Functional neuroanatomy of the parahippocampal region: The lateral and medial entorhinal areas

Abstract

The entorhinal cortex (EC) serves a pivotal role in corticohippocampal interactions, but a complete description of its extrinsic connections has not been presented. Here, we have summarized the cortical, subcortical, and hippocampal connections of the lateral entorhinal area (LEA) and the medial entorhinal area (MEA) in the rat. We found that the targets and relative strengths of the entorhinal connections are strikingly different for the LEA and MEA. For example, the LEA receives considerably heavier input from the piriform and insular cortices, whereas the MEA is more heavily targeted by the visual, posterior parietal, and retrosplenial cortices. Regarding subcortical connections, the LEA receives heavy input from the amygdala and olfactory structures, whereas the MEA is targeted by the dorsal thalamus, primarily the midline nuclei and also the dorsolateral and dorsoanterior thalamic nuclei. Differences in the LEA and MEA connections with hippocampal and parahippocampal structures are also described. In addition, because the EC is characterized by bands of intrinsic connectivity that span the LEA and MEA and project to different septotemporal levels of the dentate gyrus, special attention was paid to the efferents and afferents of those bands. Finally, we summarized the connections of the dorsocaudal MEA, the region in which the entorhinal "grid cells" were discovered. The subregional differences in entorhinal connectivity described here provide further evidence for functional diversity within the EC. It is hoped that these findings will inform future studies of the role of the EC in learning and memory.

Navigating from hippocampus to parietal cortex

“…67, figure 4 in ref. 68). This connection is relatively weak, and it is uncertain whether it is able to convey location information to possible action-preparing systems in PPC.…”

“…5). The shortest and densest projection is through the postrhinal cortex (67,68). The contribution of this pathway is uncertain because animals can still learn a simple water-maze task after relatively large lesions of the postrhinal cortex (69,70); however, no studies have tested the involvement of postrhinal cortex in path integration tasks where position can only be estimated from the animal's own motion (as in ref.…”

Towards a functional organization of the medial temporal lobe memory system: Role of the parahippocampal and medial entorhinal cortical areas

“…Areas of the parahippocampal region then connect with the hippocampus. The flow of information through the cerebral cortex, parahippocampal region, and hippocampus can be described as a hierarchy of connectivity in which widespread areas of the cerebral cortex funnel information onto multiple areas within the parahippocampal region, whose outputs converge on the hippocampus (Lavenex and Amaral, 2000;Witter et al, 2000;Kerr, 2007). Importantly, the outputs of hippocampal processing are directed back down the hierarchy to areas of the parahippocampal region, which in turn send their outputs back to the areas of the cerebral cortex from which the inputs originated.…”

An integrated neural model of semantic memory, lexical retrieval and category formation, based on a distributed feature representation

“…This model organization is also indirectly supported by anatomical studies performed in the rodent hippocampus. Burvell's et al Kerr et al 2007) report that cortical afferents to the hippocampus (and efferent from the hippocampus) separately convey complementary visual and spatial, polymodal sensory and emotional information. All this information is then linked in the hippocampus to realize episodic memory and participate to other cognitive processes.…”