Communication in the Nervous System: Study with Video Lessons, Practice Problems & Examples

Electrochemical Communication

Okay, so you guys have heard me say a couple of times now that neurons communicate via electrochemical signals, and specifically what I mean by that is that neurons communicate through electrical impulses and chemical signals. And I wanted to just take a second to kind of break down where each of these events is taking place. So, the electrical communication is actually happening within neurons. Okay. So, this is happening inside of a neuron and the main type of electrical communication that we're going to go over are called action potentials, and we will have several videos talking about those.

So, the electrical communication is happening within a neuron. Now, in contrast, the chemical communication actually happens between neurons. Okay. So, when one neuron wants to send a signal to another neuron, that is where we're going to be using chemical signaling or chemical communication, and this will involve special chemicals called neurotransmitters, and we will have several videos talking about those as well. Okay, so the big takeaway when we're saying electrochemical communication what we mean is that there is electrical communication taking place within a neuron and chemical communication taking place between neurons.

Alright. I will see you guys in our next videos to go over these types of communication in more detail. Bye-bye.

Electrical Communication – Action Potentials: Part 1

Okay, so now we're going to get started on those action potentials. We're going to cover this in 2 videos. This first video is sort of like background information, setting the stage, and then our next video will be going over the actual sequence of the action potential. I do want to give you a heads up: this material can be a bit challenging, so really take your time with it, you know, follow along closely and it can be hard but you can do hard things. So let's do it together.

Now, your body is full of things called ions, and ions are little particles that carry a small electric charge. They can be positively charged or negatively charged. Some important ions in the brain are sodium, which we're going to abbreviate as Na⁺ and potassium, which we're going to abbreviate as K⁺. As you can see, both sodium and potassium are positively charged ions. Ions can exist in neurons and they can also exist in the fluid outside of neurons.

Now, a neuron's membrane or the cell wall of a neuron has something called ion channels, and those are exactly what they sound like. They're little channels or kind of like, you know, little doors right on the cell membrane, and they allow ions to flow in or out of the cell. They allow those ions to basically flow between the neuron and the fluid surrounding the neuron. Some of these ion channels are what we call voltage-gated, and that just means that these channels will open or close when the neuron reaches a specific voltage. For example, you don't have to write down these numbers, but as an example, a voltage-gated sodium channel will open when the neuron is at negative 55 millivolts and it will close when the neuron is at about positive 30 millivolts.

Now, we also have to understand what would make ions actually move, or like what would make them move between this fluid and the inside of our neuron, and one reason that ions move is that they follow their electrical gradient, which basically states that ions move toward areas of opposite charge. It's kind of like how magnets work, you know, how with magnets opposites attract and like charges repel, ions are very similar. So a positively charged ion is attracted to a negatively charged space and vice versa. And to be clear, there are other reasons why ions move, this is just the only one that you need to know for this course.

So to give you a kind of more visual depiction of what we've been talking about, what we are doing here is basically zooming in on the cell membrane and we are looking at kind of like a super zoomed-in image of the cell wall there. Here, in this kind of light tan color, we are looking at the inside of our neuron, so we're going to just label that as our neuron, and then this kind of darker tan color is the cell membrane or the cell wall of our neuron, and then this blue color here represents the extracellular fluid and extracellular just means outside of our cell, so that's just the fluid surrounding our neurons. And you can see we have these little circular ions all over the place, so we have a couple of potassium ions in the fluid out there and a whole bunch of little potassium ions inside of our cell. We have a bunch of sodium ions in the fluid and a couple of sodium ions inside of our cell, and then you can see along the cell membrane we have these ion channels. This purple channel represents a sodium channel, so we'll label that Na⁺, and this pink one represents a potassium channel, so we'll label that as K⁺, and then just for the purposes of this illustration we decided to make the extracellular fluid more positively charged and the inside of our neuron more negatively charged.

So to give you an example of how that electrical gradient might work here, let's just say if our voltage-gated sodium channel was to open then all of these positively charged sodium ions in the fluid here would be attracted to this negatively charged space inside of our neuron. So if this channel was to open then these little sodium ions would move through the channel toward our negatively charged neuron, so that is how the electrical gradient might work there. Alright, so that is our background information and I'll see you guys in our next video to go over the sequence of an action potential. Bye bye.

Communication in the Nervous System Example 1

Okay, so we have a true or false question. If it's false, we're going to be choosing the answer that would best correct the statement, the statement being ions are attracted toward areas of opposite charge. And that is true, right? With ions, we know that they're similar to magnets, you know, opposites attract and like charges repel. So, positively charged ions will be attracted toward negatively charged spaces, and negatively charged ions would be attracted toward more positively charged spaces.

Alright, so this one is true and I'll see you guys in the next one. Bye bye.

What is the role of ion channels in a neuron?

• A. To allow vital nutrients in and out of the cell.
• B. To allow for the movement of ions in and out of the cell.
• C. To create an equal distribution of sodium and potassium on each side of the cell membrane.
• D. To protect the neuron from invading pathogens.

