Models For The Analysis Of Nerve Cells And Spatial Memory

In animal models using rodents, neural electrical activity can be monitored in the hippocampus while the test animal is running at arm radial mazes and performing tasks that require locomotion within a workplace. Experiments using this paradigm have revealed a remarkable degree of spatial selectivity. A surprisingly high number of neurons throughout the hippocampus have place fields. This means that a specific neuron within the hippocampus will fire most strongly while the rat is in a certain area of the workspace. In this way, it has been shown that different neurons within the hippocampus cover the workspace made up of the testing area. There are also some hippocampal neurons that are sensitive not only to the rat's location but also to the direction in which its head is pointing. This feature is most pronounced within the postsubiculum, an area closely linked to the hippocampus. Experiments have shown that these postsubicular neurons fire when the rat's head is pointing in the neuron's preferred direction and conversely decrease the firing rate as the orientation of the rat's head deviates from that direction.

Another example of a contribution from specific neural circuitry is new granule neurons formed during adulthood. Neuronal stem cells or progenitor cells residing within the dentate gyrus in the hippocampus continuously generate granule neurons throughout life. This phenomenon has been was shown to be under dynamic regulation. Increased hippocampal volume has been reported in birds and animals that engage in behavior requiring spatial memory, such as food storing. Another study demonstrated navigation-related structural changes in the hippocampus of taxi drivers, with an increased volume of the posterior hippocampus correlating to the time spent as a taxi driver. Laboratory animals housed in an enriched environment have an increased rate of neurogenesis within this region as compared to rats living in a standard environment. Furthermore, animals living in an enriched environment perform better on spatial memory tests. Running also induces this phenomenon and selectively enhances dentate gyrus long-term potentiation (LTP). Newborn granule cells within the adult rat hippocampus have been shown to project axons through the mossy fibers to their natural targets within the CA3 region. It has, therefore, been speculated that newborn granular cells may contribute to the improvement in spatial memory test performance. The direct link between neurogenesis within the dentate gyrus and spatial memory performance, although compelling, still needs further investigation.

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Suggested Reading

Gallagher, M., and Colombo, P. (1995). Aging: The colinergic hypothesis of cognitive decline. Curr. Opin. Neurobiol. 5, 161— 168.

Kandel, E. R., Schwartz, J. H., and Jessel, T. M. (Eds.) (1991). Principles of Neural Science. Elsevier Science, New York.

Purves, D., Augustine, G. J., Fitzpatrick, D., Katz, L. C., LaMantia, A.-S., andMcNamara, J. O. (Eds.) (1997). Neuroscience. Sinauer Ass. Inc., Sunderland, MA.

Tovee, M. J. (1998). Is face processing special? Neuron 6,1239-1242.

Wilsson, M. A., and Tonegawa, S. (1997). Synaptic plasticity, place cells and spatial memory: Study with second generation knockouts. Trends Neurosci. 20, 102-106.

Zigmond, M. J., Bloom, F. E., Landis, S. C., Roberts, J. L., and Squire, L. R. (Eds.) (1999). Fundamental Neuroscience. Academic Press, San Diego.

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