BIO254:PlaceCells
Due 12/01/06
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25 April 2024
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Place Cells, sometimes called Space Cells are a collection of neurons, found in the Hippocampus, that activate in response to an animal's position in it's physical environment. Together, a set of place cells form an abstract map of the geometry of an animal's environment. Each Place Cell has a place field, or mapping of areas to which it is most sensitive to. The firing of many different place cells, according to their place fields and the physical postion of the animal, specify the animals location and possibly it's orientation. When an animal is placed in a new environment, it's cognitive spatial map is likely to be quite disorganized. Experimentaly, this can be observed as the erratic firing of place cells, without regard to the physical location of the animal. A typical experiment might consist of a freely moving rodent implanted with extracellular electrodes and placed in a novel enclosure. In this way, the firing of many place cells can be recorded in conjunction with the animals location. Over time, an organized map with distinct place fields for different cells will emerge. Indeed, it has been found that through repeated exposures animals can progressively separate and fine tune their representations of two similar environments. Once they are tuned by experience, these spatial representations are robust and can remain stable for months. The precise mechanisms of plasticity that tune and maintain the cognitive spatial map are not known. However, it is likely that well studied mechanisms of plasticity such as LTP and LTD are involved. Animals in which molecules involved in LTP and LTD (NMDAR, CaMKII, etc.) have been perturbed genetically, often show deficits in the formation of organized place fields.
File:C:\Documents and Settings\Fernando Martinez\My Documents\My Pictures\mouse1.jpg
A schematic of an early place cell experiment and some data demonstrating the direction specific nature of this place cell's response. (From: O'Keefe and Dostrovsky)
File:C:\Documents and Settings\Fernando Martinez\My Documents\My Pictures\mouse2.jpg
The evolution of the place fields of 7 different cells as a mouse is repeatedly exposed to two different environments. (From: Wills, O'Keefe, et al.)
Wills TJ, Lever C, Cacucci F, Burgess N, O'Keefe J. Attractor dynamics in the hippocampal representation of the local environment.Science. 2005 May 6;308(5723):873-6.
Independent rate and temporal coding in hippocampal pyramidal cells. Nature. 2003 Oct 23;425(6960):828-32.
Lever C, Wills T, Cacucci F, Burgess N, O'Keefe J. Long-term plasticity in hippocampal place-cell representation of environmental geometry. Nature. 2002 Mar 7;416(6876):90-4.
The hippocampus as a spatial map. Preliminary evidence from unit activity in the freely-moving rat. Brain Res. 1971 Nov;34(1):171-5.
