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! <dab> 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. <switch on> 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.
