cingulate, parietal, and occipitalcortices is relatively weak. There is little differentia-tion across the lateral to medial bands. Piriformcortex also projects to the MEA, but the projectionterminates in the lateral and intermediate bands. Incontrast, the lateral band receives moderate to strongprojections from frontal, cingulate, parietal, and oc-cipital cortices. Projections to the medial frontal andolfactory structures tend to arise in the intermediateand medial bands. A very narrow strip of the ento-rhinal cortex that is positioned closest to the rhinalfissure gives rise to the major projections to other

cortical areas, including the lateral frontal, temporal,parietal, cingulate, and occipital cortices.

The entorhinal cortex has widespread connec-tions with subcortical structures, and it is possiblethat the subcortical afferents are as influential as thecortical afferents. Strong projections arise in claus-trum, olfactory structures, the amygdala, and dorsalthalamus. The olfactory input arises in the endopiri-form nucleus and the piriform transition area and isstronger to the LEA than the MEA. The dorsal tha-lamic input arises primarily in the midline thalamicnuclei and is stronger to MEA than to LEA. The LEAand MEA receive input from septal nuclei, thoughthe inputs are relatively small. The amygdala inputarises in all nuclei except the central nucleus andamygdalohippocampal area and is stronger to LEAthan MEA. In addition, the entorhinal cortex projectsto all amygdaloid structures except the nucleus of thelateral olfactory tract and the central nucleus(Pikkarainen and Pitkanen, 1999).

The entorhinal cortex projects to all hippocampalformation structures including the dentate gyrus, fieldsCA3, CA2, and CA1 of the hippocampus proper, andthe subiculum (reviewed in Witter and Amaral, 2004).The entorhinal projections to the dentate gyrus, CA3,and CA2 originate in layer II of the entorhinal cortex.The terminations of the layer II projections exhibit aradial topography in that the LEA terminates in theouter DG molecular layer, whereas the MEA projectsto the middle DG molecular layer (Figure 7). Theprojections to CA1 and the subiculum originate inlayer III. The terminations of the layer III projections

(a) (b)

Figure 7 Illustration of the radial and transverse topography of the entorhinal projection to the hippocampal formation.

(a) Unfolded map of the entorhinal cortex showing the LEA in light green and the MEA in dark green. (b) Line drawing of a

hippocampal section perpendicular to the long axis. Adapted fromWitter MP and Amaral DG (2004) Hippocampal Formation.In: Paxinos G (ed.) The Rat Nervous System, 3rd ed. San Diego, CA: Academic Press. The terminations of the LEA are in light

green, and the terminations of the MEA are in dark green. The projection to the DG exhibits a radial topography, whereas the

projection to CA1 and subiculum (sub) exhibits a transverse topography. See text for details.



exhibit a transverse topography such that the LEAprojects to distal CA1 and proximal subiculum, andthe MEA projects to the proximal CA1 and distalsubiculum. The organization of the CA1 and subicularprojections back to deep layers of the LEA and MEAroughly reciprocates the forward projections.

3.03.2.4 Presubiculum

The presubiculum is bordered dorsally by retrosple-nial cortex, medially by the subiculum, andventrolaterally by the parasubiculum (Figures 2(e),2(f), and 8(a)). Areas 48 and 27 according toBrodmann (1909) are both included in the presubi-culum. Area 48, the most dorsal extension of thepresubiculum, is sometimes called the postsubicu-lum. Because the presubiculum and this dorsalcomponent exhibit considerable cytoarchitectonicsimilarities, it may be more appropriate to designatethe area collectively with a single term.

Layer II of the six-layered presubiculum is thickand contains small, densely packed, and darkly stain-ing pyramidal cells (Figure 8(b)). Cells in layer IIIare even smaller, round, and also darkly staining.Whereas cells in layer II tend to form clusters, cellsin layer III have a more homogeneous look. Layer IIIis separated from the deep layers by a narrow, spar-sely populated gap that is continuous with the lamina

dissecans of the parasubicular cortex. Deep to this

cell-sparse gap are two layers. Layer V is very thin

and contains pyramidal cells. Layer VI is slightly

thicker and contains a mixture of cell types.As it turns out, acetylcholinesterase (AChE) is an

excellent marker for the presubiculum. Layer II

stains moderately darkly (Figure 8(c)). Deep to

layer II is a dark band that contains layers III, the

cell-sparse gap, and layer V. Layer VI is moderately

to lightly stained in AChE preparations. AChE is also

a good marker for the parasubiculum.The presubiculum has extensive associational,

commissural, and hippocampal parahippocampal

connections (Witter and Amaral, 2004). The septal

and temporal parts of the presubiculum are highly

interconnected. Connections with the contralateral

presubiculum are also extensive, though commissural

connectivity may be stronger ventrally than dorsally.

