—
—
Transcript Provided by YouTube:
00:00
What’s 1000 times thinner than a piece of paper, more numerous in you than grains of
00:03
sand on a beach, and proof that the smallest things can sometimes be the most powerful?
00:08
I’m talking about the synapse — the meeting point between two neurons.
00:11
If your neurons form the structure of your nervous system, then your synapses — the
00:15
tiny communication links between them — are what turn that structure into an actual system.
00:20
Because, as great and powerful as your neurons are, when it comes down to it, their strength
00:25
and their purpose lies in their connections. A single neuron in isolation might as well
00:29
not exist if it doesn’t have someone to listen or talk to.
00:32
The word “synapse” comes from the Greek for “to clasp or join.” It’s basically
00:37
a junction or a crossroads.
00:39
When an action potential — and if you don’t know what an action potential is, watch the
00:41
last episode — sends an electrical message to the end of an axon, that message hits a
00:46
synapse that then translates, or converts it, into a different type of signal and flings
00:51
it over to another neuron.
00:52
These connections are rather amazing feats of bio-electrical engineering, and they are
00:56
also ridiculously, mind-bogglingly numerous.
00:59
Consider that the human brain alone has 100 billion neurons, and each of those has 1000
01:04
to 10,000 synapses.
01:06
So you’ve got somewhere between 100 to 1,000 trillion synapses in your brain.
01:11
Each one of these hundreds of trillions of synapses is like a tiny computer, all of its
01:16
own, not only capable of running loads of different programs simultaneously, but also
01:20
able to change and adapt in response to neuron firing patterns, and either strengthen or
01:25
weaken over time, depending on how much they’re used.
01:27
Synapses are what allow you to learn and remember.
01:29
They’re also the root of many psychiatric disorders.
01:31
And they’re basically why illicit drugs — and addictions to them — exist.
01:35
Pretty much everything in your experience — from euphoria to hunger to desire to fuzziness
01:40
to to confusion to boredom — is communicated by way of these signals sent by your body’s
01:44
own electrochemical messaging system.
01:57
Hopefully, you know enough about email and texting etiquette to know that if you’re
02:00
gonna communicate effectively, you have to respect the sanctity of the group list.
02:04
It’s not a great idea to send a mass text to all of your friends first thing in the
02:08
morning to give them the urgent news that you just ate a really delicious maple-bacon donut.
02:13
Seriously, people. If you happen to have a friend who truly adores bacon, then an email would suffice.
02:17
But! If you’re out clubbing and suddenly Bill Murray shows up and starts doing karaoke…
02:22
then that would be a totally appropriate time to notify all of your friends at once that
02:26
something awesome is happening and they better be a part of it.
02:28
And in much the same way — OK, in kind of the same way — your nerve cells have two
02:32
main settings for communicating with each other, depending on how fast the news needs to travel.
02:37
Some of your synapses are electrical — that would be like an immediate group text.
02:41
Others are chemical synapses — they take more time to be received and read, but they’re
02:45
used more often and are much easier to control, sending signals to only certain recipients.
02:50
Fortunately, your nervous system has better text etiquette than your mom, and knows when
02:54
each kind is appropriate to use, and how to do it.
02:56
Your super fast electrical synapses send an ion current flowing directly from the cytoplasm
03:01
of one nerve cell to another, through small windows called gap junctions.
03:05
They’re super fast because the signal is never converted from its pure electrical state
03:09
to any other kind of signal, the way it is in a chemical synapse.
03:12
Instead, one cell and one synapse can trigger thousands of other cells that can all act
03:16
in synchrony. Something similar happens in the muscle cells of your heart, where speed
03:20
and team effort between cells is crucial.
03:23
This seems like a good system, so why aren’t all of our synapses electrical?
03:26
It’s largely a matter of control. With such a direct connection between cells, an action
03:31
potential in one neuron will generate an action potential in the other cells across the synapse.
03:35
That’s great in places like your heart, because you definitely don’t want a half a heartbeat.
