In animals, running-wheel exercise and environmental enrichment increase the number of mitochondria and synapses in nerve cells.

“And we find that running-wheel exercise, what we call environmental enrichment where we have the animals in cages where they have essentially like playground-type environment where they can maintain their mind more active. And under those conditions, exercise, mental exercise, there's an increase in the number of mitochondria in nerve cells. And associated with that, at least in some brain regions, there can even be an increase in the number of synapses between nerve cells, the connections between the nerve cells.”