The presubiculum provides a weak input to the

dentate gyrus and all fields of the hippocampus

proper. It is reciprocally connected with the subicu-

lum. The presubiculum projects to superficial layers

of the subiculum. The projection to the septal sub-

iculum is moderately strong, and the temporally

directed projection is relatively weak. The input

from the subiculum terminates in layer I.Regarding parahippocampal connectivity, the

presubiculum projects to superficial layers of the

(a) (b) (c)

Figure 8 Photomicrographs of the presubiculum (PreS) and parasubiculum (ParaS). (a) Drawing of a coronal section of the

rat brain at the level of the angular bundle (ab). The inset designates the areas shown in panels (b) and (c). (b) Nissl-stainedcoronal section showing the PreS and ParaS. Layer II is outlined for the PreS, and the combined layer II/III is outlined for

ParaS. (c) Adjacent section stained for acetylcholinesterase (AChE). AChE provides an excellent marker for these regions.



parasubiculum, but the connections with the entorhi-nal cortex are by far the strongest. The presubicular–entorhinal projection is bilateral, largely directed tomedial entorhinal cortex, and almost exclusively ter-minates in layer III. Septal presubiculum projectsmuch more heavily to the MEA than the LEA, buttemporal presubiculum projects heavily to both en-torhinal divisions. Septal presubiculum also providesa moderately heavy input to the postrhinal cortex.The dorsal extension (Brodman’s area 48, sometimestermed the postsubiculum) projects massively topostrhinal cortex. Temporal presubiculum projectsheavily to the LEA and the MEA and moderatelyheavily to perirhinal areas 36 and 35. Septal presubi-culum receives heavy input from postrhinal cortexand a moderately heavy input from the MEA portionof the lateral band. Temporal presubiculum receivesa very heavy input from the MEA portion of themedial band, weak input from the LEA, and virtuallynothing from the perirhinal and postrhinal cortices.

The heaviest neocortical input to the presubicu-lum arises in the granular retrosplenial cortex, butweaker inputs arise in prelimbic cortex, dorsomedialprefrontal areas, and the anterior cingulate cortex(Witter and Amaral, 2004). The primary subcorticalinputs are from the dorsal thalamus, specifically, theanteroventral, the anterodorsal, and the laterodorsalnuclei. The presubiculum receives subcortical inputfrom the thalamus, primarily the anterior thalamicnuclei including the anteroventral, anterodorsal, andlaterodorsal nuclei. A massive return projection tar-gets the same nuclei. There is also a strongcholinergic input arising from septal nuclei. Finally,the presubiculum has reciprocal connections with themamillary nuclei of the hypothalamus.

3.03.2.5 The Parasubiculum

For most of its rostrocaudal extent, the parasubicu-lum is bordered by presubiculum dorsally and themedial entorhinal area ventrally (Figure 2(e) and2(f)). At more caudal levels, the parasubiculum isinterposed between the postrhinal cortex and themedial entorhinal area. A broad, combined layer II/III contains large, densely packed, moderately darklystaining pyramidal cells. This layer is separated fromthe deep layers by a broad lamina dissecans. Layers Vand VI can be distinguished from one another andtend to run continuously with deep layers of themedial entorhinal area. In AChE preparations, layersI and II/III are darkly stained (Figure 8(c)). The

lamina dissecans and layer V are lightly stained, andlayer V is moderately darkly stained.

The parasubiculum has associational connectionsthat project dorsally and ventrally. The ventralprojections are heavier and more extensive thanthe dorsal ones. Commissural projections terminatein layers I and III of the contralateral homotopicregion.

The hippocampal input to the parasubiculumarises mainly in the subiculum and terminates inlayer I and superficial layer II. There are also returnprojections to the hippocampal formation. The struc-ture projects directly to the molecular layer of thedentate gyrus. This is especially interesting giventhat the parasubiculum receives strong inputs fromanterior thalamic nuclei. As has been previouslynoted, the anterior thalamic projection to the para-subiculum provides a pathway by which the anteriorthalamus can affect hippocampal processing ofincoming information at very early stages.

