Neurons, thinking cells, are generated in the human brain in the hippocampus every day. However, their production is influenced by a number of different environmental factors. For example, the consumption of alcohol has been shown to retard the generation of new neurons. Studies have shown that rats that exercise regularly can produce twice as many new cells in the hippocampus than those who lead a more sedentary lifestyle. However, unless the rats are cognitively challenged, the new neurons disappear rather quickly and do not survive indefinitely.
(Shors, Tracey J. “Saving New Bain Cells.” Scientific American, p 47-48, march 2009)