Introduction
Synapses are the connections between neurons or between neurons and muscle cells. In this video, I explain the steps of synaptic transmission—how that signal is passed from one neuron to the next.
Action Potentials Video: www.youtube.com/watch
Neuron Structure and Function Video: www.youtube.com/watch
Synapse Digital Manipulatives:
docs.google.com/presentation/d/1INd3cVYakmxY3zMfhoEuD0Kb3K-Ss3LTgj55DjYB13o/edit
^Make a copy of this Google Doc, then move around the channels, ions, and neurotransmitters to practice modeling the stages of synaptic transmission.
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Music: Up! (Storyblocks Audio)
Content
Awwwww, synapse! Synapses are the connections between neurons in your brain.
Whenever, an action potential or a nerve signal travels along an axon and reaches the end or distal part of that axon called the axon terminal, that piece of information has to get across a gap called the synaptic gap or synaptic cleft.
In order to do that.
We have to go through the process called synaptic transmission.
This, little connection between one neuron.
And the next can seem sort of insignificant - just pass the information along -.
But in reality, there’s a lot of complex stuff that can go on between those cells.
An.
Interesting application of this is the effect of drugs on the body.
When.
People, take an illegal drug or use alcohol or smoke a cigarette - and get that nicotine.
Those chemicals are directly affecting synapses in the brain.
So in this video.
We’re going to learn how synapses work.
How does that signal get transmitted from one neuron to the next? If.
You understand this, then you’ll be able to start to understand how do things like drugs and alcohol affect the brain.
How does addiction, work.
And.
What happens when someone has too few pleasure neurotransmitters in the brain, resulting in things like depression, or other mental illness., So, let’s jump to the whiteboard and get started.
So directly above me.
We have two neurons drawn;.
And we’re zooming in on the connection between the two neurons.
Now.
If you notice and you look really closely, they’re actually not physically connected.
There’s, a space in between them.
And we call that the synaptic gap or synaptic cleft.
We have the axon terminal.
The presynaptic neuron on the left.
And we have a dendrite of the postsynaptic neuron on the right.
The signal is going to get sent from the presynaptic neuron to the postsynaptic neuron.
Also drawn above me is a key that will help you keep straight these different channels and neurotransmitters and things we’re going to draw in our image.
Here.
Starting with the presynaptic neuron.
We’re going to have voltage gated sodium channels lining, the axon of the neuron.
Just distal to those, closer to the terminal.
We’re going to have voltage-gated calcium channels., Inside, the axon terminal.
We have little spheres called vesicles.
And inside those vesicles will be neurotransmitters.
That term.
Neurotransmitter just means “nerve” and “to transmit or carry information”.
These are going to carry information onto the postsynaptic neuron.
On, the dendrite of the postsynaptic neuron.
We have ligen-gated sodium channels., That term.
Ligen-Gated just means that it needs a chemical messenger.
In this case, a neuro-transmitter, to bind with it to cause it to open.
I kind of think about it like a gate and a key.
We need a key in order to open that channel.
Contrast that with the voltage-gated channels on the presynaptic neuron.
Those are going to open when the membrane reaches a certain voltage, such as during the transmission of an action.
Potential.
Finally.
We have another protein on the presynaptic neuron, and that’s called the reuptake pump.
That’s going to recycle our neurotransmitters at the end of this whole process.
Now that we have all the parts of the synapse drawn on here, let’s get into the actual steps of synaptic transmission.
The.
First, step., This, whole process has to start with an action.
Potential.
If.
You need to review action.
Potentials, I have a video of that link in the description you can check out.
As.
An action potential is traveling down, the neuron, it’s causing voltage-gated sodium channels to open.
So that more sodium rushes into the cell, which makes the neuron more positive or depolarized.
On.
The axon terminal we have voltage-gated, calcium channels.
So.
That means when the action potential (or, the depolarization) reaches the axon terminal.
It’s gonna cause the voltage-gated calcium channels to open up., That’ll cause calcium, which is more abundant on the outside than the inside of the neuron, to come rushing in to the axon terminal.
Now.
The vesicles are held in place by special proteins.
But those proteins react with calcium.
In, the presence of calcium, those proteins that are holding onto the vesicles are going to let go.
Those vesicles can now drift down to the end of the axon terminal and release their contents out into the cleft.
Their contents being those neurotransmitters.
Here in the video.
You can see those neurotransmitters being dumped out into that synaptic gap.
It looks kinda like a sneeze, like it sneezed.
Those nuerontransmitters out into the gap.
ACHOO! Remember to do the vampire sneeze;.
Don’t spread those germs, everybody! When.
Those neurotransmitters get released into the gap.
They’re going to diffuse around, and a bunch of them will latch onto (or bind with).
The receptors on those ligen-gated sodium channels on the postsynaptic neuron.
Wow that had a lot of big words in it!, In simpler, terms, the neurotransmitter will bind to the postsynaptic cell and cause the sodium channels to open.
