Now what is truly exciting is that it was found that an "enriched" environment including exercise led to a much greater number of new neural cells being produced. To quote the scientist concerned, Fred Gage, "We think voluntary exercise increases the number of neural stem cells that divide and give rise to new neurons in the hippocampus" (page 81). His work was supported by Brian Christie of the University of British Columbia who found that the individual neurons in the exercise group have more dentrites (the projections through which the neurons receive signals) and that each dendrite had more spines, representing a site where a connection can occur. The research showed that there are physical, structural reasons in the brain why learning and memory capacities are enhanced through exercise.
And another thing: the hippocampus is one of the first areas of the brain to be affected by Altzheimer's Disease; memory problems and disorientation appear among the first symptoms. Even normal, healthy aging is associated with a gradual decline in some types of memory, including episodic memory and working memory.
The science of all this is obviously highly complex, but on the surface it suggests that physical activity and reducing stress (The hippocampus contains high levels of mineralcorticoid receptors which make it more vulnerable to long-term stress than most other brain areas) both have significantly beneficial impacts on brain function as we get older.
This fascinating book has a lot more to say, especially about the mind-body issue, and I'll be writing a full review shortly.
Note: this is relatively old science now, and it's astonishing that it comes to prominence in 2009 with this book. New work from North Carolina State University have identified a gene that tells embryonic stem cells in the brain when to stop producing nerve cells called neurons. The research is a significant advance in understanding the development of the nervous system, which is essential to addressing conditions such as Parkinson's disease, Alzheimer's disease and other neurological disorders. Check out http://www.sciencedaily.com/releases/2009/11/091117102034.htm
References: 'The Plastic Mind' Sharon Begley, Constable, 2009, ISBN: 978-1-84529-674-2
P.S. Eriksson, E. Perfilieva, T.Bjork-Eriksson, A.M. Alborn, C. Nordberg, D.A. Peterson, F.H. Gage, 'Neurogenesis in the Adult Human Hippocampus', Nature Medicine, 4, 1998
H.van Praag, A.F. Schinder,, B.R.Christie, N.Toni,T.D.Palmer, F.H.Gage, 'Functional Neurogenesis in the Adult Hippocampus', Nature 415, 2002