The Nervous System, Part 3 - Synapses!: Crash Course Anatomy & Physiology #10

Summary

This script delves into the intricacies of synapses, the critical junctions between neurons that facilitate communication within our nervous system. It explains the vast number of synapses in the human brain and their role in learning, memory, and various mental states. The video also distinguishes between electrical and chemical synapses, highlighting their functions and the importance of neurotransmitters. Furthermore, it discusses the impact of drugs like cocaine on neurotransmitter balance and the potential for addiction and dysfunction, emphasizing the complexity and importance of maintaining a healthy electrochemical messaging system in our bodies.

Takeaways

• 🧠 The synapse is the meeting point between two neurons and is crucial for the nervous system's function.
• 🔗 Synapses are like tiny communication links that transform the structure of neurons into an active system.
• 🌐 The strength and purpose of neurons lie in their connections and the synapses that facilitate these connections.
• 📬 Synapses can be thought of as junctions or crossroads that convert electrical messages into chemical signals for transmission to other neurons.
• 🧬 The human brain contains an astounding 100 billion neurons, each with 1000 to 10,000 synapses, totaling 100 to 1000 trillion synapses.
• 🤖 Each synapse functions like a tiny computer, capable of running multiple programs and adapting to neuron firing patterns.
• 📚 Synapses are essential for learning and memory and are implicated in many psychiatric disorders and addictions.
• 📶 Nerve cells have two main communication settings: electrical synapses for fast transmission and chemical synapses for more controlled and selective messaging.
• 🚀 Electrical synapses are fast because they transmit signals directly without converting them to another type of signal, like in chemical synapses.
• 💊 Chemical synapses use neurotransmitters to transmit signals across a synaptic gap, allowing for various levels of control over the impulse.
• 🛑 Neurotransmitters are chemicals that can excite or inhibit neurons and are involved in a wide range of functions, from mood regulation to motor control.
• 🚫 Drugs, including cocaine, can disrupt the natural balance of neurotransmitters, leading to temporary euphoria but long-term negative effects.

Q & A

What is a synapse and why is it important?

A synapse is the meeting point between two neurons. It is crucial because it allows neurons to communicate with each other, turning the structure of the nervous system into an actual functional system.

How does the word 'synapse' originate and what does it mean?

The word 'synapse' comes from the Greek for 'to clasp or join,' essentially meaning a junction or a crossroads in the nervous system where neurons communicate.

What is the relationship between action potential and synapses?

An action potential sends an electrical message to the end of an axon, which then hits a synapse. The synapse translates this electrical message into a different type of signal and passes it to another neuron.

How many synapses are estimated to be in the human brain?

The human brain is estimated to have between 100 to 1,000 trillion synapses, given that there are 100 billion neurons each with 1,000 to 10,000 synapses.

How do synapses function in terms of learning and memory?

Synapses are responsible for learning and memory as they can change and adapt in response to neuron firing patterns, strengthening or weakening over time based on usage.

What are the two main types of synapses and how do they differ?

The two main types of synapses are electrical and chemical. Electrical synapses transmit signals directly through gap junctions, while chemical synapses use neurotransmitters to transmit signals across a synaptic gap.

Why are chemical synapses more abundant than electrical synapses in the human body?

Chemical synapses are more abundant because they offer more precise control over the transmission of signals, allowing for selective communication and the ability to modify, amplify, inhibit, or split signals.

How do neurotransmitters play a role in the function of chemical synapses?

Neurotransmitters are chemical signals that are released from the presynaptic neuron and diffuse across the synaptic gap to bind to receptors on the postsynaptic neuron, thereby transmitting the signal.

What is the significance of the synaptic cleft in chemical synapses?

The synaptic cleft is the tiny gap between the presynaptic and postsynaptic neurons where neurotransmitters diffuse to transmit signals. It is crucial for the communication process in chemical synapses.

How do drugs like cocaine affect the function of synapses and neurotransmitters?

Drugs like cocaine can block the reuptake of neurotransmitters, particularly dopamine, causing them to accumulate and leading to a temporary feeling of euphoria. However, this can deplete neurotransmitter supplies and lead to various negative effects.

What happens to neurotransmitters after they have delivered their message?

After neurotransmitters deliver their message, they typically unbind from their receptors and either degrade, get recycled, or are reabsorbed by the presynaptic neuron through a process called reuptake.