Like the presubiculum, the parasubiculum exhibitssubstantial connections with other parahippocampalstructures. The parasubiculum projects selectively tolayer II of the entorhinal cortex. The entorhinal pro-jection is much heavier to MEA than to the LEA.Interestingly, the parasubicular projection to POR iseven heavier than that to the MEA. Parahippocampalinputs arise mainly from the MEA, with the medialband providing the heaviest return projection. Thereis also a modest presubicular input.

Extrinsic connections of the parasubiculum arefew. The only neocortical afferents arise in retro-splenial cortex and visual cortex, and these inputsare quite weak. Other than the input from the ante-rior thalamus, the only other subcortical afferentsarise in the amygdala from the lateral, basal, andaccessory basal nuclei.

3.03.3 The Hippocampal Formation

The structures of the hippocampal formation aregrouped together partly because of the sequentialactivation pattern that was identified several decadesago. The entorhinal cortex activates the dentategyrus via the perforant pathway, the mossy fiberpathway from the denate gyrus activates CA3, andthe CA3 Schaffer collaterals activate CA1. Some ofthe earliest and most famous studies of the structureof the nervous system were conducted by Ramon yCajal, who used a technique developed by CamilloGolgi for darkly staining a small number of neurons



in the brain. Cajal’s elegant studies and drawings,

including the rodent hippocampus (Figure 9), pro-

vided the basis for the neuron doctrine.Because of the complex architecture of the hippo-

campus, it is helpful to describe its structure in terms

of three axes, the longitudinal, transverse, and radial

axes. As previously discussed, we use the term

septotemporal for the longitudinal axis of the hippo-campus. Along the transverse axis, which isorthogonal to the long axis of the hippocampus, thedentate gyrus can be considered the proximal limitand transverse locations designated according toposition relative to the dentate gyrus. Thus, the partof the CA3 lying in the V of the dentate gyrus isproximal CA3, and the part closest to CA1 is distal(Figure 10). Similarly, the part of CA1 closest toCA3 is proximal, and so on. Finally, the laminarstructure is perpendicular to the radial axis. In thisterminology, the molecular layers are superficial andthe layers on the opposite side of the principle celllayers are deep.

Based primarily on electrophysiological data andmapping of vasculature, Anderson and colleagues(1971) proposed that the hippocampal formationwas organized in parallel lamellae stacked along thelongitudinal axis. They further proposed that thislamellar organization would permit strips, or slabs,of the hippocampus to function as independent units.Although the lamellar hypothesis shaped research onthe hippocampus for years to come and continues toinfluence modern concepts of hippocampal function,modern neuroanatomical research has revealed thatthe hippocampal projections are much more diver-gent than is suggested by the lamellar hypothesis.Indeed, the major hippocampal and dentate associa-tional projections extend along the septotemporalaxis as well as the transverse axis.

3.03.3.1 The Dentate Gyrus

The dentate gyrus is three-layered cortex whoseprinciple cell layer is V shaped (Figure 10). Themolecular layer lies outside the V, and the poly-morphic layer lies inside the V. The beginning ofthe CA3 principle cell layer protrudes into the poly-morphic area of the dentate gyrus. This conformationhas generated some confusion over the borderbetween CA3 and the dentate gyrus polymorphiclayer, as well as the identity of these cells. In earliernomenclatures, and occasionally in modern reports,the part of CA3 next to the dentate gyrus was some-times called CA4. With modern techniques fordefining connectional characteristics, however, it isnow clear that those pyramidal cells belong to CA3.

The dentate granule cell layer contains small,very densely packed, oval cells that have a darkappearance in cell stains (Figure 10(a)). Each gran-ule cell has a small number of primary dendrites (oneto four) that are covered with spines. The dendrites

Figure 9 Drawing of the circuitry of the hippocampal

formation by Ramon y Cajal (1909). Cajal proposed that the

nervous system is made up of countless separate units, ornerve cells composed of dendrites, soma, and axons, each

of which is a conductive device. He further proposed that

information is received on the cell bodies and dendrites

and conducted to distant locations through axons.Abbreviations: A, retrosplenial area; B, subiculum; C,

Ammon’s horn; D, dentate gyrus; E, fimbria; F, cingulum; G,

angular bundle; H, corpus callosum; K, recurrent collaterals;a, axon entering the cingulum; b, cingulum fibers; c-e,

perforant path fibers; g, subicular cell; h, CA1 pyramidal

cells; i, Schaffer collaterals; collaterals of alvear fibers.



