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How Do We Know How the Brain is Wired?

Have you dissected a brain yet?  ... perhaps a frog or a sheep brain?  You can find an Internet guide to dissection of the sheep brain by clicking here.

What Is the Problem in Knowing How the Brain Is Wired?

Neurons (nerve cells) are too small to see without a microscope. And even if you use a microscope, you only see a two-dimensional view, like this computer screen.  A two-dimensional view only lets you see information on one plane. Microscopes do not allow you to see three-dimensional depth.  The third dimension extends perpendicular to the two-dimensional plane. 

Neurons often have long extensions of their cell body that go in all three dimensions. 

Look at this diagram of a neuron. At the top is the cell body with its nucleus. The cell body gives rise to a long extension (called an axon) that projects to a target, either a muscle, a gland, or another neuron. Near the end of the axon are many, many small branches.

Now, think what this would look like if you cut across the neuron and saw a two-dimensional picture of the cut at the top line.  Can you draw it? You would see a green dot on the left, followed by a large white space, followed by a blue piece of nucleus, followed by another white space, and ending in a small green dot on the right. 

Make similar two-dimensional drawings of what you would see at a cut in the middle and another cut at the bottom of the picture. Such cross cuts would not tell you much about what a neuron looks like, would it? It would show even less about where this neuron's extensions go and connect with.

So how do scientists figure out where neurons go?

Tracing Neural Pathways

Several techniques help scientists know where neurons go.  Much of this research is done on anesthetized animals, but the major pathways have been confirmed in various ways in humans.:

  • Kill the neurons (with toxins or heat) and see where else in the brain degeneration appears. Other regions of the brain, that received information from the killed neurons, will also die.
  • Electrically stimulate an area and record in various other places to see where you get responses. The responses have to be quick (less than a few thousandths of a second) to be a meaningful indicator of a direct connection. By various multiple links, you could say that everything in the brain eventually connects to everything else.
  • Inject radioactive tracers into a known area and observe where the radiation shows up. Neurons transport materials (including certain radioactive compounds) down their axons. You can not only see where the tracers end up, but the rate of their appearance tells you the transport rate.


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