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For your brain to do its job, a lot of information
needs to be communicated at lightning speed.
So your brain is full of cells that can send and receive chemical
and electrical messages to each other all the time.
But we only know that this is fundamentally how the brain works
because of early neuroscience researchers and the animals they studied.
The researchers credited with first figuring out how brain cells communicate
didn’t do it by studying mice or rats.
Instead, they worked with squids and frogs.
And as weird as it sounds, they were just the right species for the job.
Animal research is not taken lightly.
The use of every single experimental animal needs to be justified
to oversight committees, research institutions, funders,
and publishers before the research reaches your eyes or ears.
But needing specific reasons to study certain animals doesn’t mean
you can’t study a wide range of animals.
Take Santiago Ramon y Cajal: he analyzed at least 58 different species
in the late 19th and early 20th centuries, from snails to oxen.
He labeled some of their brain tissues with the hottest new cell stains of the day,
including Golgi staining, which highlights only selected cells out of the crowd.
This way, he was able to see how an individual cell stretched out
between two other cells, as if they were playing telephone.
His work sparked the idea that information traveled
from the end of one cell to the beginning of the next.
But we didn’t know exactly how cells were sending and receiving information
until scientists from the University of Cambridge observed
the electricity running through the squid’s giant axon.
It’s tough to do precise experiments on the scale of a brain cell,
especially the teeny tiny ones that some animals have.
Human axons, the outstretched parts that transmit information to the next cell,
only get up to about 10 micrometers wide.
But in the squid, an axon can be as big as 1 millimeter wide.
That’s 100 times bigger!
Thanks to the squid, researchers discovered that an exchange of ions,
or electrically charged atoms, generates an electrical current within a cell.
Those ions, mainly sodium and potassium, can flow across the cell membrane.
When the cell is at rest and not sending messages,
there’s more potassium inside the cell and more sodium outside the cell.
And when it initiates communication to another cell, the membrane opens up
to let sodium rush in and potassium rush back out to reach a balance.
This ion movement gives that part of the cell membrane
a different electrical charge, which then moves like a wave down the cell’s axon.
And that’s an action potential.
So the current allows one cell to signal to another cell.
But current isn’t the only thing that changes when an action potential is generated.
Voltage also plays a role.
So these researchers kept the voltage constant using a voltage clamp that
adds a counterweight to whatever the cell generates.
And that let them focus only on how the current changes,
so they could measure how the electricity flows in a cell.
But while the squid helped take our understanding of electrical signaling to the
next level, the frog was the first animal shown to have electricity in its body at all.
Back in the 19th century, experiments on frogs showed that
their leg muscles would twitch when stimulated with electricity.
Their large, muscley legs made them a great animal to study in that experiment.
And years later in 1921, researchers were still studying frogs,
among other organisms, to figure out the rest of the puzzle.
You see, a researcher suspected that chemicals are
also involved in cell-to-cell communication.
This was because sometimes the cell receiving the message reacts
by becoming more active, but other times it becomes subdued.
And he didn’t see that kind of specific effect being controlled by electricity alone.
So he set out to test his idea that cellular communication
can be triggered by chemicals.
He studied two frog hearts that were held in
individual containers of a fancy saltwater solution.
One heart still had nerves attached to it and the other did not.
He stimulated the nerves of the heart that still had them,
then transferred the solution it had been bathing into the second heart.
The second heart responded as if the nerves had just been stimulated because it
was responding to the chemicals that the first heart had released into its solution.
Those chemical messengers are what we now call neurotransmitters.
We know today that certain neurotransmitters lead to more communication,
others lead to less, and some can lead to either depending on the situation.
Like glutamate usually activates a pathway within the cell
that encourages sodium to flow in and potassium to flow out.
Another neurotransmitter, GABA, usually initiates a different pathway
that brings more potassium into the cell, keeping it from sending messages.
So both electricity and chemicals are powerful signaling tools.
And because they were both found in frogs, we know that within the same animal,
cells use both electricity and chemicals to send signals to each other.
After initiating an action potential, a cell releases neurotransmitters to the next cell.
So thanks to squids, frogs, and many other organisms,
we know much more about how brain cells communicate with each other,
and how our own brains do what they do.
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