Name: 神經系統，第三部分--突觸!速成班 A&P #10 (The Nervous System, Part 3 - Synapses!: Crash Course A&P #10)
Uploaded: 2021-01-14T05:32:13.000Z
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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

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

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

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

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

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

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

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

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

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