(host) This is what a thought looks like, or many thoughts, thanks to a special microscope that can visualize activity inside single nerve cells.

And even though this brain belongs to a tiny fish, your thoughts work in exactly the same way.

Everything that you think and do comes from neurons talking to each other.

In your brain, there's about 86 billion neurons, each exchanging signals with hundreds or thousands of others, building a network with more possible connections than there are stars in a thousand Milky Way galaxies.

That is pretty dang cool.

But, like, what is a thought, really?

I mean, how do neurons actually work?

What are these messages that they send inside our bodies?

And how fast do those messages travel?

And what does it have to do with a cockroach?

Electricity.

Every thought, every move you make, everything you see, hear, and smell, every heartbeat, all the love, pain, humor, wonder you've ever felt, every dream, every memory-- they all happen thanks to electricity.

And today, I'm going to show you with some real neuroscience experiments how all that happens down on its most basic level, in this incredible cell called the neuron.

[light orchestral music] Hey, smart people.

Joe here.

So you're a multicellular creature, which is pretty great, but it gives our bodies this problem to solve.

Our cells have to talk to each other.

But to explain why that's a problem, I want to stop talking about biology for a second and talk about William Henry Harrison.

Yeah, like the ninth president of the United States.

So in the mid-19th century, the young country now stretches from the Atlantic to the Pacific, and getting a message from one end to the other back then took forever.

So William Henry Harrison was famously inaugurated on this cold, wet day in March 1841.

He refuses to wear an overcoat, gives the longest inaugural address of all time, parades on horseback instead of in a carriage, and catches pneumonia and dies after just 31 days in office.

What's crazy is, it took 110 days for news of his death to reach California.

That's, like, three times longer than he was president.

Because the speed of communication was limited by the speed of a horse until this happened.

Beginning in 1861, when the transcontinental telegraph was completed, people on opposite coasts could communicate almost instantaneously.

This changed everything.

I mean, sure there were stations along the way where the message had to be decoded and passed on, but instead of the speed of a horse, the telegraph was only limited by the speed of electricity.

Okay, now we can talk about biology again.

Just like New York and California in the 1800s, your body is faced with this problem.

How do cells that are super far apart talk to each other?

Well, they can use chemicals.

That's what single-celled things like bacteria do, and your body does it too.

You ever had butterflies in your stomach?

That's caused by a chemical released into your blood and distributed by diffusion.

That chemical communication is kind of like William Henry Harrison's death finally reaching California.

Over long distances, it's slow.

I mean, if you stepped on something hot or sharp, you wouldn't want to depend on chemicals to send the signal to your brain.

Nerve cells solve this problem.

They let different parts of our body talk to each other fast.

One way that they do this is, nerve cells are stretched way out so two cells that want to talk can just be closer to each other.

Chemical signals between cells don't have to diffuse very far, so they can trade signals pretty fast.

But now we have this new problem.

How do you get a signal from one end of this stretched-out cell to the other and fast?

Electricity, just like that telegraph we talked about.

There's something like 60 kilometers of neurons in your body shooting tiny pulses of electricity to the other.

But it's not like electricity that powers a lamp or a cybertruck.

It's living electricity, and that part of our story actually begins in Italy in the late 1700s with a frog, only the frog is dead, and actually, it's just the frog's legs.

Okay so this is the end of the Enlightenment, and for the first time, people were systematically trying to explain how the universe worked by taking things apart down to their fundamental bits-- things like gravity, light, chemistry, and electricity.

So doctors of that time sort of viewed the human body as a machine, where, if you understood all of its parts maybe you could understand how the whole thing worked, which means they were really into dissecting bodies.

And that's where the frog legs come in, thanks to this guy, Luigi Galvani.

He has this weird idea that maybe electricity is alive.

Like when you rub a piece of amber, which is basically fossilized tree sap, why does it attract stuff, or why can some fish give off electric zaps?

Where does this electricity in living things come from?

One day, Galvani's cutting up some frog legs, and he gets this little static electricity shock, and suddenly, the frog's leg twitches.

It also worked when a storm was nearby too.

Wire up a lightning rod, and the legs kick.

This was a big discovery.

A body's movement is linked to electricity, not psychic fluids or magic or whatever people thought before that.

But one day, something weird happened.

Galvani just touched the legs with a couple of metals and they twitched, no lightning, no spark, and this made Galvani conclude that we are full of some electrical fluid, that the electricity that made things move was already inside the body.

He called it animal electricity, and this idea made Galvani super famous.

One of his fans was this young, British writer named Mary Shelley, who wrote a book about it.

Maybe you've heard of it.

But it also got the attention of another Italian guy, Alessandro Volta.

Yeah, that's the guy we named the volt after.

