, 2005). The firing fields formed a grid-like pattern, and the cells were referred to as grid cells (Figure 1). The size of each grid field and the spacing between them were Tenofovir research buy found to increase progressively from small in dorsal to large in ventral MEC (Fyhn et al., 2004, Hafting et al., 2005 and Sargolini et al., 2006). At the dorsal tip, the spacing was approximately 30 cm in the rat; at the ventral tip, it was more

than 3 m (Brun et al., 2008). The position of the grid vertices in the x,y plane (their grid phase) appeared to vary randomly between cells at all dorsoventral locations, but each grid maintained a stable grid phase over time. The cells fired at the same x,y positions irrespective of changes in the animal’s speed and direction, and the firing fields persisted in darkness, suggesting that self-motion

information is used actively by grid cells to keep track of the animal’s position in the environment (Hafting et al., 2005 and McNaughton et al., 2006). This process, referred to as path integration, may provide the metric this website component of the spatial map. Grid cells were soon found to colocalize with several other specialized cell types. A substantial portion of the principal cells in layer III and layers V and VI of the MEC were tuned to direction, firing if and only if the animal’s head faced a certain angle relative to its immediate surroundings (Sargolini et al., 2006). Similar cells were already known to exist in other parahippocampal and subcortical regions (Ranck, 1985 and Taube, 2007), but the entorhinal head direction cells were different in that many of them exhibited grid-like activity at the same time (conjunctive grid × head direction cells). In addition, approximately 10% of the active entorhinal cell population was found to fire selectively in the vicinity of geometric borders such

as the walls of a recording enclosure or the edges of a table (Savelli et al., 2008 and Solstad et al., 2008). We have referred to these cells as border cells (Solstad others et al., 2008). Collectively, grid cells, head direction cells, and border cells are thought to form the neural basis of a metric representation of allocentric space (Moser et al., 2008). The entorhinal spatial representation is different from the hippocampal map in that cell assemblies maintain their intrinsic firing structure across environments. If two grid cells have similar vertices in one environment, they will fire at similar locations also in another environment (Fyhn et al., 2007 and Hafting et al., 2005). If two border cells fire along adjacent borders in one enclosure, they will do so in other boxes, too (Solstad et al., 2008). In the hippocampus, in contrast, different subsets of neurons are recruited in different environments (Muller et al.