Researchers have been stunned to find the remarkable linkage between exercise and your brain. The brain is incredibly dependent upon exercise, but it actually goes far beyond simple dependence: exercise can help you both maintain and even rebuild your brain. We all need improved memory, retention and learning and exercise is king when it comes to natural solutions.
Aerobic exercise raises levels of a brain chemical called BDNF, which can actually promote the formation of new synapses.  As you may recall, synapses are the connections between neurons and so the more synapses, the more interconnectedness that your brain has. This interconnectedness is one of the major factors that makes our brain the incredible computing and processing machine that it is.
However, it doesn’t just stop there. Exercise can actually cause the rebirth of new neurons in the hippocampus, which is the seat of short term memory. One study on mice showed that aged mice could still experience 50% of the neurogenesis (in the hippocampus) of younger mice!  This is astonishing for all you middle aged and beyond guys out there: if you stop and think about it, your brain is anything but dead if you will but exercise.
One question researchers asked themselves was if these new neurons that resulted from exercise were actually able to grow into fully functional, mature, usable neurons. Again, the research has extended a very encouraging affirmation and one study of mice showed that adult neurogenesis does in fact lead to functional neurons that can be used by the brain. 
This is born out by many studies on humans which show the tremendous benefits of exercise to the brain. One of the most amazing occurred on 59 individuals aged 60 to 79. These individuals did brisk walking (NOT relaxed, lazy-paced walking) only three times a week and after six months actually significantly increased the grey and white matter content of their brains.  This is an incredible discovery: you can literally, through intense exercise, re-grow your precious grey matter!
How could such an incredible transformation occur? Exercise does its magic through many different pathways. First of all, it creates what I would call a super-neurotransmitter cocktail. There is strong evidence that it increases dopamine , seratonin  and acetylcholine , all of which are known to improve learning and/or memory. Researchers from the University of Illinois believe that exercise works its wonder through these mechanisms by “1) increasing capillary growth around neurons, 2) increasing synaptic density and 3) promoting positive cholinergic effects”.  In other words, exercise feeds neurons, increases the cross linking between neurons and boosts the brain’s key neurotransmitter. If you’ve looking for a Cognitive Fountain of Youth, you’ve just found it!
Exercise also boosts what some researchers an all-critical brain chemical named BDNF, which has been shown to “stimulate neurogenesis, increase resistance to brain insult and improve learning and mental performance”.  Researchers have also found that exercise significantly increases blood flow in the brain, thus bathing it in critical nutrients, and blood flow is correlated to neurogenesis.  You don’t need Gingko to increase blood flow – you just need to get your butt moving!
Exercise improves so many abilities in the brain that it’s difficult to even keep up with all the research. For example, one researcher found that it greatly increases something called “P3 latency”, which is kind of like your attention speed. Normally, this declines with age but one study showed that exercise actually decreased the P3 latency levels of older adults to better than that of young, sedentary individuals!  All you seniors out there should also take note of a meta-analysis that showed that exercise conclusively improved cognition, i.e. thinking and learning in your age group. 
Exercise is also known for boosting working memory, especially after aerobic exercise.  And on a Creative Note, exercise also increases creativity. It has long been known that exercise lifts mood and mood enhances creativity. However, one study in the British Journal of Sports Medicine  showed that exercise enhances creativity independently of mood. Exercise is also known for improving concentration, recall, productivity, test/exam speed – the list goes on and on.
Finally, exercise is also one of the few proven natural ways to prevent Alzheimer’s and dementia.  One study from the Canadian Study of Health and Aging, for example, found that “Compared with no exercise, physical activity was associated with lower risks of cognitive impairment, Alzheimer disease, and dementia of any type. Significant trends for increased protection with greater physical activity were observed. High levels of physical activity were associated with reduced risks of cognitive impairment, Alzheimer disease, dementia of any type“.  And this appears, by the way, to occur not just because of exercise’s neuron-building and neuron-promoting properties. A study at the University of Chicago showed mice that exercised had 80% less Alzheimer’s plaque than mice that did not exercise.
1) Newsweek, 2/10/07, p.68-72]
2) The Jour of Gerontology 6) Cardiovasc Res,Apr 2002,54(1):25-35
3) Arch Neurol,2001,58:498-504
4) Saving Your Brain, Jeff Victoroff, p. 135
5) Rev Endocr Metab Disord,Dec 2006,7(4):225-35
6) Cardiovasc Res,Apr 2002,54(1):25-35
7) PNAS, 2009, 106:1255 126, “Correction for Caloric restriction improves memory in elderly humans”, appeared in issue 4, January 27, 2009 of Proc Natl Acad Sci USA and first published January 26, 2009
9) Achieving Optimal Memory, Aaron Nelson, Ph. D, p.48, McGraw-Hill, 2005
10) Achieving Optimal Memory, Aaron Nelson, Ph. D, p.152, McGraw-Hill, 2005
11) Ann Intern Med,2006,(144):73 81; Am J Epidemiol,2005,161:639 651; Arch Int Med,2001,161:1703 1708; J Am Med Assoc,2004,(292):1454 1461
12) British Journal of Sports Medicine, 1997, 31:240-245, “Exercise enhances creativity independently of mood”
13) The Journal of Comparitive Neurology, 2001, 435:406-417, “Adult Neurogenesis Produces a Large Pool of New Granule Cells in the Dentate Gyrus”
14) The Journal of Neuroscience, Sep 21 2005, 25(38):8680-8685; “Exercise Enhances Learning and Hippocampal Neurogenesis in Aged Mice”
15) Exp Neurol, 2003 Nov;184(1):31-9, “Can the brain be protected through exercise? Lessons from an animal model of parkinsonism”
16) Acta Physiol Scand, 1989 Jul, 136(3):473-81, “Effect of sustained exercise on plasma amino acid concentrations and on 5-hydroxytryptamine metabolism in six different brain regions in the rat”
17) Behav Brain Res, 1991, 46:123 133, “Enhancement of spatial learning in F344 rats by physical activity and related learning-associated alterations in hippocampal and cortical cholinergic functioning”
18) Trends Neurosci, 2002 Jun, 25(6):295-301, “Exercise: a behavioral intervention to enhance brain health and plasticity”
19) Psychophysiology, 2002, 39:3:303-312, “The relationship of age and cardiovascular fitness to cognitive and motor processes”
20) Med Sci Sports Exerc, 2009 Apr, 41(4):927-34 “The effect of acute aerobic and resistance exercise on working memory”
21) J Appl Physiol. 2006 Oct, 101(4):1237-42, Epub 2006 Jun 15, “Exercise, cognition, and the aging brain”
22) PNAS, Mar 27 2007, 104(13):5638-5643, “An in vivo correlate of exercise-induced neurogenesis in the adult dentate gyrus”