As you might expect, exercise is a fairly stressful activity for your muscles, however it’s also stressful for your brain. Muscle cells and neurons release progrowth, or survival signals when they are under stress; so that they can perform to the level being demanded of them. These signals are in the form of proteins released from the cells, and so are relatively broad acting: they affect everything nearby. Your brain, has individual neurons that specifically activate one muscle fiber. Coordinated movements use gobs of muscle fibers, both individually and together in a coordinated effort. Exercise activates all of these neurons to release these ‘grow and do better’ factors throughout your whole brain and body. New neurons are born to regulate the coordination of muscle fibers/groups, which in effect de-age neurons nearby these new neurons.
So; you are repairing/pimping up your brain, because the body is lazy, and only wants to perform to the level that it gets challenged to. The brain is very similar to a muscle, if you want it to get stronger, you have to use it, and stress it.