Volta checks Galvani's notes, does his own experiments, and realizes that certain metals, when they touch, can create electrical current thanks to charge passing between the metals.

And this leads him to invent the first real battery, the voltaic pile.

One of these.

This is a replica of one of Volta's earliest batteries, a voltaic pile, and I made this one myself.

It's not very big, but it's impressive.

I made this one out of some common household items: some zinc-plated washers that I got at the hardware store, regular old pennies coated in copper, a saltwater solution-- just regular table salt will work--and these absorbent paper circles cut about the same size as our pennies and washers.

Okay, let's build a battery.

We'll start off with a little sheet of aluminum.

This is just regular aluminum foil.

It's only going to act like a conductor.

Then we take a zinc washer, put that there, we take a circle of our paper, dip it in our saltwater solution, dab it off not too much, stack it on top of our zinc washer, and put a penny on top.

Okay, I've got my meter set here to DC voltage.

Let's see if we've got anything.

Wow, so from one penny and one washer, I've already got over half a volt.

How does this work?

Electricity is basically moving charges.

Some of the zinc atoms from the washer turn into zinc ions and dissolve into the salty solution, leaving behind two electrons.

When we close the circuit, those electrons flow through to the copper and come to rest in a different molecule.

Those flowing charges are electricity.

The force driving electrons to fall down from one metal to the other is called voltage.

So stacks and stacks of electrons wanting to fall from one side to the other-- well, that adds up, and we can get a big rush of electricity through it when we connect the ends.

That's how batteries work.

All right, that is ten stacks, so let's see what kind of voltage we're cooking with now.

All right, in our little homemade battery here, I'm creating over two and a half volts.

Pretty awesome.

But what can we do with it?

Well, I've got an idea, and it involves some cockroaches-- well, just their legs, really.

We can replicate one of Galvani's famous experiments using one of these guys, Hi.

Where are you?

Come out.

It's going to be fun.

Come do some science with us.

Guys... One strong roach.

Jeez, this roach didn't skip leg day.

Okay, I've got one of our roach friends, and I'm just going to need its legs, so first I'm going to drop it in some ice water.

This is basically going to put the roach to sleep.

You guys don't want to see this.

Dab him off, not too wet.

So we are going to remove one of this cockroach's legs, but don't worry, they'll grow back.

They're so small.

And there we go.

We got the leg.

Let me put this guy back with his friends.

So I've got our cockroach leg here on our little platform.

Grab a couple of these pins.

One down here in the bottom part of the leg, one up here at the top part of the leg.

Now we'll hook these cables up to our battery right here.

Touch these to one of our wires over here.

And when we touch the second wire to the pin... Did you see that?

The leg twitched.

This is so cool.

Voltage from our homemade battery is stimulating muscle activity inside of this cockroach leg because it's making nerves fire.

Do-do-do-do.

If you thought that was cool, you can even do this with music because the signal coming through a headphone cable is basically just a voltage that speakers would turn into sound, but we can turn it into this.

Look at that.

The leg is twitching along to the beat.

That's a sick beat, man.

The voltage from our musical signal is stimulating that cockroach leg.

This is incredible.

We just replicated one of the very first experiments in all of neuroscience, although I don't think Galvani and Volta had hip-hop.

But anyway, now let's do something different.

Let's listen.

Okay.

Now I'm going to plug these electrodes in to my special neuron detector box here.

Flip it on.

Now, the signal you're hearing right now is mostly just background noise from the lights, from my computer plugged in, all this electric stuff around me.

But watch what happens when I push on this roach's leg.

[static spikes and dips] There is no external electricity this time.

There is no battery attached to this.

This is electricity coming from inside of this leg.

Now, how do neurons detect stronger signals versus weaker signals?

Well, they actually do that by the rate at which they fire.

The harder I push on this leg, the faster those spikes go.

[scats "Shave and a Haircut"] All right.

I'm basically the most accomplished neuroscientist of the 1790s now.

So Galvani and Volta got in this big fight.

Galvani is all like, "Animals can make their own electricity."

And Volta says, "No, that's ridiculous.

"The salt inside the frog legs "and the metals you touched them with "created the electricity, and that's why the muscles twitched."

They were kind of both right, because outside electricity, like from a battery, does make nerves fire.

But we do also have a form of electricity inside our bodies, in our nerves, just not in the way that Galvani thought.

So how does that electricity inside our bodies happen?

Well, first we need to get to know the hardware of your nervous system, the neuron.

There are lots of different kinds of neurons, but they're all built pretty much the same: a cell body with the nucleus inside and these things sticking off called dendrites, which are how neurons listen for messages from other neurons; this long part, called an axon, which acts like a wire to send the signal from the listening end down here to this end; the synapse, the gap where one neuron can pass the signal to the next.

Now, we don't find neurons in plants or fungi or anything else, only animals.

