A Grid Cell Grid

30 January 2014
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Extracellular recordings have provided detailed phenomenology of spatial dis-charge patterns (place cells, grid cells, head direction cells) in the rodent brain. At the same time we know very little about the underlying microcircuits because extracellular recordings do not identify to recorded cellular elements. We devised methods that allow the identification of neurons in freely moving animals, which we apply the rodent entorhinal cortex. We reference cells relative to the patchy architecture of layer 2 in medial entorhinal cortex. Anatomical analysis reveals that calbindin-positive pyramidal neurons layer 2 in medial entorhinal cortex are arranged in a regular and often hexonal grid. Across animals this grid of patches shows a consistent alignment to the parasubiculum, cholinergic inputs and the layer 1 axons also run along a grid axis. I will provide evidence that grid cells correspond largely to these calbindin-positive pyramidal neurons in layer 2. I conclude that layer 2 grid discharges originate in a spatiotemporally highly organized microcircuit, a pyramidal ‘grid-cell-grid.