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Scientists have known for a long time that animals can generate new brain nerve cells (neurons) in a process called neurogenesis. But they’ve always believed that the more complex and highly evolved human brain, after reaching maturity, cannot produce new neurons. Most people remember learning this in biology class. The going scientific theory was that tissues like skin and bone can heal themselves because they have the “stem” cells that trigger the growth of new nerve cells, but that the brain lacks those magic stem cells. Therefore, while the adult human brain can compensate for damage by making new connections among surviving nerve cells, it can’t actually repair the damage by growing new cells.

Recent advances by two neuroscientists, Fred H. Gage of the Salk Institute in San Diego and Peter S. Eriksson of the Göteborg University Institute of Clinical Neuroscience, indicate that the old belief is wrong. Gage and Eriksson collected the first convincing evidence that the brains of adult humans, even elderly people, regularly regenerate neurons in the hippocampus. Why is the hippocampus of interest? It appears to control which experiences are filed away into long-term memory and which pass into oblivion. It is not only where memories are stored, but it also controls which bits of information become memories. Simply put, it is the control panel for learning.

Of course, if the ultimate goal is to stimulate neural growth and regeneration in damaged human brains, simply proving that new cells are grown is not enough. Scientists need to identify whether the new neurons are working, and whether they’re sending and receiving messages appropriately. Luckily, they realized that human neurogenesis in the hippocampus mirrors the same process in rodents, so they can turn to mice and rat studies for clues.

Rodent studies analyzing the effects of environment on brain anatomy and learning have been instructive. In the early 1960’s, Mark Rosenzweig and colleagues at the University of California-Berkeley removed rats from their standard cages, and put them into an enriched environment where they had large cages, other rats for company, and a continually changing environment with varieties of exercise wheels and toys that were rotated in and out of the cage. Compared with rats kept in standard cages, these showed heaver brains, greater thickness in certain brain structures, more connections between nerve cells, and increased branching of neural projections. And they performed better on learning tests. For example they were better at navigating mazes than the control group.  Since the Rosenzweig study, scientists have been convinced that enriching the environment of adult rodents enhances brain wiring in ways that increase brainpower. A 1997 study revealed that adult mice in enriched living conditions grew 60% more neurons than did genetically identical control animals, and they did better on a learning tasks. These results held even for elderly mice, which typically produce new neurons at a much lower rate than younger mice.

Some scientists are trying to identify which features of an enriched environment have the strongest effect on hippocampal nerve cell production. Dr. Gould, of Princeton University, has shown recently that participation in a learning task, even outside of an enriched environment, enhances the survival rate of new nerve cells, resulting in an elevation in the number of new neurons.

In addition to environmental enrichment, animal studies in the last several years have identified other factors that influence neurogenesis. Stress has shown signs of being an inhibitor, while exercise tends to increase new nerve cell production. However, these links are still being investigated and are quite preliminary. The truth is that there are many steps to neurogenesis, and there are multiple factors that affect it at any stage of the game.

While this discovery might not yield immediate answers in medicine, the possibilities are staggering. For example, if researchers can learn how to stimulate existing stem cells to produce new working nerve cells in chosen parts of the brain, neurological diseases such as Alzheimer’s and Parkinson’s, and stroke trauma could be eased. The discovery also holds great promise of uncovering new knowledge about how adults learn. It points toward the exciting conclusion that designing stimulating and enriching environments for learning can actually enhance the brain’s effectiveness.

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