Exercise and the Brain
Inner IDEA: Exciting discoveries underscore how exercise benefits brain health and boosts lifelong learning.
Exercise improves our physical and mental health—that is now beyond debate. The physical benefits are obvious; we know that exercise lowers blood pressure, decreases cholesterol, reduces fat, adds muscle and improves cardiovascular function. But how is it that exercise also reduces stress, anxiety and depression and allows us to maintain focus at work and to think clearly?
You might easily assume that improved physical health drives improved mental health; that a healthy body breeds a healthy mind. But the truth is, we know much more about how exercise affects the body than how it affects the mind. That brings us to the issue of measuring the mind and to the proverbial “mind-body problem”: Is the mind separate from the body? It is a question that has tormented philosophers since René Descartes suggested nearly 400 years ago that mind and body were clearly separate but the brain (the pineal gland to be precise) was the place where mind interacted with body. This Wizard of Oz sort of view suggests that a peek behind the neurobiological curtain will unveil some ethereal forces pulling levers.
Regardless of your view on the issue, accept for the time being that our cognitive, perceptual and emotional faculties—indeed, our sense of self—are all nestled within the 100 billion or so neurons (nerve cells) that make up the brain. So if exercise can have lasting effects on the mind, then exercise must also affect the brain. As it turns out, the brain is incredibly dynamic. It is not hard-wired, as we once believed, and it responds to exercise in much the same way that heart, lungs and muscles do. The brain can change its structure and function by adding new neurons, making new connections between neurons (synapses) and even creating brand-new blood vessels, all in response to different forms of exercise (see Figure 1).
Over the last two decades, neuroscientists have begun to reveal how physical activity—whether it be endurance, strength or skill training—can change the neurochemistry, structure and function of the brain. We are starting to understand how these changes in brain biology affect our cognitive, sensory, motor and emotional behaviors. We are also discovering that the neurobiological imprint of exercise can help treat and possibly even prevent a number of psychiatric disorders (such as depression and anxiety) in addition to neurological disorders (such as stroke, Alzheimer’s disease and Parkinson’s disease).
Exercise Improves Cognitive Function
Although the American College of Sports Medicine (ACSM) recommends at least 30 minutes of moderately intense aerobic exercise 5 days per week (ACSM & AHA 2007), alarmingly it is estimated that 74% of all Americans fail to meet this requirement, and the lack of activity represents a major contributor to rising healthcare costs. Aside from the obvious effects on physical health, there is mounting evidence that a sedentary lifestyle also affects the brain—and in turn lessens mental capacity. Sibley and Etnier (2003) found a clear connection between how much schoolchildren exercised and their cognitive performance: the more aerobic exercise the children engaged in, the better they performed on verbal, perceptual and mathematical tests. The same pattern of results was found in older adults: aerobic training improved cognitive performance (Colcombe & Kramer 2003), and active lifestyles decreased age-related risks for cognitive impairment and dementia (Yaffe et al. 2009). Not surprisingly, these cognitive effects were accompanied by clear changes in brain structure and function.
Exercise Changes Brain Function
The fact that exercise enhances cognition suggests it must have some effects on the brain that outlast the exercise experience itself. That is, exercise must somehow change brain function in a lasting manner. Indeed, research supports this hypothesis; the reduced cognitive capacity in sedentary individuals is also associated with different patterns of brain activity—both at rest and while performing mentally challenging tasks—than those observed in active subjects.
Although a number of brain areas are involved in the complex cognitive tasks that we engage in on a daily basis, the cerebral cortex is a major player. Compared with sedentary people, active individuals show greater baseline levels of cortical activity (Dustman et al. 1990) and more activity in various brain regions when performing cognitive tests (Polich & Lardon 1997). Some cortical areas show increases in activity when we are struggling with a particular task. One such area that is especially sensitive in this regard is the anterior cingulate cortex (ACC). Part of the brain’s limbic system, the ACC has connections with numerous brain areas involved in processing sensory, motor, emotional and cognitive information. This brain area becomes very active during moments of indecision or confusion when we are posed with a problem. After a 6-month walking intervention, people showed decreased ACC activity relative to nonaerobic toning or stretching groups (Colcombe et al. 2004), the implication being that less activity in the ACC contributes to the enhanced cognitive function resulting from exercise.
Exercise-related changes in brain function are not limited to areas of the cortex concerned with cognitive function. Brain areas that are engaged during movement are also affected. One key cortical area is the motor cortex. This strip of tissue contains neurons that send information down to the spinal cord to cause muscle contraction. Individuals engaged in regular exercise show reductions in the amount of activity within the motor cortex when performing simple movements (Voelcker-Rehage, Godde & Staudinger 2010). While this might seem counterintuitive, one interpretation is that the cortex is more efficient at controlling movement and therefore requires less effort to produce movement.