Outlines

00:00

🧠 The Power of Synapses

This paragraph introduces synapses as the critical meeting points between neurons, emphasizing their role in the nervous system's functionality. Synapses are described as the 'tiny communication links' that transform the structure of neurons into an operational system. The paragraph explains that the strength of neurons lies in their interconnectedness, and without synapses, neurons would be ineffective. The concept of synapses as junctions or crossroads is introduced, along with the process of action potentials being translated into different signals at synapses. The vast number of synapses in the human brain is highlighted, with each neuron having thousands of synapses, contributing to the brain's computational and adaptive capabilities. Synapses are also identified as the basis for learning, memory, and the root of many psychiatric disorders, as well as the mechanism behind the effects of illicit drugs and addictions.

05:05

🚀 Synaptic Communication and Neurotransmitters

The second paragraph delves into the two main types of synapses: electrical and chemical. It explains that electrical synapses allow for immediate communication through gap junctions, enabling rapid and synchronized responses, as seen in the heart's muscle cells. However, the lack of control with electrical synapses could lead to overstimulation and exhaustion of the nervous system. Chemical synapses, on the other hand, are slower but more precise and selective, using neurotransmitters to convey messages across a synaptic gap. The process involves the release of neurotransmitters from synaptic vesicles, their diffusion across the cleft, and binding to receptors on the postsynaptic neuron, which can either excite or inhibit the neuron's activity. The paragraph also discusses the importance of neurotransmitters in various bodily functions and how drugs can exploit the synaptic mechanisms to create their effects, using cocaine as an example of how it disrupts the natural balance of neurotransmitters.

10:06

🎬 Behind the Scenes of Crash Course Production

The final paragraph provides credits and acknowledgments for the production of the Crash Course episode. It mentions the support from Logan Sanders and Dr. Linnea Boyev, and encourages viewers to support free education through Subbable.com. The writing and production team behind the episode is credited, including the writer Kathleen Yale, script editor Blake de Pastino, consultant Dr. Brandon Jackson, directors Nicholas Jenkins and Michael Aranda, and the graphics team from Thought Café. This paragraph serves as a closing note, recognizing the collaborative effort that goes into creating educational content.

Keywords

💡Synapse

A synapse is the junction where a signal passes from one neuron to another. It is central to the video's theme as it explains how synapses function as the communication links in the nervous system, turning its structure into a functional network. The script uses the term to describe how electrical messages from neurons are translated into different signals at the synapse, highlighting their importance in learning, memory, and the root of various psychiatric disorders.

💡Neuron

Neurons are the nerve cells that make up the nervous system. The video emphasizes the role of neurons in forming the structure of the nervous system and their reliance on synapses for communication. The script mentions that a single neuron is ineffective without connections, illustrating the interconnected nature of neurons and the significance of their synapses.

💡Action Potential

An action potential refers to the electrical signal that travels along the length of a neuron. The video script explains that when an action potential reaches the end of an axon, it hits a synapse, which then converts this electrical signal into another type of signal. The concept is foundational to understanding how neurons communicate and is used to illustrate the process of signal transmission at synapses.

💡Neurotransmitter

Neurotransmitters are the chemical messengers used in chemical synapses to transmit signals across the synaptic gap. The video describes neurotransmitters as couriers that bind to receptors on the postsynaptic neuron, playing a crucial role in the conversion of electrical signals to chemical signals and vice versa. The script also discusses how drugs like cocaine can disrupt neurotransmitter function.

💡Electrical Synapse

Electrical synapses are a type of synapse that allows for direct electrical communication between neurons. The video explains that these synapses are fast because they do not convert the signal from electrical to chemical, as seen in the heart muscle cells where quick, synchronized responses are necessary.

💡Chemical Synapse

Chemical synapses are the more common type of synapse that uses neurotransmitters for signal transmission. The video script highlights that these synapses are slower but more precise and selective, allowing for complex control over signal transmission, which is essential for the nervous system's proper functioning.

💡Presynaptic Neuron

The presynaptic neuron is the cell that sends the signal across a synapse. The video describes how this neuron uses its presynaptic terminal to release neurotransmitters into the synaptic cleft. The concept is key to understanding the mechanism of signal transmission in chemical synapses.

💡Postsynaptic Neuron

The postsynaptic neuron is the cell that receives the signal at the synapse. The video explains that this neuron has receptor sites where neurotransmitters bind, causing ion channels to open and potentially leading to the generation of a new action potential. This concept is central to the discussion of how signals are received and processed in the nervous system.

