The ability to find one's way depends on the brain's ability to integrate information about location, direction and distance. A key component of the brain network subserving such integration is the ‘grid’ cell. Grid cells fire selectively at regularly spaced positions in the environment such that, for each cell, activity is observed only when the animal is at places that together define a repeating triangular pattern tiling the entire environment covered by the animal. Grid cells are known to co-localize in the medial entorhinal cortex (MEC) with head-direction cells, conjunctive grid × head direction cells, and border cells, which each contribute to a dynamically updated metric representation of current location. In the present talk I will show that grid cells are abundant also in other parahippocampal structures, including pre- and parasubiculum. The proportion of grid cells decreases from MEC to parasubiculum, and from parasubiculum to presubiculum. The symmetry of the grid pattern, and its relationship to the theta rhythm, follows a similar gradient. Grid cells in pre- and parasubiculum intermingle with head-direction cells and border cells, as in layers III-VI of MEC, suggesting that the parahippocampal cortex forms one extended functional network cutting across variations in network architecture and cellular response properties. I will next show that the accuracy of grid representations in the parahippocampal network depends on interactions with other space-movement-representing networks such as the posterior parietal cortex (PPC); however, although PPC inactivation causes gradual blurring of grid cell firing fields, the basic hexagonal geometry and the directional orientation of the grid remain roughly intact in the absence of PPC input, as might be expected if the grid structure were generated intrinsically in the parahippocampal cortex. Finally, I will present preliminary results from a study using optogenetic tools to determine the identity and quantity of entorhinal functional cell types with monosynaptic projections to the hippocampus. The results so far suggest that hippocampal place cells receive a combination of monosynaptic inputs from grid cells, head direction cells, conjunctive grid × head direction cells, and border cells.
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