Neural Plasticity: What is it?
Essentially, plasticity
is a word that neuroscientists use to refer to physical changes in the brain
that seem to correspond to learning and memory. We don't completely know
how it works, or what the connection is between individual neurons and memories
or thought processes, but they definitely appear to be related.
Neurons perform a variety of functions in the
nervous system, but two of them are important here: they form a relay network,
and they cause things to happen as a result of relayed information. A prototypical
neuron is composed of multiple parts, as shown in the diagram to the right.
There are three important ones, though. There's a cell body (or soma), which contains parts that
keep the neuron alive; there are axons, and there are dendrites. Axons and
dendrites are a sort of reciprocal network; axons send signals out, and
dendrites receive signals and interpret them. There are a lot more details
to this process, but most of them aren't really important to understand
right now. What's important is to remember how neurons perform their functions:
a combination of chemical and electrical signaling that relays messages
throughout the nervous system. An electrical impulse in one part of the
neuron causes chemical messaging in another part of the cell, and those
chemical cues are picked up by still more neurons. Eventually, out of all
of this, you get behavior.
The link between a single neuron and, for
example, a muscle fiber response is relatively easy to picture. The neuron
transmits its signal (called an action
potential) down the axon, and when it gets to the end, the muscle
fiber twitches. The picture below illustrates a slightly more complete pathway:
say a touch receptor on your fingertip is stimulated. That nerve sends its
action potential to the brain, possibly by crossing some other neurons on
the way (especially in the spinal cord). Then the brain sends a response
back (like a twitch to move a fly off your hand, for example) and the signal
goes back down a second pathway, complementary to the first. Action potentials
only go one way on each neuron--this is why it takes one path to get
to the brain and another to return to the hand. We can replicate this in
living or dead organisms.
In theory, the brain works the same way; each axon transmits
information to other neurons, and eventually this ricocheting electricity
causes things to happen. But old models of the brain, in which neuron structure
was fixed, didn't explain one vital aspect of behavior: it changes.
Rats learn how to navigate their way through water mazes; human beings learn
to drive. When you were three years old, seeing cookies on the table might
have made you excited; now, you think about fat and clogged arteries and
dying of a heart attack at thirty -- you might go ahead and eat it anyways,
but all of the things that go into your mental image of "cookie"
have changed. If each neuron's structure is fixed, how do we account for
this kind of change?
Well, there are two solutions. The first one is to conclude
that there is some kind of mystical connection between thought, behavior
and learning that just doesn't happen in the brain. It happens in your soul,
perhaps. Or in the collective unconscious, for all we know. These kinds
of speculations are perfectly valid, but they don't get neuroscientists
grant money, either. And so they began to investigate learning even further.
And they discovered this nifty little thing called Long-Term
Potentiation.
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Memory main page