💡Depolarization

Depolarization is the process by which the inside of a neuron becomes more positive, bringing it closer to the threshold needed to generate an action potential. The video uses this term to describe how excitatory neurotransmitters increase the likelihood of a neuron firing, contributing to the overall message of how signals are amplified in the nervous system.

💡Hyperpolarization

Hyperpolarization is the process by which the inside of a neuron becomes more negative, moving it further from the threshold needed to fire an action potential. The video explains that inhibitory neurotransmitters cause hyperpolarization, making it harder for a neuron to become excited and pass along a signal.

💡Reuptake

Reuptake is the process by which a neurotransmitter is reabsorbed by the presynaptic neuron after it has transmitted its signal. The video script uses the concept of reuptake to explain the normal cycle of neurotransmitter release and retrieval, and how drugs like cocaine can disrupt this process by blocking reuptake, leading to an accumulation of neurotransmitters and altered neuronal function.

Highlights

Synapses are the meeting points between two neurons and are crucial for the nervous system's function.

A single neuron's effectiveness is determined by its connections, emphasizing the importance of synapses.

The word 'synapse' originates from the Greek for 'to clasp or join', highlighting its role as a junction.

Action potentials are electrical messages that synapses convert into different signals for neuron communication.

The human brain contains 100 billion neurons with 1000 to 10,000 synapses each, totaling 100 to 1,000 trillion synapses.

Synapses function like tiny computers, capable of running multiple programs and adapting based on neuron firing patterns.

Synapses are essential for learning, memory, and are linked to psychiatric disorders and addictions.

Nerve cells have two communication settings: electrical and chemical synapses for fast and controlled messaging.

Electrical synapses are fast but lack the control of chemical synapses, which are more precise and selective.

Chemical synapses use neurotransmitters to transmit signals across the synaptic gap to the postsynaptic neuron.

Neurotransmitters can modify, amplify, inhibit, or split signals, allowing for complex control of neuronal impulses.

The presynaptic neuron sends signals through the presynaptic terminal, releasing neurotransmitters into the synaptic cleft.

Postsynaptic neurons receive neurotransmitters through receptor sites, usually on dendrites or the cell body.

Neurotransmitters短暂地绑定到受体上，然后要么降解，要么被回收，影响药物的作用机制。

Cocaine and other drugs interfere with neurotransmitter reuptake, leading to imbalances and potential addiction.

Drugs can exploit the synaptic system by exciting or inhibiting neurotransmitter functions, mimicking natural signals.

Synaptic health is vital for proper nervous system function, and imbalances can lead to various dysfunctions.

Transcripts

What’s 1000 times thinner than a piece of paper, more numerous in you than grains of

sand on a beach, and proof that the smallest things can sometimes be the most powerful?

I’m talking about the synapse -- the meeting point between two neurons.

If your neurons form the structure of your nervous system, then your synapses -- the

tiny communication links between them -- are what turn that structure into an actual system.

Because, as great and powerful as your neurons are, when it comes down to it, their strength

and their purpose lies in their connections. A single neuron in isolation might as well

not exist if it doesn’t have someone to listen or talk to.

The word “synapse” comes from the Greek for “to clasp or join.” It’s basically

a junction or a crossroads.

When an action potential -- and if you don’t know what an action potential is, watch the

last episode -- sends an electrical message to the end of an axon, that message hits a

synapse that then translates, or converts it, into a different type of signal and flings

it over to another neuron.

These connections are rather amazing feats of bio-electrical engineering, and they are

also ridiculously, mind-bogglingly numerous.

Consider that the human brain alone has 100 billion neurons, and each of those has 1000

to 10,000 synapses.

So you’ve got somewhere between 100 to 1,000 trillion synapses in your brain.

Each one of these hundreds of trillions of synapses is like a tiny computer, all of its

own, not only capable of running loads of different programs simultaneously, but also

able to change and adapt in response to neuron firing patterns, and either strengthen or

weaken over time, depending on how much they’re used.

Synapses are what allow you to learn and remember.

They’re also the root of many psychiatric disorders.

And they’re basically why illicit drugs -- and addictions to them -- exist.

Pretty much everything in your experience -- from euphoria to hunger to desire to fuzziness

to to confusion to boredom -- is communicated by way of these signals sent by your body’s

own electrochemical messaging system.

