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Grid cells and the medial-entorhinal space network

      The medial entorhinal cortex is part of a neural system for mapping of self-location. One of the first components to be detected in this internal map was the grid cell. Grid cells fire electric impulses when animals are at particular locations that together tile the environment in a periodic hexagonal pattern, like in a Chinese checkerboard. The medial-entorhinal circuit of grid cells was soon found to include a wider spectrum of functional cell types, such as head direction cells, speed cells, and border cells, intermingled among the grid cells. In this lecture, I will show that additional specialized cell types are present when spatial behavior is tested in environments with salient objects or landmarks. A subset of medial entorhinal cells fires in a vector-like manner at distinct distances and directions from objects inserted in the recording enclosure, irrespective of where in the enclosure the object is located, and irrespective of the identity of the object. I will next show that grid cells retain spatial relationships not only across recording environments but also from awake exploration to sleep, consistent with the idea that grid cells, and the entorhinal network as a whole, are part of a network-generated attractor-like representation of local space. I will discuss possible roles of inhibitory networks in this representation and show that different functional cell types may be regulated by distinct classes of GABAergic interneurons. Finally, I will discuss how the entorhinal-hippocampal navigational circuit evolves during the formation of the nervous system during the first weeks and months of life and I will discuss how immaturity of the circuit at early developmental stages may influence properties of medial entorhinal cell types.
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