extend into the molecular layer all the way to thehippocampal fissure. In the subgranular region,between the granule cell layer and the polymorphiclayer, there are several cell types, most of whichare immunoreactive for gamma-aminobutyric acid(GABA). A subset of these cells is also immunoreac-tive for the calcium-binding protein, parvalbumin(Figure 10(c)). The most prominent types are thebasket cell and the axo-axonic, or chandelier, cell.The basket cell interneurons are quite large with asingle, aspiny apical dendrite extending into the mo-lecular layer and several basal dendrites extendinginto the polymorphic layer. Axo-axonic cells have adendritic tree of radial branches extending throughthe molecular layer. The axon arborizes extensivelyin the granule cell layer.

The molecular layer contains mostly dendritesfrom cells of the granule and polymorphic layers. Inmaterial stained for heavy metals using the Timm’smethod, it is possible to visualize the three sublayersof the molecular layer (Figure 10(b)). The inner thirdof the molecular layer, next to the lightly coloredgranule cell layer, stains a reddish brown. The middlesublayer stains yellow, and the outer sublayer stainsorange. Though the molecular layer mostly containsdendrites, there are a few cell types that stain for VIP,GABA, and parvalbumin. One interesting type ofGABA-immunoreactive cell, with its soma in themolecular layer, has an axon restricted to the outertwo-thirds of the molecular layer. Thus, its terminalfield coincides with the perforant path terminations inthe dentate molecular layer.

The polymorphic layer contains a number of celltypes, but the best characterized is the mossy cell.These large multipolar cells are so named because oftheir large dendritic spines, the so-called thornyexcrescences. The mossy fiber axons from the gran-ule layer terminate on these spines. There are anumber of interneurons with somata in the poly-morphic layer that make inhibitory connections onthe dentate granule cells. One such cell type is thehilar perforant-path (HIPP) associated cell, whichhas a dendritic tree in the polymorphic layer butextensive axonal arbor in the outer two-thirds ofthe molecular layer where the perforant path inputterminates. Another type is the hilar commissural-associational pathway (HICAP) related cell, whichinnervates the inner molecular layer, the molecularlayer perforant-path (MOPP) associated cell. Thedendritic tree and axonal arbor of this cell occupythe outer two-thirds of the molecular layer, wherethe perforant path input terminates.

gcl

gcl

gcl

gcl

Figure 10 Photomicrographs of the hippocampalformation. (a) Nissl-stained coronal section showing the

dentate gyrus (DG) and hippocampus proper, comprising

fields CA3, CA2, and CA1. (b) Adjacent section stained for

heavy metals using the Timm’s method. (c) Parvalbumin-stained section showing the same regions. Layers of the DG

are the outer molecular layer (ml), the granule cell layer (gcl),

and the polymorphous layer (pol). The CA fields contain theouter stratum lacunosum-moleculare (slm), the stratum

radiatum (sr), the gcl, and the stratum oriens (so). CA3 also

contains the stratum lucidum (sl). The dashed line

demarcates the border between the DG and CA3. The solidline shows the location of the hippocampal fissure and

demarcates the border between DG and CA1.



The entorhinal cortex provides the only corticalinput to the dentate gyrus through the perforantpathway. There is substantial subcortical input fromthe septal nuclei, the hypothalamus, and the brainstem modulatory systems. The cholinergic inputfrom the septal region originates in the medial septalnucleus and in the nucleus of the diagonal band ofBroca. It is topographically organized such that cellsin the medial septal areas tend to terminate septally,and cells in lateral septal structures terminatetemporally. The projection terminates in the poly-morphic layer. The primary hypothalamic input isfrom the supramamillary area and terminates in anarrow band of the molecular layer just superficialto the granule cell layer. This projection is probablyexcitatory and appears to target both granule cellsand interneurons. The dentate gyrus receives inputfrom each of the modulatory neurotransmitter sys-tems in the brainstem. The noradrenergic input isfrom the pontine nucleus of the locus coeruleus andterminates in the polymorphic layer. The serotoner-gic input is from several of the raphe nuclei and alsoterminates in the polymorphic region. Minor dopa-minergic inputs arise in the ventral tegmental areaand the substantia nigra.

