Probably the best-understood type of physical change in the brain associated with learning has to do with Long-term Potentiation (LTP). LTP is a mechanism by which individual neurons and sets of neurons increase their responsiveness to particular signals. If a particular axon sends signals to a particular dendrite repeatedly, the receiving dendrite will change its structure so that it responds with greater sensitivity. This usually occurs by bringing the axon and dendrite closer together, by increasing the surface area of the dendrite, or by making the surface of the dendrite more densely populated with receptors.

Basically, if the dendrite gets busy, it pays more attention to the axon terminal that's signalling it. Think of it like an internet connection: if you don't use it very much, and it's expensive, you might keep one phone line for both the phone and the internet, and use a cheaper, slower modem. But say you start needing it for research or work. Then it might be worth installing a second phone line; you can get more done with less effort! If suddenly your entire family is using the internet constantly? Then it might be worth getting cable or DSL, and installing a LAN to make everybody happy.

It's important to remember these changes are both permanent and reversible. That might sound contradictory, but it isn't. Say you've upgraded all the way to a T1 and LAN, because you need the internet so much it's vital. And then all the family members needing it...stop using it. They go to college. They decide it's only for geeks. Or that it's a tool of Satan. It's expensive to keep a T1 line up, so after a while you decide your money would be better spent elsewhere; you go back to two phone lines. And eventually to only one, because you just don't need it that much. Are you back where you started? Well...almost. The connection is the same strength. But all the lines you installed are still there. The holes in the wall have been drilled, and they're not going away. So if, in a few years, your children should move back into the house because they've just graduated with English majors and have to work at McDonald's, the potential for the upgrade is still there. It'll take a little while, and some money, but less time or money than it took the first time.

That's exactly what scientists see in the brain, too. A neuron that's changed its structure to associate strongly with another neuron can have that learning overwritten, if the connection is no longer important and other connections become vital. It only has a limited amount of metabolic energy (the biological equivalent of time and money), after all. So after a long period of little stimulation, the strength of the connection might decline. But if, suddenly, the connection becomes important, the LTP occurs again, and much more quickly than it did the first time.

The connections between LTP and learning have been verified experimentally in a couple of different ways. Primarily they rely on classical neuroscientifical technique: disrupting a normal process and seeing what happens. For example, if researchers inject a substance called AP5 into the hippocampus (one part of the brain, explained in the next section), one of the receptors responsible for LTP no longer functions. And when that happens, experimental subjects have trouble acquiring certain types of memories. It doesn't interfere with using previously established memories at all (Campeau et al 1992; Fanselow & Kim 1994).

And at about this point, you're wondering why I've bothered explaining all of this; pages of text without a single mention of stem cells! Fear not, all shall be revealed to you:

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