1. Structural and Functional Organization
The hippocampus (HPC) has a highly organized structure in which most excitatory neurons lay in a single ordered layer, either granule or pyramidal, while interneurons (INs) are either excitatory or inhibitory and appear dispersed in the plexiform layers around the principal cells. HPC is organized in two structures, the dentate gyrus (DG) and the Ammon’s horn (Cornu Ammonis (CA)) (Fig 2A). In the distal part of the CA1 field, there is a transition zone to the entorhinal cortex (Ent). This transition zone represents subicular complex, which is further subdivided. In the subiculum, the pyramidal cells appear to be decomposed into a multiple cell layer (Fig 2A).
Most neurons in DG are organized in a granular layer and appear as an arrow tip. Outside the granule layer is the molecular layer, which contains the dendritic projections and the terminals of the granule cells (GCs). Among these terminals, the main …show more content…
In a simplified description, afferents from the medial Ent enter the HPC through the performant pathway (PP) and targets the distal dendrites of GCs in DG to form the first synapse (Fig 2A). The second synapse is located in the stratum lucidum of the CA3 and between the mossy terminal, which is arising from GCs and the proximal dendrites of the pyramidal cell. Then, the pyramidal cells of CA3 send a myelinated axon that runs to the contralateral CA1 and an ipsilateral collateral named as Schaffer’s collateral that target the proximal dendrites of the CA1 pyramidal cells, which represent the third synapse. Then, the CA1 pyramidal cells project back to the inner layers of Ent. All along this pathway, excitatory and inhibitory intrinsic circuits shape the intrinsic and flow activity of the HPC system (see Fig