The only output of the dentate gyrus is the gran-ule cell mossy fiber projection to CA3. Axoncollaterals of each cell project to the full extent ofthe transverse axis. This is the one component of thehippocampal circuitry that does show a lamellar pro-jection pattern. The fibers travel along or within theCA3 pyramidal layer, eventually reaching the stra-tum lucidum. In addition to the CA3 projection, themossy fibers give rise to an associational connectionconsisting of about seven collaterals that terminate inthe dentate gyrus polymorphic layer before enteringCA3. The mossy fiber axons can be easily identifiedin Timm’s preparations in which they stain verydarkly (Figure 10(b)).

3.03.3.2 The Hippocampus Proper

The hippocampus proper consists of three fields.Proximal to distal from the dentate gyrus, they arethe Ammon’s horn fields CA3, CA2, and CA1(Figure 10). Like the dentate gyrus, these fields area three-layered cortex consisting of a principle layerlocated between cell-sparse layers. Deep to the prin-ciple cell layer is the stratum oriens. Superficial tothe principle cell layer are the stratum radiatum andthe stratum lacunosum-moleculare. CA3 has an addi-tional thin layer, the stratum lucidum, which lies just

superficial to the principle cell layer and deep to thestratum radiatum.

The principal cell layer consists, primarily, ofpyramidal cells. The pyramidal cells in CA3 arelarger and the layer thicker compared with CA1.The small and often-overlooked field CA2 containslarger pyramids, similar to CA3, but is similar to CA1in other ways. For example, it lacks mossy fiber input.Each pyramidal cell has an apical dendritic arborextending upward through the stratum radiatumand the stratum lacunosum-moleculare and a basaldendritic arbor extending into the stratum oriens.CA3 cells proximal to the dentate gyrus have smallerdendritic trees than the cells distal to the dentategyrus, but overall, the dendritic arbors of cells inCA3 are larger than those in CA1. The dendriticarbors of pyramidal cells in CA2 are mixed, somewith large arbors, similar to CA3, and some withsmall arbors, similar to CA1.

Interneurons undoubtedly play an importantrole in the regulation of local circuits in the hippo-campus proper. Hippocampal interneurons differ inmorphology, immunoreactivity, synaptic properties,laminar location, and connectivity. Hippocampal in-terneurons are GABAergic, but they may also beimmunoreactive for somatostatin, neuropeptide Y,vasopressin, cholecystokinin, parvalbumin, calbindin,and calretinin. Most hippocampal interneurons haveshort axons, but there are also interneurons with longaxons that project outside the hippocampal forma-tion. We mention a few types here, but for a fulldiscussion, see Freund and Buzsaki (1996).

One prominent interneuron type in the pyramidalcell layer is the chandelier or axo-axonic cell. Theapical dendrites are radially oriented and span allsuperficial layers to the hippocampal fissure. Thebasal dendrites form a thick arbor in the stratumoriens. There is also a heterogeneous group of basketcells whose apical dendrites extend into the stratummoleculare and whose extensive basal dendrites spanthe entire depth of the stratum oriens. The axo-axonic cells and the basket cells are convenientlypositioned to receive excitatory input from all affer-ents of the hippocampus proper.

Some hippocampal interneuron cell types havecell bodies in stratum oriens and innervate principalcell dendrites, similar to those described for thedentate gyrus. The oriens lacunosum-moleculare(O-LM) cells have a dense axonal arbor restrictedto the stratum lacunosum moleculare. The dendritictree is localized to layers that receive recurrentcollaterals. There are many other interneuron



types including the bistratisfied, horizontal trilami-nar, and radial trilaminar cells.

The CA3 pyramidal cell axons are highly col-lateralized and project to all CA fields bothipsilaterally and contralaterally. There is also asmall collateral projection to the polymorphiclayer of the dentate gyrus. CA3 does not, however,project to the subiculum, presubiculum, parasubi-culum, or entorhinal cortex. The projection to CA1is called the Schaffer collateral projection, the pro-jections to CA3/CA2 are called the associationalprojections, and the projections to contralateralstructures are called the commissural projections.The Schaffer collateral projection exhibits a topo-graphy such that proximal CA3 cells (closer to thedentate gyrus) tend to project to levels of CA1 thatextend farther in the septal than temporal direc-tions. Distal CA3 cells (farther from the dentategyrus) tend to project farther in the temporaldirection. In addition, projections of proximalCA3 cells tend to terminate more superficially instratum radiatum, whereas distal CA3 cells tend toterminate more deeply in stratum radiatum and instratum oriens. The associational projections exhib-it complex transverse and radial topographies, butin general the CA3-CA3 associational projectionsterminate extensively along the septotemporal axis(Witter and Amaral, 2004). The pattern of theterminations of the commissural projections mirrorsthose of the associational projections.

