A revolution recently occurred in neuroscience that has far reaching implications for our future.
According to all the textbooks in neuroscience, we are born with a full complement of around 100 billion neurons and that it is all downhill from there. This was a discouraging "fact".
Fred Gage, a colleague of mine at the Salk Institute, has discovered that new neurons are born every day in the hippocampus, an important part of the brain for long-term memory of facts and events, even in adults. This was first found in rats, but has now been shown in monkeys and humans, and not just in the hippocampus, but also in the cerebral cortex, the storehouse of our experience and the fountainhead of our creativity. This was widely reported, but what has emerged since then is even more encouraging.
First, the new neurons in the hippocampus don't survive unless the animal exercises; a running wheel in an otherwise standard lab cage is enough to keep new neurons alive and well in mice. Second, the increase in the strengths of connections between neurons in the hippocampus that occurs when they are strongly activated, called long-term potentiation, is twice as strong in mice that had access to a running wheel. Finally, the mice with exercise were smarter at memory tasks. We still do not know how all this happens, but the bottom line is that something as basic as exercise can make you smarter. Recess in schools and executive gyms help not only the body, but can also make the mind sharper.
These results have implications for graceful aging.
Until recently, the dominant view of how the brain develops made the assumption that experience selects neurons and connections from a fixed, pre-existing repertoire. This view had some plausibility when it was thought that all of the neurons you will ever have are present at birth, coupled with evidence of neuron death and pruning of connections during childhood. However, if the brain makes new neurons in adults then this cannot be the whole story, since the growth of new connections must also be occurring, and doing so in an experience-dependent way.
This discovery, coupled with increasing evidence that new connections can grow even between old neurons as a consequence of an enriched environment, means that an active mind along with an active body predisposes us for a lifetime of learning. This is good news for the baby boomers who have embraced health clubs and challenging new experiences, but bad news for couch potatoes who are exercise phobic.
In short, "Use it or lose it".
An active lifestyle and a rich social life are the best insurance against premature senility. We will learn much more about the how the brain renews itself in the next century as neuroscientists reveal more about the mechanisms and circumstances that make new neurons survive and grow stronger connections. Ultimately, this will lead to greater productivity from the elderly, the fastest growing segment in western societies.