03:40
But if every synapse in your body activated all of the neurons around it, your nervous
03:44
system would basically always be in “group text” mode, with every muscle fiber and
03:48
bit of organ tissue always being stimulated and then replying-all to the whole group which
03:53
would stimulate them even more until everyone just got maxed out and exhausted and turned
03:57
off their phones for good…which would be death.
03:59
So that would be bad, which is partly why we have chemical synapses. They are much more
04:03
abundant, but also slower, and they’re more precise and selective in what messages they send where.
04:09
Rather than raw electricity, these synapses use neurotransmitters, or chemical signals,
04:14
that diffuse across a synaptic gap to deliver their message.
04:17
The main advantage chemical synapses have over electrical ones is that they can effectively
04:21
convert the signal in steps — from electrical to chemical back to electrical — which allows
04:26
for different ways to control that impulse.
04:28
At the synapse, that signal can be modified, amplified, inhibited, or split, either immediately
04:33
or over longer periods of time.
04:35
This set-up has two principal parts:
04:37
The cell that’s sending the signal is the presynaptic neuron, and it transmits through
04:41
a knoblike structure called the presynaptic terminal, usually the axon terminal.
04:46
This terminal holds a whole bunch of tiny synaptic vesicle sacs, each loaded with thousands
04:51
of molecules of a given neurotransmitter.
04:54
The receiving cell, meanwhile, is, yes, thankfully the postsynaptic neuron, and it accepts the
04:59
neurotransmitters in its receptor region, which is usually on the dendrite or just on the cell body itself.
05:04
And these two neurons communicate even though they never actually touch. Instead, there’s
05:09
a tiny gap called a synaptic cleft between them — less than five millionths of a centimeter apart.
05:14
One thing to remember is that messages that travel via chemical synapses are technically
05:18
not transmitted directly between neurons, like they are in electrical synapses.
05:23
Instead, there’s a whole chemical event that involves the release, diffusion, and
05:27
reception of neurotransmitters in order to transmit signals.
05:30
And this all happens in a specific and important chain of events.
05:33
When an action potential races along the axon of a neuron, activating sodium and potassium
05:37
channels in a wave, it eventually comes down to the presynaptic terminal, and activates
05:42
the voltage-gated calcium (Ca2+) channels there to open and release the calcium into
05:47
the neuron’s cytoplasm.
05:48
This flow of positively-charged calcium ions causes all those tiny synaptic vesicles to
05:53
fuse with the cell membrane and purge their chemical messengers. And it’s these neurotransmitters
05:58
that act like couriers diffusing across the synaptic gap, and binding to receptor sites
06:03
on the postsynaptic neuron.
06:04
So, the first neuron has managed to convert the electrical signal into a chemical one.
06:08
But in order for it to become an action potential again in the receiving neuron, it has to be
06:13
converted back to electrical.
06:14
And that happens once a neurotransmitter binds to a receptor. Because, that’s what causes
06:18
the ion channels to open.
06:20
And depending on which particular neurotransmitter binds to which receptor, the neuron might
06:24
either get excited or inhibited. The neurotransmitter tells it what to do.
06:28
Excitatory neurotransmitters depolarize the postsynaptic neuron by making the inside of
06:33
it more positive and bringing it closer to its action potential threshold, making it
06:37
more likely to fire that message on to the next neuron.
06:40
But an inhibitory neurotransmitter hyperpolarizes the postsynaptic neuron by making the inside
06:45
more negative, driving its charge down — away from its threshold. So, not only does the
06:49
message not get passed along, it’s now even harder to excite that portion of the neuron.
06:53
Keep in mind here: Any region of a single neuron may have hundreds of synapses, each
06:58
with different inhibitory or excitatory neurotransmitters. So the likelihood of that post-synaptic neuron
07:03
developing an action potential depends on the sum of all of the excitations and inhibitions in that area.
07:09
Now, we have over a hundred different kinds of naturally-occurring neurotransmitters in
07:13
our bodies that serve different functions. They help us move around, and keep our vital
07:17
organs humming along, amp us up, calm us down, make us hungry, sleepy, or more alert, or
07:21
simply just make us feel good.
07:22
But neurotransmitters don’t stay bonded to their receptors for more than a few milliseconds.
