Grid cells

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A grid cell is a type of neuron found in the entorhinal cortex (EC) that fires strongly when an animal is in specific locations in an environment. Grid cells were discovered in 2005 and it is hypothesized that a network of these cells constitute a mental map of the spatial environment (Hafting et al., 2005).

Description

Grid cells are found in the dorsocaudal part of medial entorhinal cortex in rats. Cells that are located more ventral (more distant from the border of entorhinal cortex with postrhinal cortex) have wider firing fields and correspondingly greater spacing between the fields.

While cells with this firing pattern are found in all layers of EC, layer II has cells that are most grid-like, independent of the head direction of the animal. As one proceeds to deeper layers, grid cells show increasing dependence on head direction (i.e. in EC layer V there are cells with a grid-like pattern that fire only when the animal is facing a particular direction).

It has been suggested that a place code is computed in the entorhinal cortex and fed into the hippocampus, which may make associations between place and events which is needed for the formation of memories.

In contrast to a hippocampal place cell, a grid cell has multiple firing fields, with regular spacing, which tessellate the environment in a hexagonal pattern. The unique properties of grid cells are as follows

1. Grid cells have firing fields dispersed over the entire environment (in contrast to place fields which are restricted to certain specific regions of the environment)
2. The firing fields are organized into a hexagonal grid
3. Firing fields are generally equally spaced apart, such that the distance from one firing field to all six adjacent firing fields is approximately the same (though when an environment is resized, the field spacing may shrink or expand different in different directions; Barry et al. 2007)
4. Firing fields are equally positioned, such that the six neighboring fields are located at approximately 60 degree increments

The grid cells are anchored to external landmarks, but persist in darkness, suggesting that grid cells may be part of a self-motion based map of the spatial environment.

Cellular Electrophysiology

Stellate cells recorded from the grid cell-containing region of entorhinal cortex in vitro have been shown to have intrinsic, subthreshold oscillations and resonant properties (Giocomo et al. 2007). When the membrane potential of such a cell is brought near threshold, the membrane potential begins to oscillate at a frequency partially dependent on how depolarized the cell is. Similarly, when current is injected into such a cell at a range of frequencies, the magnitude of the cell's response depends on the frequency of the input, with a largest magnitude of response at the resonance frequency.

The oscillation and resonant frequencies of these cells decrease along the dorsoventral axis of entorhinal cortex, correlating to the changing spatial periodicity (the increasing spacing of the grid fields) along the same axis. This suggests that these temporal oscillations may relate to the spatial properties of grid cells, in agreement with a prediction from an earlier model of grid cells (Burgess et al. 2007).

References

• Barry C, Hayman R, Burgess N, Jeffery KJ. Experience-dependent rescaling of entorhinal grids. Nat Neurosci. 10(6):682-4. (2007). PMID 17486102

• Burgess N, Barry C, O'Keefe J. An oscillatory interference model of grid cell firing. Hippocampus. 17(9):801-12. (2007). PMID 17598147

• Fyhn M, Molden S, Witter MP, Moser EI, and Moser MB. Spatial representation in the entorhinal cortex. Science 305, 1258-64 (2004). PMID 15333832

• Giocomo LM, Zilli EA, Fransen E, Hasselmo ME. Temporal frequency of subthreshold oscillations scales with entorhinal grid cell field spacing. Science 315(5819):1719-22. (2007). PMID 17379810

• Hafting T, Fyhn M, Molden S, Moser MB, and Moser EI. Microstructure of a spatial map in the entorhinal cortex. Nature 436, 801-6 (2005). PMID 15965463

• Sargolini F, Fyhn M, Hafting T, McNaughton B, Witter MP, Moser EI, and Moser MB. Conjunctive representation of position, direction, and velocity in entorhinal cortex. Science 312, 758-62 (2006). PMID 16675704