Extrinsic connections of CA3 are not robustexcept for the substantial projection to the lateralseptal nucleus. The major subcortical input to CA3is cholinergic and arises in the medial septal nucleusand the nucleus of the diagonal band of Broca. Thereis a GABAergic component of the projection thatterminates primarily on the GABAergic interneuronsof the stratum oriens. Temporal CA3 receives aminor input from the amygdala basal nucleus,which terminates in the stratum oriens and the stra-tum radiatum. Inputs from piriform cortex have alsobeen reported. A noradrenergic projection arises inthe locus coeruleus, and a serontonergic input arisesin the raphe nucleus.

Field CA2 can be differentiated from CA3 by thelack of mossy fiber input and the associated thornyexcrescences (Figure 10(b)). The pyramidal celllayer also stains more intensely for parvalbumin(Figure 10(c)). The intrinsic projections are similarto those of CA3, although the topographies may notbe the same. For example, like CA3, CA2 also pro-vides a small collateral projection back to the dentate

gyrus. Not much is known specifically about CA2extrinsic connections, but available evidence suggeststhat the region is differentiated from CA3 byhypothalamic input from the supramamillary area.

Field CA1 exhibits only a weak associational/commissural connection, a feature that is in strikingcontrast to the robust associational network presentin CA3. This difference has been interpreted asunderlying some of the putative functional differ-ences in CA3 and CA1. Other intrahippocampalconnections, however, are extensive. CA1 interneu-rons project to CA3 and to the polymorphic layer ofthe dentate gyrus. The major projection from CA1,however, is to the subiculum, and that projectionexhibits a strict topography. Distal CA1 projects toproximal subiculum, and proximal CA1 projects todistal subiculum. The mid-CA1 projection termi-nates in midproximodistal subiculum.

Field CA1 receives substantial cortical and subcor-tical input from extrahippocampal structures. Corticalinput arrives from the perirhinal, postrhinal, and en-torhinal cortices. Subcortical input to CA1 is grosslysimilar to the subcortical input to CA3 but differs inthe details. The septal input is weaker and terminatesin stratum oriens. The input from the amygdala ismore substantial, especially to distal CA1. Amygdalainput arises in the basal and accessory basal nuclei.There is a prominent input from the nucleus reuniensof the thalamus that terminates in the stratum lacuno-sum moleculare. Like CA3, CA1 receives weaknoradrenergic input from the locus coeruleus andweak serotonergic input from the raphe nucleus.There is also a weak dopaminergic input.

Of the hippocampal CA fields, CA1 has the morerobust extrinsic projections. The cortical projectionsinclude the perirhinal, postrhinal, entorhinal, retrosple-nial cortices, preinfralimbic, and medial prefrontalcortex. In general, the septal half projects more heavilyto postrhinal cortex, the medial entorhinal area, andretrosplenial cortex, whereas the temporal half of CA1projects more heavily to perirhinal cortex, the lateralentorhinal area, and infralimbic cortex. Temporallevels also project to the anterior olfactory nucleus,the hypothalamus, nucleus accumbens, and the basalnucleus of the amygdala.

3.03.3.3 The Subiculum

The subiculum is widely considered the outputstructure of the hippocampal formation. In this way,it differs from its parahippocampal neighbors, thepre- and parasubiculum, which are considered to be



input structures. Like the CA fields of the hippocam-pus proper and the dentate gyrus, the subiculum is athree-layered cortex with a deep, polymorphic layer,a pyramidal cell layer containing the principle cells,and a molecular layer, which is continuous with thestratum lacunosum moleculare of field CA1. Thesubiculum can be distinguished from the proximallysituated CA1 and the distally situated presubiculumby a principle cell layer that is more loosely packed.The border with CA1 is further demarcated by thewidening of the middle layer of the subiculum.