You can see that happening in the diagram right? Now.
As.
Those sodium channels open, sodium is gonna rush into that postsynaptic neuron.
Which is going to cause it to depolarize a little bit.
If.
You remember from your study of action potentials, that depolarization causes the voltage to get a little bit more positive.
If.
The voltage of that post-synaptic neuron reaches threshold.
Suddenly sodium channels will start opening up very quickly and that’ll cause a new action potential to travel down that postsynaptic neuron.
We call this an excitatory synapse, because it’s causing the next neuron to be more likely to send its own action potential further along in the process.
But.
We also have something called an inhibitory synapse.
Here’s.
What happens in an inhibitory synapse.
Instead of the positive ion sodium coming into the dendrite, in an inhibitory synapse, a negative ion, chlorine in this case, is going to come into the synapse.
If.
You think back to our graph of membrane voltage, this would cause the membrane voltage to go down or become more negative, which gets it farther from that threshold.
Voltage.
And.
Now this post-synaptic neuron is less likely to send its own action.
Potential.
It’s kind of like a tug-of-war between inhibitory and excitatory synapses.
If, you’ve ever seen a cartoon with a devil and an angel on the shoulder and they're each telling you kind of opposite things to do.
It’s kind of like that.
But.
One of them will win and either cause the postsynaptic neuron to send a signal or to not send a signal.
And again to summarize, excitatory, synapses use sodium.
And those will make the neuron more likely to send a new action.
Potential.
And inhibitory.
Synapses use a negative ion, such as chlorine, which makes the postsynaptic neuron less likely to send its own action.
Potential.
Now.
Those postsynaptic channels, won’t stay open.
Forever.
In, fact.
Those neurotransmitters will just bind for a little bit.
And then after some time, they’ll leave and they’ll start to diffuse away.
The.
Last step in all.
This is our bodies being efficient with the resources that we have.
We.
Don’t want these neurotransmitters to get away.
So while some of them will diffuse away from the synapse.
A lot of them will be recycled or re-uptaken into the presynaptic.
Cell.
This is accomplished by the reuptake pump.
That’s going to pump that neurotransmitter back into the axon terminal, where it gets repackaged in a vesicle.
So the next time, an action potential, happens.
It’s ready to sneeze out, *I, mean*, ready to release these neurotransmitters back into the gap.
So.
Those are the steps of synaptic transmission, or the communication of a signal from one neuron to the next.
Now.
We looked at the example of communication between two neurons.
This could also take place between a neuron and another cell, such as a muscle cell.
If.
It’s communicating with the muscle cell, such as a signal from my brain telling my arms to move around.
We would call that a neuromuscular junction.
Neuromuscular - “nerve” “muscle.” But.
It works pretty much the same way.
All.
The stuff we learned about, a synapse between two neurons, also applies to a synapse between a nerve cell and a muscle, cell., So, let’s go back and recap the steps of synaptic transmission.
First thing - we have to have an action potential in the presynaptic neuron.
That causes sodium channels to open, and sodium to rush in and depolarize.
The neuron.
This causes voltage-gated, calcium channels to open and let calcium float into the terminal.
The, calcium will cause those proteins that are holding the vesicles to release them.
The vesicles can then diffuse down to the membrane of the axon terminal and release out their neurotransmitters into this synaptic gap.
Those neurotransmitters will diffuse around.
And some of them will bind with ligen-gated receptors on the postsynaptic neuron.
Those, ligen-gated channels will then open up.
If.
It’s an excitatory synapse, that causes sodium to rush in and depolarize, the postsynaptic neuron, making it more likely to send its own action potential further along., If.
It’s an inhibitory synapse.
Then a negative ion (such as chlorine) will rush into the cell.
This will cause that neuron’s membrane voltage to become more negative, or hyperpolarized, preventing an action potential from occurring in that postsynaptic neuron.
After that happens, the neurotransmitters will release.
And those channels on the postsynaptic neuron will close back up.
Those neurotransmitters will diffuse away.
And a lot of them will get recycled or reuptaken into the presynaptic cell.
They’ll, get repackaged into vesicles until their ready for the next action potential.
And the next synaptic transmission to happen.
This whole process takes place very quickly.
We’re, talking terms of milliseconds., Great! So.
You’ve heard me talk through the steps.
Now, test yourself.
I’m going to replay through the whole animation without the steps, listed.
I”m, not going to talk.
What I want you to do is to narrate, and see if you can describe what’s happening in each step.
If, you find that you can do that.
Then, you know, this process pretty well.
So, go ahead and try to describe the steps, and I'll see you in a minute! {music} All.
This talk about synapses, and these substances that affect your synapses makes me thirsty.
Let me, drink.
Some of this coffee.
Oh, yeah!, Caffeine!, That’s.
Another one that affects your synapses! {music} Cheers to your brain and to your synapses! [Terry.
The Torso model] “If I only had a brain…”.
