Architecture of the Entorhinal Cortex A Review of Entorhinal Anatomy in Rodents with Some Comparative Notes

Abstract

The entorhinal cortex (EC) is the major input and output structure of the hippocampal formation, forming the nodal point in cortico-hippocampal circuits. Different division schemes including two or many more subdivisions have been proposed, but here we will argue that subdividing EC into two components, the lateral EC (LEC) and medial EC (MEC) might suffice to describe the functional architecture of EC. This subdivision then leads to an anatomical interpretation of the different phenotypes of LEC and MEC. First, we will briefly summarize the cytoarchitectonic differences and differences in hippocampal projection patterns on which the subdivision between LEC and MEC traditionally is based and provide a short comparative perspective. Second, we focus on main differences in cortical connectivity, leading to the conclusion that the apparent differences may well correlate with the functional differences. Cortical connectivity of MEC is features interactions with areas such as the presubiculum, parasubiculum, retrosplenial cortex (RSC) and postrhinal cortex, all areas that are considered to belong to the “spatial processing domain” of the cortex. In contrast, LEC is strongly connected with olfactory areas, insular, medial- and orbitofrontal areas and perirhinal cortex. These areas are likely more involved in processing of object information, attention and motivation. Third, we will compare the intrinsic networks involving principal- and inter-neurons in LEC and MEC. Together, these observations suggest that the different phenotypes of both EC subdivisions likely depend on the combination of intrinsic organization and specific sets of inputs. We further suggest a reappraisal of the notion of EC as a layered input-output structure for the hippocampal formation.

Concentric ring topology of mammalian cortical sectors and relevance for patterning studies

“…This omitting trend among experts maybe owes partly to a subliminal idea that there is little functional relevance of the morphological mesocortical ring concept (i.e., is it a mere histologic curiosity?). However, Witter et al () point out in their review on the entorhinal cortex that there is presently a wide consensus on a crucial role of the entorhinal, postrhinal, perirhinal and parahippocampal areas as a connectional interphase between neocortex and hippocampal allocortex . This underlines an important functional role for the mesocortex.…”

Modelling the influence of the hippocampal memory system on the oculomotor system

“…The ERC, in primates and in humans, receives input through two functional pathways from PRC (to anterolateral ERC) and PHC (to posteromedial ERC) (Naber et al, 1997;Suzuki & Amaral, 1994, Maass et al, 2015, and in turn, project to distinct areas of the CA1, pre-, and para-subiculum (Witter, 2017;Caballero-Bleda & Witter, 1993). Research regarding the functional significance of the distinct regions of the ERC in human and non-human primates is just emerging; however, in primates, cells in posterior ERC code for saccade direction during visual exploration (Killian, Potter, Buffalo, 2015).…”

Stereological analysis of the rhesus monkey entorhinal cortex

“…Given its central role in memory function, the entorhinal cortex has been the focus of very intense investigation in animal models of human memory processes, in particular in rats and monkeys. However, although the general functional organization of the entorhinal cortex is conserved across species (Insausti, Herrero, & Witter, ; Witter et al, ), there are clear differences in the number, the relative development, and the structural characteristics of different subdivisions of the entorhinal cortex between rats, monkeys, and humans (Amaral et al, ; Amaral & Lavenex, ; Insausti et al, ; Insausti, Tunon, Sobreviela, Insausti, & Gonzalo, ). It is therefore important to obtain reliable estimates of the fundamental neuroanatomical characteristics of the entorhinal cortex in these different species in order to be able to extrapolate the findings obtained in experimental studies in animals and create realistic models of the basic principles of human memory function (Witter & Moser, ).…”

“…Similarly, following the scheme developed in primates and the detailed analysis of the connectivity of this region, Insausti et al () described six subdivisions in the rat entorhinal cortex: the dorsal lateral entorhinal field (DLE), the dorsal intermediate field (DIE), the amygdalo‐entorhinal transitional field (AE), the ventral intermediate entorhinal field (VIE), the medial entorhinal field (ME), and the caudal entorhinal field (CE). Despite the similarities in the overall organization of the hippocampal‐cortical connectivity in rats and monkeys, this nomenclature is rarely used (Witter et al, ). Instead, most neuroanatomical and functional studies consider a simpler parcellation of the rat entorhinal cortex that includes the lateral entorhinal cortex (LEC), which comprises the fields DLE, DIE, AE, and VIE, and the medial entorhinal cortex (MEC), which comprises the fields ME and CE.…”