Place Cells

Place cells are the hippocampal pyramidal cells, and they derive their name from the fact that they only fire when an animal is in the cell's firing field, i.e. in a part of its environment that the cell responds to (Kentros et al, 1998, 2121). Place cells, which are found in hippocampal area CA1, receive positional information from two different areas: the intrahippocampal network in area CA3, and directly from the entorhinal cortex (Brun et al, 2002, 2243). It is thought that these cells are characterized by location-specific firing, and that the angular position of their firing fields suggests that visual cues may activate them (Save, Cressant, Thinus-Blanc and Poucet, 1998, 1818). However, these cells have also been found to be active during sleep, and in blind rats, which indicates that the true nature of these cells has not yet been elucidated (Wilson and McNaughton, 1994, 676; Save, Cressant, Thinus-Blanc and Poucet, 1998, 1818). This paper will look at some of the investigations that have been carried out to try and elucidate the true nature and role of hippocampal place cells in location mapping.

Kentros et al (1998) investigated the effects of N-methyl-D-aspartate (NMDA) blockade on the glutamate receptors, and how this affected the NMDA-based plasticity of synapses in the formation and maintenance of firing fields . Encoding occurring in place cells in the rat hippocampus is thought to map out new environments by the fact that each cell fires only when the

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s shown that these burst are initiated in the CA3 area of the hippocampus, and that the outer layers of the entorhinal cortex show neural activity correlated with CA1 area electroencephalogram sharp wave activity. This all suggests that during the sleep cycle, place cells replay representation of locations visited prior to sleep to reinforce the neuronal states encoded by them. Save, Cressant, Thinus-Blanc and Poucet (1998) furthered this work, and other studies which have shown that place cells fire during the sleep cycle by looking at the activity of these cells in blind rats. The fact that a given place cell only fires in a specific area of its environment, that if a target is rotated, then the field rotates to match it, and the fact that this firing is retained even after the lights are turned off in an experimental setting suggests that there is something other than visual cues which is stimulating these cells. These researchers decided to investigate this by testing place cells of rats made blind shortly after birth. The rats were tested in a circular apparatus which had three-dimensional objects placed at the periphery to provide spatial cues. Surprisingly, recordings from place cells in these rats were

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