In fact, all animals have neurons except sponges and--and whatever these are.

And some of these neurons can be huge.

I mean, the biggest animals are millions of times more massive than the smallest ones.

The neurons running down a giraffe leg can be a few meters long, and there is one axon in a blue whale-- scientists think it could be the longest axon of any animal, a single cell more than 25 meters long.

But there is another huge axon in this animal, the North Atlantic squid.

It's like a millimeter in diameter, like a thousand times the diameter of a human neuron.

And this squid neuron is really important to the history of neuroscience because it let us figure out this.

It's called the action potential.

Action potentials are the zaps in our nerve cells, our living electricity.

Now remember how I told you a battery makes electricity by separating charges and then letting them flow downhill?

That's exactly what happens inside of a neuron.

This is a cross section of a neuron's axon.

And there is this pump that connects the outside of the cell to the inside, and it's constantly pumping charged atoms, or ions, in and out like a revolving door.

It's pumping positively-charged sodium out of the cell and positively-charged potassium into the cell.

And outside of the cell are also all these negative chloride ions.

In the inside of the cell, we find a ton of negatively-charged molecules like proteins and stuff.

So a neuron is like a banana in the ocean.

It's full of potassium in this salty outside world.

When you add all of these charges inside and out, a neuron just sitting there, not doing anything, is negative inside.

And thanks to huge neurons like the one from that squid that we were talking about, scientists have been able to actually stick tiny wires in and measure that voltage difference.

It's about minus-70 millivolts.

So we have this separation of charges like a battery.

A neuron is more negative inside than outside.

But we also have a chemical potential.

Okay, what does that mean?

Well, sodium wants its concentration to be the same on both sides of this wall, so sodium wants in.

And potassium-- it wants its concentration to be the same inside and out, so it wants to leak out of the cell.

But the membrane doesn't let that happen, except there is these little doors in the wall.

Some doors only let sodium through, some only let potassium through, and they only open if the voltage is just right.

Now you're ready to see how an action potential works.

Up here, a dendrite receives a little splash of a chemical from the neuron next door, and that signal says, "Let a little sodium in," and that ticks the voltage up just a tiny bit.

Blip, signal, a little bit of sodium.

Blip, signal, a little bit of sodium.

But if the cell body gets a big enough signal from its neighbor and the voltage hits this magical threshold, something incredible happens.

All these sodium-only doors suddenly open, and positive sodium rushes in, and the voltage inside the cell shoots way up in, like, a millisecond.

But then the sodium-only doors slam shut, and these potassium-only doors open.

So potassium rushes out, so the voltage drops way down.

And then the sodium-potassium revolving door pump chugs along and gets everything back to where we started, minus-70 millivolts.

All of this happens in, like, 5 milliseconds, and one little action potential explosion leaks down the axon.

Boom, hits the threshold, sodium doors open.

Bam, they shut.

Potassium doors open.

Bam, they shut.

And this explosion causes another action potential down and down the axon, a chain reaction of chemical electricity traveling from cell body to synapse.

Whew.

And at the synapse, a splash of chemicals released sent over to the neuron next door, and the chain reaction goes on.

All of these little living electrical messages happen in just a few thousandths of a second.

When I tell my hand to move, it feels like that signal travels to my hand instantly.

But not even light, the fastest signal in the universe, travels instantaneously.

So how fast is a nervous system?

Is it faster than a car?

Faster than a plane?

Faster than my phone?

Well, ever notice when you stub your toe, you can feel the impact almost instantaneously, but the pain takes a couple of seconds before you feel it?

That's because these two signals, touch and pain, travel on two different types of nerve fiber with very different speeds.

In your slower neurons, an action potential chain reaction can move down the axon between one-half and two meters per second, around four and a half miles per hour.

But some nerves can speed up this chain reaction by being wider, the same way a wider pipe can let more water flow through, or by wrapping themselves in this insulation called myelin, kind of like insulation around a wire.

That myelin around the axon lets an action potential chain reaction jump down an axon from node to node way faster.

Incidentally, "Nodes of Ranvier" would make a great band name.

"Nodes of Ranvier."

I don't know.

We'll do something with that.

In these insulated nerves, signals can travel down an axon at 80 to 120 meters per second.

That's like 270 miles per hour.

So depending on the neurons, the speed of thought can be a slow jog or a screaming race car.

You're made up of dozens of different types of cells, from bones to skin to blood to spleen-- whatever a spleen does-- but neurons have to be the most amazing cells in your entire body.

Stretched like wires, they can make their own electricity.

They can transmit signals from head to toe in fractions of a second.

And if you get enough of them together in one place and give them a few million years of evolution to wire themselves up, they can figure out the entire universe.

They can even understand themselves.

At least, I hope you do now.

Stay curious.