The connectivity between the entorhinal cortex and the hippocampal formation traditionally has been viewed as the entorhinal cortex mediating input and output pathways of the hippocampus, where entorhinal superficial layers provide the main cortical input to the hippocampus and deep layers of the entorhinal cortex receive hippocampally processed information to convey that back to the cortex. In my talk I will summarize several sets of experimental data, indicative for a need to re-define this by now traditional circuit model. These include evidence that inputs from the pre- and parasubiculum, which distribute to superficial layers, also target principal neurons in deep layers. Neurons in deep layers, like those in superficial layers, contribute a specific input component to the hippocampal formation. Some inputs thought to distribute selectively to deep layers, like from the retrosplenial cortex, have a bisynaptic indirect component targeting principal neurons in superficial layers. Deep and superficial layers show strong interconnectivity, mediating hippocampal output to become integrated as part of the input signal. The potential relevance of this latter circuit is supported by observations that during development, hippocampal fibers strongly distribute to superficial layers, showing a strikingly modular terminal distribution.
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