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Thanks for watching this episode of SciShow! And thank you again to The Kavli Prize for supporting this episode. The Kavli Prize in Neuroscience is awarded for outstanding achievement in advancing our knowledge and understanding of the brain and nervous system. To learn more about Dr. Marder’s work, you can visit page:

We used to think neural circuits were rigid and robotic, but now we know that's not true -- thanks to crab stomachs.

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This episode was made in  partnership with The Kavli Prize.

The Kavli Prize honors scientists for  breakthroughs in astrophysics, nanoscience, and neuroscience – transforming our understanding  of the big, the small, and the complex. In the early 1970s, computers  were suddenly everywhere.

People saw a lot of promise in these  machines that took an input like a keystroke, passed messages along a series of wires, and resulted in an output, like  displaying that letter on a screen. Neuroscientists also saw  something that appeared familiar: the wiring of the human nervous system. The way our neurons take an input, pass  electrical messages between neurons in a specific sequence, and then deliver an  output seemed very similar to a computer.

Scientists came to think of the brain that  way, and of behaviors as being “hardwired.” But as satisfying as the one input,  one circuit, one output comparison was, researchers soon realized that the millions  of things our nervous system can do would require more hardwired circuits  than it could possibly contain. An innovative neuroscientist named Eve  Marder had a completely different theory for how our nervous systems work. And to prove it, she needed—of  all things—crab stomachs. [♪ INTRO] Think about walking for a second.

All the muscles in your legs, ankles, and  feet have to be able to move up, down, left, right, at a 37 degree bend, faster,  slower, with more or less pressure so that you can adjust to the  terrain without losing your balance. Walking is a behavior – an action  or response to the environment. And understanding how those behaviors are produced is important for knowing how  to fix them when they go wrong.

We’ve known for a long time that  brain activity involves electricity. Anatomists could see the networks of nerves connecting to one another and  to muscles throughout the body. So it was appealing to imagine electricity traveling along those just like  it does wires in a circuit.

The brain would produce an instruction,  the “wires” would carry the signal, and the result would be some behavior. That intuitive idea was the base assumption  in neuroscience around the 1970s. But it began to fall apart as researchers developed a better understanding of neuroanatomy.

If human bodies were wired like a  circuit board, you’d have one circuit for each of those kinds of walking – go  slow, go fast, go up a slope, and so on. Soon, researchers could see there weren’t  enough neurons in the body for that. Second, everyone’s nervous system  is “wired” a little differently.

But give or take some differences  in ability between us, all our “circuits” always produce  the same behavioral results. Small differences in individual  bodies get overridden. Finally, while individual  neurons live for our whole lives, they’re still cells that have parts  that wear out and need to be replaced.

But neurons aren’t like Formula 1 cars that get to take a quick break to swap out parts. Somehow they manage to put new tires  on while they’re still driving. None of those things made sense if you  had one circuit producing one behavior.

A circuit board just couldn’t  explain all that adaptability. It wouldn’t keep working as it’s being repaired, or work at all if the system didn’t  have an impossible number of circuits. So it was kind of a mystery how the  nervous system accomplished these feats.

And it was a mystery Brandeis  University professor Eve Marder was determined to solve. Marder’s research focused on the  stomatogastric ganglion, or STG, a group of about 30 neurons  connected to a crustaceans’ stomach. The STG neurons control chewing, sort of.

Crabs and lobsters don’t chew with  their mouths and then swallow. Instead, muscles in their  stomachs expand and contract to grind their dinner into digestible pieces. And one nice thing about these STG neurons  is that they keep doing their thing even when you pull them from the  organism and study them in a lab.

Marder researched the activity of  the neurons as they were influenced by neuromodulators – molecules  that influence what neurons do, like the neurotransmitter dopamine. At the time, most neuroscientists  thought of neuromodulators as working on individual neurons. Molecule A goes into neuron slot  B and makes that neuron send out an electrical signal that tells the next  neuron what to do, and so on down the line.

They were thought to just be carrying  the signal to create the behavior, like an old-timey bucket  brigade putting out a fire. In the circuit-board view, that  same chain of the exact same neurons would pass a signal every time  molecule A went into neuron slot B, and it would always end in the same behavior. But Marder found that  neuromodulators were influencing the entire group of STG neurons together.

Instead of a circuit that just passed  a signal along, the neuromodulator was talking to everybody at once, and  everybody had a slightly different reaction! One neuron might have its activity turned up and another turned down by  the exact same molecule. That meant the muscles in those  crustacean stomachs could produce a whole bunch of different outcomes  with one very simple neural circuit.

The implications for our own  nervous systems were clear. Marder could now prove how we  get really fine-tuned behavior, like the many different ways to walk, without needing a nearly infinite  number of different neural connections. Her work also addressed how different individuals could produce the same behaviors.

In general, the crabs and lobsters she worked with weren’t bred to be as identical as  possible, like most lab animals are. Instead she got them from the  wild, straight from fishing boats. And like most wild creatures, those crustaceans had a lot of  variability in their nervous systems.

Since all crabs do crab things, studying  wild crabs allowed her to explore how different circuits could be and  still result in the same behavior. As it turned out, some of the  individual crabs had connections between some of their neurons that were  six times stronger than other crabs. But the difference didn’t matter.

Both circuits had the same result. So there are multiple  solutions, multiple pathways, that can get you to the same point. Which isn’t just important between individuals.

It’s also important within individuals. This explained how, as individual  parts of our neurons wear out and are swapped, they keep functioning. They can create the same end result from  a different circuit or part of a circuit.

Since Marder’s discovery,  neuroscientists have been busy using that new understanding  of how the nervous system works to investigate all kinds of  problems with the human nervous system. For example, understanding how neuromodulators create flexibility in the nervous  system could benefit stroke treatment. When someone experiences a stroke, part of their brain gets cut  off from blood flow and oxygen.

If it’s cut off long enough, those neurons die. But we know that after a stroke, the brain can make new connections that  allow it to compensate for what was lost. It doesn’t have to rebuild the same circuits.

Marder’s insights could help  researchers figure out how to rebuild those new connections to  fill in the gaps more efficiently. And as Marder continues to  study wild-caught animals, she’s also interested in how they  are affected by the changing climate. For her work describing how nervous  systems both change and stay the same, Eve Marder was awarded the 2016  Kavli Prize in Neuroscience.

And deservedly so! Because science is kind of like the way  we’ve thought about our nervous system. When we keep doing things the  same way, we get the same outcome.

One input, one circuit, one output. But with a few changes to  how we think about things, entirely new pathways are possible. Thanks for watching this episode of SciShow!

And thank you again to The Kavli  Prize for supporting this episode. The Kavli Prize in Neuroscience is  awarded for outstanding achievement in advancing our knowledge and understanding  of the brain and nervous system. They also award a nanoscience  and astrophysics prize, honoring researchers for transforming our  understanding of the science of atomic, molecular and cellular structures, and  advancing our knowledge of the origin, evolution and properties of the universe.

If you want to learn more about Dr. Marder’s work, you can visit her  page on the Kavli Prize website by clicking the link in the description. [♪ OUTRO]