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’s pretty dang cool.
But… like, what is a thought… like, really?
I mean how do neurons actually work?
What are these messages they send inside our bodies?
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 gonna show you, with some real neuroscience experiments, how all that
happens, down at its most basic level: In this incredible cell called
a neuron.
[OPEN] 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.
Like, the 9th 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.
But 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!
That’s 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.
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.
Ok, 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.
Ever had butterflies in your stomach?
That’s caused by a chemical released into your blood and distributed by diffusion.
But that chemical communication is kinda like
William Henry Harrison’s death finally reaching California.
Over long distances, it’s slow.
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 they do this:
nerves cells are stretched way out, so two cells that wanna 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 km (37 miles) of neurons in your body, shooting tiny pulses
of electricity from one end 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.
Ok, 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.
Quick side note: I’ve got this whole episode on some of those crazy early electricity experiments,
you should go watch that.
Anyway…
So doctors of that time sort of viewed the human body as a machine, where if you understood
all 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’s has this weird idea, that maybe electricity is alive.
Like, when you rub a piece of amber, 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’re 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 is this young British writer named Mary Shelley, who writes a book about
it.
Maybe you’ve heard of it.
But it also got the attention of another Italian guy: Alessandro Volta (yeah, 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 early batteries.
A voltaic pile.
I made this one myself.
It’s not very big but it’s impressive!
I made this one out of some common household items.
Zinc washers from 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.
Ok let’s build a battery!
Let’s start with a little sheet of aluminum, just regular aluminum foil.
It’s only gonna act as a conductor.
Take a zinc washer, take a circle of our paper, dip it in our saltwater solution, dabit off,
not too much, stack it on top of our zinc washer, and put a penny on top.
Ok, I’ve got my meter here set to DC voltage.
Let’s see if we’ve got anything.
Wow!
So from 1 penny and 1 washer I’ve already got over half a volt.
How does it 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 (in the washer)
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".
Stacks and stacks of electrons wanting to fall from one side to the other, that all
adds up and when we connect the two ends, it can send a big rush of electricity through
when we connect the ends.
That’s how batteries work!
Ok that is ten stacks!
Let’s see what kind of voltage we’re cookin’ with now.
Alright in our little homemade battery here I’m creating more than 2.5V, pretty awesome!
But what can we do with it?
Well I’ve got an idea, and it involves some cockroaches.
Just their legs, really.
We can replicate one of Galvani’s famous experiments using these guys.
Hi, where are you?
Come out.
It’s gonna be fun, come do some science with us!
That is one strong roach.
Jeez this roach didn’t skip leg day.
Ok I’ve got one of our roach friends and I’m just gonna need its leg, so I’m gonna
drop it in some ice water.
This is basically gonna put the roach to sleep.
You guys don’t wanna see this.
Dab him off, not too wet.
Ok so we’re gonna remove one of this cockroach’s legs.
Don’t worry, they’ll grow back.
They’re so small.
There we go!
We’ve got the leg, I’m gonna put this guy back with his friends.
Ok so we’ve got our cockroach leg here on our little platform.
Let’s grab a couple of these pins, one down here in the bottom part of the leg, one here
in the top of the leg.
Now we’ll hook these cables up to our battery.
One here.
One of our wires here.
And when we touch the second wire to our pin… did you see that?
The leg twitched!
This is so cool.
Voltage from our homemade battery is stimulating muscle activity inside this cockroach leg,
because it’s making nerves fire.
[beats]
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 normally turn into sound.
But we can turn it into this.
[beats, music]
Look at that!
The leg is twitching along to the beat.
Sick beat man!
The voltage from our musical signal is stimulating our 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.
Now I’m going to plug these electrodes into my special neuron detector box here.
Flip it on.
The signal you’re hearing right now is mostly just background noise from these lights, from
my computer, from all this electrical stuff plugged in.
But watch what happens when I push on this roach’s leg.
[sounds] There’s no external electricity this time,
there’s no battery attached to this.
This is electricity coming from inside this leg.
[sounds] How do neurons detect stronger signals versus
weaker signals?
They actually do that by the rate at which they fire.
The harder I push on that leg, the faster these spikes go.
[funny pokes] Alright, I’m basically the most accomplished
neuroscientist of the 1790s now.
So Galvani and Volta got in this big fight.
Galvani says that “animals can make their own electricity.”
And Volta says “No that’s ridiculous
the salt inside the frog’s legs
and the metals you touched them with
created the electricity,
and that’s why the muscles twitched”
And in the end, 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?
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.
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 to here, to this end: the synapse, the gap where one neuron can pass the signal
to the next.
We don’t find neurons in plants or fungi or anything else.
Only animals.
In fact, all animals have neurons except sponges 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’s 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’s another huge axon in this animal, the North Atlantic squid,
it’s like a millimeter in diameter, like 1000 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:
[Action potential blip]
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 a neuron.
This is a cross section of a neuron.
And there’s 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 all these negative chloride ions, and 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 a salty outside world.
When you add up all 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 we were talking about, scientists
have been able to stick tiny wires in and measure that voltage difference.
It’s about -70 mV.
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.
OK… what does that mean?
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’s 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 tiiiiny bit.
Blip… signal… little bit of sodium.
Blip, signal, 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 (woosh), 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 (woosh) so the voltage drops way down.
And that sodium/potassium revolving door pump chugs along and gets everything back to where
we started: -70 mV.
And this all happens in like 5 milliseconds!
And one little action potential explosion leaks down the axon, boom, it 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!
And at the synapse, a splash of chemical is released, and 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, or faster than a cell phone?
Ever noticed, when you stub your toe, you can feel the impact almost instantaneously,
but the pain takes a couple 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
0.5-2 meters per second.
That’s about 4.5 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)
In these insulated nerves signals can travel down an axon at 80-120 meters per second.
That’s about 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, 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.
Want more science content?
Then you’ll want to check out PBS’ new show Animal IQ.
Hosted by Trace Dominguez and Dr. Natalia Borrego, Animal IQ features deep dives on
animal minds to find out just how smart the animal kingdom really is.
We know that humans are clever, but can you find your friends in a crowd as well as a
baby penguin?
Drive a car as well as this rat?
Sense Earth’s magnetic field like a fox?
Head on over to PBS Terra to find out, and be sure to tell them that Joe sent you.