Hope: New Nerve Cells -- Even in Old Age
 

http://www.sciencedaily.com/releases...0506121751.htm

"The precise factors that influence the reactivation of dormant stem cells are not yet clear. The cells can, however, be stimulated to divide again. The scientists observed more newborn hippocampal neurons in physically active mice than in their inactive counterparts. "Consequently, running promotes the formation of new neurons,"

This is effectively documented in Spark: The Revolutionary New Science of Exercise and the Brain by John Ratey.