Lex Luthor wrote:
The entire brain is functioning at all times. It's always exercising. There's always inputs from all 5 senses. If by "strengthened" you mean "better trained for a particular task", then you would be correct.
It's a little more complicated than just that. Different parts of the brain are, in fact, more active than others at any given moment, depending on what you're doing. And when you make your brain "better trained for a particular task", what this generally means at a biological level is that you've increased the number of neural interconnections in a particular region of the brain.
As consequence, more work (in the liter, physics sense) is being done by that part of the brain. We can tell because of increased vascular structure (to meet the increased energy demands) in those regions of individuals skilled a particular task vs. the unskilled. And in a more general sense, we can actually see the increased vascular
activity more or less in real-time using a functional MRI (fMRI). Different regions of the brain are clearly more active than others depending on task, and the amount of activity in a particular region of the brain correlates well with the skill set that maps to that region.
There's an interesting case of boy (well, probably a grown man by now) who had a congenital abnormality in which his arms never developed (i.e., he was born with no arms, basically). He taught himself to write, brush his teeth, and do just about everything else using his feet instead of his hands. He agreed to be part of a study on the brain, and what they found was fascinating -- the regions of the brain corresponding to motor control and tactile response for his feet had an incredibly high number of neural connections compared to an average person. His brain's map of his feet was as detailed as a typical person's map of their hands. I wish I could remember his name, but it's been quite a few years since I saw the documentary he was featured in.
Long story short, there's ample evidence of the brain's plasticity, and that specific regions of the brain can, in fact, be made "stronger" (more connections, more robust blood supply) merely through practice, and lots of it. Exactly
how plastic the specific regions are, and under what conditions, is a an active area of neuroscience right now with a lot of applications for rehabilitation/therapy, education, childhood development, etc.
A few other interesting facts/general observations:
It really isn't the case that the entire brain is functioning all of the time. As pointed out above, at a particular time, some regions are active and some are not. But beyond that, at any given moment, only a very small portion of your synapses are actually engaged in transmitting a signal. It's good that your entire brain isn't functioning at once -- you'd be having a grand mal seizure (
http://www.straightdope.com/columns/read/640/do-we-really-use-only-10-percent-of-our-brains)
The brain accounts for only about 2% of your body mass. And yet, it accounts for about 15% of your blood supply, 20% of your oxygen supply, 25% of your blood glucose. (
http://www.acnp.org/g4/gn401000064/ch064.html)