Exercise Changes Brain Structures
Although the structure of the brain is highly complex, it can be broken down into two general components. Gray matter contains all of the neurons and supporting cells, while white matter consists of the axons of these neurons (nerve cell fibers) that carry signals from one area to another. One might compare this to the way in which most large cities are organized, with houses and buildings connected by streets and freeways.
Magnetic resonance imaging (MRI) allows for the measurement of gray and white matter and can reveal something about the way that exercise influences the overall structure of the brain. MRI scans have shown that exercise boosts overall brain volume (Colcombe et al. 2006), increasing both gray matter (Colcombe et al. 2006) and white matter (Gordon et al. 2008). Interestingly, these changes can occur over relatively short periods of time with what might seem like minimal amounts of training. After learning to juggle for only a few weeks, for example, study subjects showed increases in gray matter within regions of the brain concerned with integrating visual and motor information (Draganski et al. 2004).
For more on how exercise affects health and learning, see the sidebars.
Our knowledge of the effects of exercise on the body and mind continues to expand. The development of new technologies is indeed allowing us to “peek behind the cerebral curtain” to see which levers are being pulled—to understand the biology of how exercise improves both body and mind. This knowledge will allow us to harness the brain’s endogenous capacity to adapt to experience and will guide the development of new therapies to treat the damaged or diseased brain as well as to improve our general quality of life. This has become increasingly important as our population ages and the stressors of this fast-paced world mount.
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Neurons are arguably the most high-maintenance cells in the body. They require a constant supply of glucose and oxygen or they begin to die. The brain represents 3% of total body weight (Love & Webb 1992) but uses 20% of total blood supply and 25% of total oxygen supply. Neurons are constantly being bombarded by hundreds of neurochemicals, and the DNA must work incredibly hard to keep up with making all of the necessary proteins to maintain function.
There is one family of neurochemicals known as growth factors. So named because they can make neurons “grow,” these neurochemicals have been clearly shown to increase in the brain in both number and size during exercise (van Praag, Kempermann & Gage 1999). Think of growth factors as “fertilizer” for the brain. They act to keep neurons healthy and reduce their susceptibility to cell death, which may account for why exercise appears to combat the onset of many neurological diseases, including Parkinson’s disease (Xu et al. 2010) and Alzheimer’s disease (Scarmeas et al. 2010).
One of the more exciting discoveries in neuroscience in the last 20 years has been that the adult brain can continue to make new neurons throughout the lifespan. It doesn’t happen equally in all brain areas, for reasons that are not totally understood, but it happens readily in one specific area: the hippocampus. This is an evolutionarily older part of the brain that is concerned with forming memories and processing emotion, which may help explain some of the cognitive and emotional benefits of exercise.
Interestingly, aerobic exercise can increase neurogenesis (generation of new neurons) within the hippocampus at many stages of development, including in the neonatal (Kim et al. 2007), juvenile (Lou et al. 2008) and adult brains (van Praag, Kempermann & Gage 1999). The fact that the hippocampus is a critical brain structure used in memory may explain why aerobic exercise can enhance learning (Vaynman & Gomez-Pinilla 2006). Furthermore, we know that stress reduces neurogenesis, an effect that may contribute to depression and anxiety (Lucassen et al. 2010). Therefore, the enhanced neurogenesis brought about by exercise may represent the neurobiological mechanism by which regular exercise reduces depression.
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Colcombe, S.J., et al. 2004. Cardiovascular fitness, cortical plasticity, and aging. Proceedings of the National Academy of Sciences, 101 (9), 3316–21.
Colcombe, S.J., et al. 2006. Aerobic exercise training increases brain volume in aging humans. Journal of Gerontology, Series A Biological Sciences and Medical Sciences, 61 (11), 1166–70.
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Sibley, B.A. & Etnier, J.L. 2003. The relationship between physical activity and cognition in children: A meta-analysis. Pediatric Exercise Science, 15 (3), 243–56.
van Praag, H., Kempermann, G., & Gage, F.H. 1999. Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus. Nature Neuroscience, 2 (3), 266–70.
Vaynman, S., & Gomez-Pinilla, F. 2006. Revenge of the “sit”: How lifestyle impacts neuronal and cognitive health through molecular systems that interface energy metabolism with neuronal plasticity. Journal of Neuroscience Research, 84 (4), 699–715.
Voelcker-Rehage, C., Godde, B., & Staudinger, U.M. 2010. Physical and motor fitness are both related to cognition in old age. European Journal of Neuroscience, 31 (1), 167–76.
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Yaffe, K., et al. 2009. Predictors of maintaining cognitive function in older adults: The Health ABC study. Neurology, 72 (23), 2029–35.
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