The principle cell layer contains large pyramidalcells. The basal dendrites terminate in the deep partof the principle layer, and the apical dendrites extendinto the molecular layer. The pyramidal cells arelarge and of uniform shape. Electrophysiologicalfindings suggest that there are two populations ofpyramids, though they cannot be distinguished mor-phologically. So-called regular spiking cells tend tobe located superficially, and bursting cells tend to belocated deep in the layer. Although both types areprojection cells, it is possible that only the burstingcells project to the entorhinal cortex. Among thepyramids are numerous smaller cells, probably repre-senting varied types of interneurons. Perforantpathway fibers contact GABAergic cells that stainfor parvalbumin. Not much is known about subicularinterneurons, but in general, the population of inter-neurons appears similar to that observed in field CA1(Witter and Amaral, 2004).

The associational connections of the subiculumextend temporally from the point of origin and ter-minate in all layers. There is no commissuralprojection. There are also local associational connec-tions confined to the pyramidal layer and the deepestpart of the molecular layer. The available data sug-gests that the bursting pyramidal cells form acolumnar network that is roughly interconnected.

Connections of the subiculum with other hippo-campal structures is limited to input from CA1,which is massive. The projection exhibits a topogra-phy such that proximal CA1 projects to distalsubiculum, midproximodistal CA1 projects to mid-subiculum, and distal CA1 projects to proximalsubiculum. The projection is not truly lamellar, how-ever, as any part of CA1 projects to about one-thirdof the septotemporal extent of the subiculum.

The parahippocampal connections of the subicu-lum are more diverse, but the best-characterizedprojection is to deep layers of entorhinal cortex.Septal levels of the subiculum provide substantialinput to the lateral and medial entorhinal cortices.

Septal subicular input to the postrhinal cortex isequally strong, but input to perirhinal cortex, espe-cially area 35, is modest. Temporal subiculumprovides massive input to the entorhinal cortex andmoderate input to perirhinal and postrhinal cortices.The subiculum also receives a substantial input fromthe entorhinal cortex. The entorhinal lateral bandprojects more strongly to septal subiculum, and theentorhinal intermediate and medial bands projectmore strongly to temporal subiculum. There is alsomodest input from the perirhinal and postrhinal cor-tices. Perirhinal cortex projects relatively morestrongly to temporal subiculum, and the postrhinalcortex projects relatively more strongly to septalsubiculum. The subiculum also projects heavily tothe pre- and parasubicular cortices, though the returnprojections are modest (O’Mara et al., 2001).

The most prominent neocortical projections are toretrosplenial and prefrontal cortices. The distal andseptal part of the subiculum projects to the ventralretrosplenial cortex. The presubiculum projections tofrontal areas include the medial orbital, prelimbic,infralimbic, and anterior cingulate cortices. The ret-rosplenial projection is reciprocated, but availableevidence suggests that the frontal projections are not.

The diverse subcortical projections target the sep-tal complex, the amygdala, the nucleus accumbens,the hypothalamus, and the thalamus. All septotem-poral levels of the subiculum project to the lateralseptum, but the projection arises primarily in theproximal part. Septal input arises mainly in thenucleus of the diagonal band. The amygdala projec-tion arises primarily in the temporal subiculum andtargets the posterior and basolateral nuclei. The pro-jection to the nucleus accumbens is topographic suchthat the proximal part of the septal subiculum pro-jects to rostrolateral nucleus accumbens, and theproximal part of the temporal subiculum projects tothe caudomedial nucleus accumbens. The hypotha-lamic projection also arises primarily in the temporalsubiculum. It terminates in the medial preoptic areaand the ventromedial, dorsomedial, and ventral pre-mammillary nuclei. Finally, there are documentedconnections with thalamic nuclei, though the detailsare not well described. The proximal part of theseptal subiculum projects to the anteromedialnucleus of the thalamus, but the distal part projectsto the anterior thalamic complex. The latter projec-tion is reciprocated. Temporal subiculum receivesinput from the nucleus reuniens. Available evidencesuggests that the anteroventral nucleus of the thala-mus also projects to the subiculum.



3.03.4 Conclusions

3.03.4.1 The Flow of Sensory Informationthrough the Hippocampal System

The entorhinal cortex is widely recognized as theprimary way station for sensory information on itsway from the neocortex to the hippocampus. Muchof the neocortical input arrives by way of the peri-rhinal and postrhinal cortices, but there are also directneocortical inputs to the entorhinal cortex. The pre-subiculum and parasubiculum are also consideredinput structures for the hippocampal memory system.The distinct patterns of cortical afferentation to para-hippocampal structures, the intrinsic connections, andthe topography of the parahippocampal–hippocampalconnections suggest that parahippocampal structuresare involved in the preprocessing of sensory informa-tion provided to the hippocampus, and that there isfunctional diversity within the parahippocampalregion. The view that parahippocampal structureshave different functions is consistent with emergingevidence that there is also substantial functional diver-sity among hippocampal formation structures.

