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This is the fifth in an occasional series called 'Ask a Neuroscientist.' Today, we take audience-submitted questions to David Eagleman, a neuroscientist at Baylor College of Medicine in Houston, and William H. Frey II, director of the Alzheimer's Research Center at Region's Hospital  in St. Paul, to learn more about how the brain works.

 We don't know. Part of modern neuroscience's quest is to answer that. One theory goes that, in the same way brains control muscle movement, your brain controls your arms and legs and mouth and so on. Thought might be, essentially, covert muscle movement. In other words, it's going through the same routine that says 'bend this, flex that, extend that' - except that it's not controling a muscle. Instead, it's controling something conceptual.

It's unclear if that's true, but what is clear is brains only exist in animals that have motor movements. If you look at a plant, which doesn't move around, it doesn't have a brain. In fact, there are some species of sea creature - some kind of barnacle, I think - that has a brain and moves around in the ocean until it finds a place to latch on to. And when it gets to its place and settles in, it eats its own brain for nutrition because presumably you don't need a brain if you're not moving.

So what we know is brains seem to have evolved for the purpose of moving around and in more sophisticated animals, thought is something that lives on top of that.

What is the relationship between increased physical exercise and cognitive processes, such as higher executive functions, learning or working memory?

We know that physical exercise and fitness improves memory. It also causes the size of certain parts of the brain that are key to memory to increase in size. And this includes a part of the brain called the hippocampus, which is very important to us for memory and cognition. 

We know this occurs, in part, because exercise increases the production in our body of nerve growth factors. These are natural proteins that actually help the brain to grow and be healthy.

They also protect the brain from disease and degeneration. Actually, people with Alzheimer's who have poor memory and poor cognition are known to be deficient in certain ones of these growth factors. For example, insulin is a growth factor very important to the brain. These are quite low in people with Alzheimer's.

When you run, for example, you actually elevate the concentration of an insulin-like growth factor in the blood. And this growth factor can enter the brain and it can bind to receptors in certain areas of the brain and signal the brain to increase growth. These are important signals that are produced that actually go to the brain and signal the brain to grow and replace worn-out parts.

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Tom Weber

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Neuroscientists answer your questions: How does a brain think? And what is the relationship between increased physical exercise and cognitive processes

Ask a Neuroscientist: Thinking and physical activity

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