Cellular, columnar and modular organization of spatial representations in medial entorhinal cortex

- Grid cells in medial entorhinal cortex provide a metric representation of space.
- Functional and anatomical evidence points to principal cell diversity, modularity and columnar organization in medial entorhinal cortex.
- Intracellular recordings show that grid activity arises from recurring ramps of depolarization.

Spatial discharge patterns in medial entorhinal cortex consist of grid, head direction, border and spatial-band cells. These firing patterns differ from the single-peaked fields of hippocampal place cells, in that they have well-defined geometries and extend throughout the available space. Such discharge properties could contribute to a metric representation of space. Both functional and anatomical evidence point to principal cell diversity, modularity and columnar organization, but linking entorhinal anatomy and physiology remains challenging. Layer 2 microcircuits consist of pyramidal neurons and a stellate cell network, which lacks recurrent excitation and is coupled by disynaptic inhibition. Intracellular recordings showed that periodic, grid-like firing emerges from depolarization ramps, whereas theta-oscillations determine spike timing. Interference with various inputs to entorhinal cortex abolishes grid activity, often without concomitant loss of hippocampal place activity.