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TOM COSTELLO: Take a walk through the halls of the
American Museum of Natural History. You're really traveling
back in time, following the evolution of creatures that have
swum, crawled and walked on Earth for hundreds of millions
of years. For as long as life has been evolving on earth,
the brain has been evolving too, following a twisted path from
species to species.
MELINA HALE: Most animals have brains. Insects have brains.
Things like crabs and lobsters have brains. All vertebrates,
like us, and fish have brains.
COSTELLO: Melina Hale is a neurobiologist at the University
of Chicago who is funded by the National Science Foundation. She
studies the brains of different types of fish that have evolved
over time in order to unlock the secrets of how the brain works.
To accomplish this, Hale focuses on one of the brain's main jobs:
generating movement.
MELINA HALE: Really, everything animals do, involves movement.
If we smell something that smells wonderful,
we might move to it. If we sense something that may be
dangerous, we may move away from it.
COSTELLO: But getting an animal's limbs, head or tail to
move isn't as simple as it might seem. The brain has to take in
information from the body, process it, then send signals to
the muscles, so they can contract and move the body's
bones – all in the space of a few milliseconds. Scientists
know that messages are carried throughout the nervous system
along nerve cells called neurons, and that neurons are
organized into circuits that generate particular functions.
But how these neural circuits work remains a mystery.
MELINA HALE: We want to understand what they are, how
they're connected together. What are the subtypes of interneurons
that you need to build a neural circuit,that can drive movement?
COSTELLO: To better understand how neural circuits work, Hale
and her team of graduate students study a species with a
brain that is much simpler than the human brain: the zebrafish.
MELINA HALE: We actually aren't even studying the large
zebrafish, that are about this big. Instead, we're studying
their larvae, which are teeny-tiny. You can barely see
them, they're about three milimeters long.
COSTELLO: It helps that zebrafish larva are completely
transparent, letting you peer right inside their brains.
When Hale and her graduate student Richard laser-scan the
brain of a genetically-modified zebrafish larva, they can
actually see the neurons in the fish's brain firing as
different parts become active.
MELINA HALE TO RICHARD: Whoa! That looks good. I am kind of
surprised that there's this whole band of activity.
COSTELLO: Amazing new technologies are letting
scientists, like Hale observe – for the very first time – what
neural circuits look like when an animal moves. Like in this
video which shows eighty percent of the neurons firing inside
a larval zebrafish's brain.
MELINA HALE: I love watching the cells be active, while the
animal is behaving. It's just remarkable to me that we can see
the brain work and try to understand how it's functioning.
COSTELLO: Hale and her students examine an even smaller part of
the zebrafish's brain: a simple circuit called the "startle
circuit," which may hold clues to how all neurons work.
MELINA HALE: One of the great things about the startle circuit
for scientists is that it's very simple. So it allows the animal
to respond really quickly to a threatening stimulus.
COSTELLO: They also startle an Australian lungfish and observe
how it moves. It's one of the few survivors of a prehistoric
family of ancient vertebrae -- making it our distant cousin.
MELINA HALE: They have basically the same startle circuit that
evolved over five hundred million years ago, really.
COSTELLO: By carefully studying how a simple neural circuit
controls movement in an ancient fish -- and a tiny one, Hale is
piecing together how nervous systems have evolved in all
animals, and how they interact with the animal's bodies to
generate movement. Scientists won't fully understand how the
circuitry in the human brain works for a long time. But by
studying circuits in the simplest and oldest of animals,
they're hoping to one day answer to the vast questions
we humans have about the brain.
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