Hopefully, you know enough about email and texting etiquette to know that if you’re

gonna communicate effectively, you have to respect the sanctity of the group list.

It’s not a great idea to send a mass text to all of your friends first thing in the

morning to give them the urgent news that you just ate a really delicious maple-bacon donut.

Seriously, people. If you happen to have a friend who truly adores bacon, then an email would suffice.

But! If you’re out clubbing and suddenly Bill Murray shows up and starts doing karaoke...

then that would be a totally appropriate time to notify all of your friends at once that

something awesome is happening and they better be a part of it.

And in much the same way -- OK, in kind of the same way -- your nerve cells have two

main settings for communicating with each other, depending on how fast the news needs to travel.

Some of your synapses are electrical -- that would be like an immediate group text.

Others are chemical synapses -- they take more time to be received and read, but they’re

used more often and are much easier to control, sending signals to only certain recipients.

Fortunately, your nervous system has better text etiquette than your mom, and knows when

each kind is appropriate to use, and how to do it.

Your super fast electrical synapses send an ion current flowing directly from the cytoplasm

of one nerve cell to another, through small windows called gap junctions.

They’re super fast because the signal is never converted from its pure electrical state

to any other kind of signal, the way it is in a chemical synapse.

Instead, one cell and one synapse can trigger thousands of other cells that can all act

in synchrony. Something similar happens in the muscle cells of your heart, where speed

and team effort between cells is crucial.

This seems like a good system, so why aren’t all of our synapses electrical?

It’s largely a matter of control. With such a direct connection between cells, an action

potential in one neuron will generate an action potential in the other cells across the synapse.

That’s great in places like your heart, because you definitely don’t want a half a heartbeat.

But if every synapse in your body activated all of the neurons around it, your nervous

system would basically always be in “group text” mode, with every muscle fiber and

bit of organ tissue always being stimulated and then replying-all to the whole group which

would stimulate them even more until everyone just got maxed out and exhausted and turned

off their phones for good...which would be death.

So that would be bad, which is partly why we have chemical synapses. They are much more

abundant, but also slower, and they’re more precise and selective in what messages they send where.

Rather than raw electricity, these synapses use neurotransmitters, or chemical signals,

that diffuse across a synaptic gap to deliver their message.

The main advantage chemical synapses have over electrical ones is that they can effectively

convert the signal in steps -- from electrical to chemical back to electrical -- which allows

for different ways to control that impulse.

At the synapse, that signal can be modified, amplified, inhibited, or split, either immediately

or over longer periods of time.

This set-up has two principal parts:

The cell that’s sending the signal is the presynaptic neuron, and it transmits through

a knoblike structure called the presynaptic terminal, usually the axon terminal.

This terminal holds a whole bunch of tiny synaptic vesicle sacs, each loaded with thousands

of molecules of a given neurotransmitter.

The receiving cell, meanwhile, is, yes, thankfully the postsynaptic neuron, and it accepts the

neurotransmitters in its receptor region, which is usually on the dendrite or just on the cell body itself.

And these two neurons communicate even though they never actually touch. Instead, there’s

a tiny gap called a synaptic cleft between them -- less than five millionths of a centimeter apart.

One thing to remember is that messages that travel via chemical synapses are technically

not transmitted directly between neurons, like they are in electrical synapses.

Instead, there’s a whole chemical event that involves the release, diffusion, and

reception of neurotransmitters in order to transmit signals.

And this all happens in a specific and important chain of events.

When an action potential races along the axon of a neuron, activating sodium and potassium

channels in a wave, it eventually comes down to the presynaptic terminal, and activates

the voltage-gated calcium (Ca2+) channels there to open and release the calcium into

the neuron’s cytoplasm.

This flow of positively-charged calcium ions causes all those tiny synaptic vesicles to

fuse with the cell membrane and purge their chemical messengers. And it’s these neurotransmitters

that act like couriers diffusing across the synaptic gap, and binding to receptor sites

on the postsynaptic neuron.

So, the first neuron has managed to convert the electrical signal into a chemical one.

But in order for it to become an action potential again in the receiving neuron, it has to be

converted back to electrical.

And that happens once a neurotransmitter binds to a receptor. Because, that’s what causes

the ion channels to open.

And depending on which particular neurotransmitter binds to which receptor, the neuron might

either get excited or inhibited. The neurotransmitter tells it what to do.

