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
What Is the Problem in Knowing How the Brain Is
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
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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