07:27
After they deliver their message, they just sort of pop back out, and then either degrade or get recycled.
07:31
Some kinds diffuse back across the synapse and are immediately re-absorbed by the sending
07:36
neuron, in a process called reuptake.
07:38
Others are broken down by enzymes in the synaptic cleft, or sent away from the synapse by diffusion.
07:43
And this mechanism is what many drugs — both legal and illegal — so successfully exploit,
07:48
in order to create their desired effects.
07:49
These drugs can either excite or inhibit the production, release, and reuptake of neurotransmitters. And
07:55
sometimes they can simply mimic neurotransmitters, tricking a neuron into thinking it’s getting
07:59
a natural chemical signal, when really it’s anything but.
08:02
Take cocaine, for example. Don’t take cocaine.
08:04
Once it hits your bloodstream, it targets three major neurotransmitters —
08:07
serotonin, dopamine, and norepinephrine.
08:10
Serotonin is mainly inhibitory and plays an important role in regulating mood, appetite,
08:14
circadian rhythm, and sleep. Some antidepressants can help stabilize moods by stabilizing serotonin levels.
08:20
And when you engage in pleasurable activities — like hugging a loved one, or having sex,
08:24
or eating a really, really great donut — your brain releases dopamine, which influences
08:28
emotion and attention, but mostly just makes you feel awesome.
08:32
Finally, norepinephrine amps you up by triggering your fight or flight response, increasing
08:36
your heart rate, and priming muscles to engage, while an undersupply of the chemical can depress a mood.
08:42
So in a normal, sober state, you’ve got all these neurotransmitters doing their thing
08:45
in your body. But once they’ve delivered their chemical payloads, they’re usually
08:49
diffused right back out across the synapse to be absorbed by the neuron that sent them.
08:53
But cocaine blocks that reuptake, especially of dopamine, allowing these powerful chemicals
08:58
to float around and accumulate — making the user feel euphoric for a time, but also paranoid and jittery.
09:04
And because you have a limited supply of these neurotransmitters, and your body needs time
09:08
to brew more, flooding your synapses like this eventually depletes your supply, making
09:12
you feel terrible in a number of ways.
09:14
Cocaine and other drugs that target neurotransmitters trick the brain, and after prolonged use may
09:20
eventually cause it to adapt, as all those synapses remember how great those extra chemicals feel.
09:25
As a result, you actually start to lose receptors, so it takes even more dopamine, and finally
09:30
cocaine, to function normally.
09:32
Sometimes the best way to understand how your body works is to look at how things can go
09:35
wrong. And when it comes to your synapses, that, my friends, is what wrong looks like.
09:39
In their natural, healthy state, your synapses know when to excite, when to inhibit, when
09:44
to use electricity and when to dispatch the chemical messengers.
09:46
Basically, a healthy nervous system has the etiquette of electrical messaging down to,
09:51
well, a science.
09:52
Today you learned how electrical synapses use ion currents over gap junctions to transmit
09:57
neurological signals, and how chemical synapses turn electrical signals into chemical ones,
10:02
using neurotransmitters, before converting them to back electrical signals again. And
10:05
you learned how cocaine is a sterling example of how artificial imbalances in this electrochemical
10:10
system can lead to dysfunctions of all kinds.
10:13
This episode of Crash Course was brought to you by Logan Sanders from Branson, MO, and
10:17
Dr. Linnea Boyev, whose YouTube channel you can check out in the description below. Thank you
10:21
to Logan and Dr. Boyev for supporting Crash Course and free education. Thank you to everyone
10:26
who’s watching, but especially to our Subbable subscribers, like Logan and Dr. Boyev, who make
10:31
Crash Course possible. To find out how you can become a supporter, just go to Subbable.com.
10:35
This episode was written by Kathleen Yale, the script was edited by Blake de Pastino,
10:39
and our consultant, is Dr. Brandon Jackson. It was directed by Nicholas Jenkins and Michael
10:43
Aranda, and our graphics team is Thought Café.
—
This post was previously published on YouTube.
—
Photo credit: Screenshot from video.