Excitatory neurotransmitters depolarize the postsynaptic neuron by making the inside of

it more positive and bringing it closer to its action potential threshold, making it

more likely to fire that message on to the next neuron.

But an inhibitory neurotransmitter hyperpolarizes the postsynaptic neuron by making the inside

more negative, driving its charge down -- away from its threshold. So, not only does the

message not get passed along, it’s now even harder to excite that portion of the neuron.

Keep in mind here: Any region of a single neuron may have hundreds of synapses, each

with different inhibitory or excitatory neurotransmitters. So the likelihood of that post-synaptic neuron

developing an action potential depends on the sum of all of the excitations and inhibitions in that area.

Now, we have over a hundred different kinds of naturally-occurring neurotransmitters in

our bodies that serve different functions. They help us move around, and keep our vital

organs humming along, amp us up, calm us down, make us hungry, sleepy, or more alert, or

simply just make us feel good.

But neurotransmitters don’t stay bonded to their receptors for more than a few milliseconds.

After they deliver their message, they just sort of pop back out, and then either degrade or get recycled.

Some kinds diffuse back across the synapse and are immediately re-absorbed by the sending

neuron, in a process called reuptake.

Others are broken down by enzymes in the synaptic cleft, or sent away from the synapse by diffusion.

And this mechanism is what many drugs -- both legal and illegal -- so successfully exploit,

in order to create their desired effects.

These drugs can either excite or inhibit the production, release, and reuptake of neurotransmitters. And

sometimes they can simply mimic neurotransmitters, tricking a neuron into thinking it’s getting

a natural chemical signal, when really it’s anything but.

Take cocaine, for example. Don’t take cocaine.

Once it hits your bloodstream, it targets three major neurotransmitters --

serotonin, dopamine, and norepinephrine.

Serotonin is mainly inhibitory and plays an important role in regulating mood, appetite,

circadian rhythm, and sleep. Some antidepressants can help stabilize moods by stabilizing serotonin levels.

And when you engage in pleasurable activities -- like hugging a loved one, or having sex,

or eating a really, really great donut -- your brain releases dopamine, which influences

emotion and attention, but mostly just makes you feel awesome.

Finally, norepinephrine amps you up by triggering your fight or flight response, increasing

your heart rate, and priming muscles to engage, while an undersupply of the chemical can depress a mood.

So in a normal, sober state, you’ve got all these neurotransmitters doing their thing

in your body. But once they’ve delivered their chemical payloads, they’re usually

diffused right back out across the synapse to be absorbed by the neuron that sent them.

But cocaine blocks that reuptake, especially of dopamine, allowing these powerful chemicals

to float around and accumulate -- making the user feel euphoric for a time, but also paranoid and jittery.

And because you have a limited supply of these neurotransmitters, and your body needs time

to brew more, flooding your synapses like this eventually depletes your supply, making

you feel terrible in a number of ways.

Cocaine and other drugs that target neurotransmitters trick the brain, and after prolonged use may

eventually cause it to adapt, as all those synapses remember how great those extra chemicals feel.

As a result, you actually start to lose receptors, so it takes even more dopamine, and finally

cocaine, to function normally.

Sometimes the best way to understand how your body works is to look at how things can go

wrong. And when it comes to your synapses, that, my friends, is what wrong looks like.

In their natural, healthy state, your synapses know when to excite, when to inhibit, when

to use electricity and when to dispatch the chemical messengers.

Basically, a healthy nervous system has the etiquette of electrical messaging down to,

well, a science.

Today you learned how electrical synapses use ion currents over gap junctions to transmit

neurological signals, and how chemical synapses turn electrical signals into chemical ones,

using neurotransmitters, before converting them to back electrical signals again. And

you learned how cocaine is a sterling example of how artificial imbalances in this electrochemical

system can lead to dysfunctions of all kinds.

This episode of Crash Course was brought to you by Logan Sanders from Branson, MO, and

Dr. Linnea Boyev, whose YouTube channel you can check out in the description below. Thank you

to Logan and Dr. Boyev for supporting Crash Course and free education. Thank you to everyone

who's watching, but especially to our Subbable subscribers, like Logan and Dr. Boyev, who make

Crash Course possible. To find out how you can become a supporter, just go to Subbable.com.

This episode was written by Kathleen Yale, the script was edited by Blake de Pastino,

and our consultant, is Dr. Brandon Jackson. It was directed by Nicholas Jenkins and Michael

Aranda, and our graphics team is Thought Café.