Nowadays, scientists have all kinds of tricks and tools to study how the brain works.

We've got optogenetics and green glowing mice, and we've even mapped out every single synapse in the C. elegans nervous system.

But all of those inventions are pretty new.

So what did the neuroscientists of yesteryear use instead?

Our ink-credible cephalopod friends.

Yep, as wild as it sounds, scientists have actually learned a lot from... squid.

And, more specifically, the giant squid axon.

Now, just to clarify, I mean the giant squid axon, not the giant squid axon.

We're not talking about the kraken here.

Release the kraken!

Nope, just the humble, ordinary squid.

See, back in the mid-1900s, we didn't have all of those fancy tools to let us study brain activity.

So scientists had to get creative when it came to figuring out how brains actually, you know, work.

At that time, options were pretty limited, because of course you couldn't just go digging around inside people's heads.

Although doctors did perform a lot of lobotomies around that time.

But those weren't very precise.

Or, you know, evidence-based.

We were still relying on things like lesion studies to help us understand what different parts of the brain did.

And we didn't fully understand how, exactly, neurons send signals.

These days, we know that neurons send electrical signals called action potentials using ionic currents and pumps to change the electrical potential along the cell surface in the axons of neurons.

What that means is that the inside of the neuron is more negative than the outside.

There are more negative ions bumping around inside, which keeps the voltage at about negative 70 millivolts.

But when a neuron receives an incoming signal, that ionic balance gets disrupted, and the voltage starts to creep upward.

Once it reaches the tipping point of about negative 55 millivolts, it sets off a cascade, throwing open the gated ion channels and causing the electrical potential to spike, thus creating the action potential.

This spike then travels along the axon, where it can push the axonal terminal to release chemicals called neurotransmitters, to pass the message on to the next neuron in the chain.

But when neuroscientists Alan Hodgkin and Andrew Huxley were first studying action potentials in the mid-1900s, they didn't have the tools necessary to measure these potentials in humans.

Instead, they chose to experiment with the longfin inshore, a small squid.

Now, why in the world did they choose a squid instead of something less slimy and squishy?

The decision came down to one thing.

Calamari nuts!

Just kidding.

It's because of the squid's giant axon.

Although these creatures are only one to two feet long, so like that big, the giant axon measures up to 1.5 millimeters in diameter, which is about the diameter of this pipe cleaner, making it more than a thousand times wider than a typical human axon.

Why would a dinky little squid need such a huge axon?

Well, the axon connects to the water jet propulsion system, and in the wild, this allows the squid to rapidly escape from predators.

Wide axons transmit electrical signals much faster than thin ones, as the added thickness allows a larger number of electrons to flow through at any given time.

Humans can get around this limitation because we have myelination around most of our axons, which basically acts as insulation for the neurons and helps us transmit signals faster.

Instead of myelin, squids just evolved an enormous axon.

After all, the faster they could escape, the more likely they were to survive and be able to pass on that trait to their offspring.

This wide axon of the longfin inshore squid made them the perfect model organism for studying action potentials, because you could easily insert a wire into the axon in order to measure the electrical potential.

Back in 1930, a dude named John Zachary Young was studying the nervous systems of sea creatures, pretty much any species he could get his hands on.

And during his research, he realized that these long, stringy structures that had previously been mistaken for blood vessels were actually neurons.

He'd discovered the giant squid axon.

A bit later, Hodgkin and Huxley came along and jabbed an electrode in it.

This allowed them to record the first ever action potential, the spike of a neuron sending a signal.

To do this, they used technology called a voltage clamp.

When an action potential shoots through the axon, sodium and potassium ions flow in and out of the cell pretty rapidly, depending on how the ion channels open and close.

With the voltage clamp, Hodgkin and Huxley could measure how much current they needed to inject in order to keep the voltage constant while that process was happening.

By doing so, they could track how much charge was flowing into and out of the cell at any given time.

With this experimental strategy, they were able to see that the neuron first experiences depolarization, that spike I talked about earlier, and then hyperpolarization, where it actually overshoots the normal resting potential of negative 70 millivolts, dipping even lower down and creating a barrier that makes it just a little bit harder for the neuron to send another action potential and helping keep signals distinct.

But even though they could now see the shape of the action potential, they didn't actually know how the action potential was being formed.

That is, which ions were going where to create that quintessential spike.

And obviously you can't just see ions, so testing it proved to be kind of a challenge.

But Hodgkin and Huxley had another tool in their back pocket.

They added chemicals called tetrodotoxin and tetraethylammonium to the solution they were keeping the squid axons in.

These compounds block sodium and potassium channels, respectively.

Fun fact, tetrodotoxin is the toxic compound produced by pufferfish and other poisonous sea creatures that makes them so deadly.

It blocks the sodium channels that allow your nerve cells to send their signals.

If in fact you've consumed the venom of the blowfish, and from what the chef has told me it's quite probable, you have 24 hours to live. 24 hours?!

Well, 22.

I'm sorry I kept you waiting so long.

By looking at the current when these channels were blocked, or when they removed sodium and potassium from the medium, Hodgkin and Huxley were able to figure out that sodium channels open first and cause the depolarization phase of the action potential, and that afterwards potassium channels open and the outward flow of potassium ions causes the hyperpolarization.

This discovery was a huge advance in helping us understand how neurons send action potentials, and how those action potentials could be controlled and manipulated for future experiments.

In fact, it won them the Nobel Prize in Physiology or Medicine in 1963.

Though their choice of model organism may seem a bit strange, Hodgkin and Huxley made no mistake.

They found the perfect organism for the experiments they wanted to run, with just the right morphology to help us understand how our own nervous system functions.

Today, squids still play an important role in research, but they aren't the only sea scientists studying memory, and the African killifish has become an integral part of modern aging research.

So it's pretty clear that researchers can go beyond mice, worms, and flies when it comes to choosing animal models for studying the brain.

I want to give a huge thank you to the International Youth Neuroscience Association for their help researching and writing the script for this video.

Founded at the 2016 International Brainbee, the IYNA is an next generation of neuroscientists.

They're a really cool organization, and I highly recommend checking them out if you're in high school or early on in college and love all this kind of stuff.

To learn more about IYNA and how you can get involved, check out the link down below.

Thanks for watching this episode of Neuro Transmissions.

Until our next transmission, I'm Alie Astrocyte.

Over and out.