In Figure 11, we have attempted to schematizethe flow of sensory information through the hippo-campal memory system. Beginning with the inputstructures, perirhinal area 36 receives sensory inputfrom visual, auditory, and somatosensory regions.Longitudinal intrinsic connections integrate acrossmodalities before transmission to perirhinal area 35.This polymodal input to area 35 is joined by olfac-tory information and then passed on to entorhinalcortex, primarily the lateral band of the LEA. Thepostrhinal cortex receives visual and visuospatialinput from posterior parietal, retrosplenial, and visualassociation regions, along with a small input fromauditory association cortex. That information isintegrated and transmitted to entorhinal cortex, pri-marily to the lateral band of the MEA. Presubiculumis targeted by the subiculum in a topographical man-ner such that septotemporal levels of the subiculummap onto septotemporal levels of the presubiculum.Subicular input is integrated with direct visuospatialinput to the presubiculum, especially the septal com-ponent. That information is forwarded to theparasubiculum, the postrhinal cortex, and the MEA.

Late

ral

band

Are

a 35

Are

a 36

Inte

rmed

iate

band

Med

ial

band

Figure 11 Diagram of the flow of sensory information through the hippocampal memory system. Black arrows indicate the

primary pathways by which sensory information traverses the hippocampal memory system. Note that only the strongerconnections are indicated and that emphasis is on the feedforward connections as opposed to the feedback connections.

Abbreviations: CA, CA field; DG, dentate gyrus; LEA, lateral entorhinal area; MEA, medial entorhinal area; PER, perirhinal

cortex; POR, postrhinal cortex; sub, subiculum.



The most septal component of the presubiculum, thearea sometimes termed the postsubiculum, projectsmassively to the postrhinal cortex. Finally, the para-subiculum, which receives direct, but modest, visualand visuospatial input along with its subicular input,projects to postrhinal cortex and the MEA.

To summarize, information from all modalitiesreaches the full septotemporal extent of the hippo-campus, but the degree of processing of differentmodalities is weighted differently along the septo-temporal axis. Visual and visuospatial input isprocessed and elaborated along a pathway thatincludes the postrhinal cortex, lateral MEA, the sep-tal hippocampal formation, septal presubiculum,parasubiculum, and then back to postrhinal cortexand lateral MEA. Olfactory information is less segre-gated but follows a pathway that includes theperirhinal cortex, medial LEA, and temporal hippo-campal formation structures. Thus, there appears tobe functional diversity in the processing of sensoryinformation that is organized along the septotem-poral axis; visual and visuospatial information ispredominant in the septal hippocampus, and olfac-tory information is predominant in the temporalhippocampus.

3.03.4.2 The Comparative Anatomyof the Hippocampal System

As indicated earlier, all components of the parahip-pocampal region and the hippocampal formation arerepresented in both the rodent and the primate brain.Many of the connectional principles are also con-served. Taking into account the differences in brainsize and sensory processing needs, cortical afferenta-tion of the parahippocampal structures is similaracross species. Additionally, the available evidencesuggests that the architecture of the perforant path-way is similar in the primate and rodent brains. In themonkey and the rat, the perforant pathway projec-tions to the dentate gyrus, CA3, and CA2 originate inlayer II of the entorhinal cortex. Also in both, theprojections to CA1 and the subiculum originate inlayer III. In addition, the terminations of the projec-tions originating in entorhinal layer III exhibit atransverse topography. The rostral entorhinal cortexin the monkey and the lateral entorhinal area in therat project to the border of the CA1 and subiculum;the caudal entorhinal cortex in the monkey and themedial entorhinal area in the rat project to proximalCA1 and distal subiculum (Witter, 1986, 1993,Amaral, 1993). The intrinsic connections of the

monkey entorhinal cortex also exhibit patterns simi-

lar to the lateral to medial bands of intrinsic

connectivity observed in the rodent entorhinal cor-

tex. Taken together, the evidence suggests that both

the rat and monkey hippocampal memory systems

are excellent models for the medial temporal lobe

memory system in the human brain.

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