Electrical Communication – Action Potentials: Part 2

Okay. So now that we have all that background information out of the way, let's go over action potentials. So our neuron is going to start at what we call resting potential, which is when our neuron has an internal voltage of about -70 mV. And resting potential is exactly what it sounds like. Our neuron is basically just resting, it's not communicating, and it's not having an action potential yet.

So what's going to happen is as other neurons are trying to send us a signal, that basically bumps up the internal voltage of our neuron just a little bit. So every time a neuron is trying to be like, hey, hey, I have a little message for you, that's going to bump up the voltage of our neuron, and when our internal voltage reaches -55 mV, also known as threshold, that is when an action potential is going to occur. Alright, so -55 mV will basically trigger an action potential to fire. Now, an action potential is literally just a rapid change in voltage within our neuron, and we're going to go over what that change looks like in just one second and it's going to serve as the basis for neural signaling. Action potentials are basically the electrical change or the electrical communication that we see happening within our neurons, and one important thing to know about action potentials is that they are what we call an all or nothing event.

That basically means you either have an action potential or you don't. You can't have a weak one. You can't have a strong one. You can't have a half of one. If your neuron reaches -55 mV or threshold, it is going to fire an action potential and all action potentials are the exact same, like strength or magnitude.

All right, so we're going to go over what the actual sequence of an action potential looks like, or what that change in voltage looks like. So here we have this graph, as you can see, along the y-axis we have our membrane potential or the voltage, along the x-axis we have time, this is happening in milliseconds very quickly, of course, and along the top, we have our voltage-gated sodium and potassium channels. So you can see if those channels are closed or if they are open. As a reminder, purple is sodium, and pink is potassium. So basically what's going to happen is that change in voltage is going to be because of the flow of our sodium and potassium ions, and you'll see what that looks like in a second.

So I am going to first draw in our resting potential and threshold stage with purple here. So our neuron is going to start off at -70 mV at rest. As you can see at this stage both our sodium and potassium channels are closed so there's no ion movement. So we're starting off at resting potential and as other neurons are trying to communicate with us that is bumping our voltage up just a little bit up to about -55 mV. Of course, that is threshold and as you can see something very exciting happens at -55 mV.

That is when our voltage-gated sodium channels are going to open and that is going to trigger the first stage of our action potential, which is called depolarization. So basically what happens is when our sodium channels open, remember, opposites attract, so we have all these positively charged sodium ions in the fluid outside of our neuron and once that channel opens they're going to be very attracted to the inside of our neuron because it's very negatively charged right now. So we're going to have positively charged sodium ions rushing into our cell, and that is going to make our voltage more positive. So I'm going to draw this in with orange. So our voltage is going up up up all the way to about +30 mV.

Okay? So sodium has flooded into our cell and now our voltage is more positive. Now at +30 mV our voltage-gated sodium channels close, so sodium movement has stopped at that point, but our voltage-gated potassium channels open, and that is going to trigger the next phase of our action potential, which is called repolarization. So what's basically happening here is the voltage-gated potassium channel opens. Now at this point, the inside of our neuron is very positive. And remember, opposites attract, but like charges repel.

So all of the positively charged potassium inside of our neuron is like, you know, I don't want anything to do with this, basically. So once that channel is open, potassium is going to flood out of our cell. That positively charged space is no longer attractive for them, and as positively charged potassium ions leave, our voltage is going to become more negative. So I'm going to draw that in with blue, and our voltage is going to dip back down and get more and more negative. Now what's actually going to happen here is phase 3 of our action potential, which is called a refractory period, and during the refractory period, the voltage actually gets more negative than the resting potential.

So it's going to dip down to like -80 or -90 mV. So I'm just going to have it dip down like that with my green, and at this point, the potassium channels will close, so ion movement has completely stopped. Now the point of this refractory period is basically so that our neuron will take a break. So during this time, our neuron can't fire another action potential, so it's kind of like a little mandatory break period for our neuron in between action potentials. Okay, so we have that little refractory period where we are now more negative than rest and then very quickly our neuron will recover from that and go back to resting potential at -70 mV and again both of our voltage-gated, sodium, and potassium channels are closed at that time.

Alright, so that is the general sequence of an action potential. We start at rest, we get bumped up to threshold -55 mV, our cell or our neuron will get more positive, it will then get more negative, it will then dip very negative below resting potential very briefly, and then return to rest. Now I know that the terms depolarize and repolarize are not the most intuitive to remember. The way I always remember this back in undergrad was when I think polar, I think cold, so I kind of think, like, negative temperatures or negative voltage. So if we are depolarizing, we are basically getting warmer, or into the positive temperatures or positive voltage.

And then if we are repolarizing, we're basically getting colder again, back down to negative temperatures or negative voltage. So that's how I used to remember it. Hopefully, that is helpful to you as well. Okay. I did want to just quickly show you what this actually looks like as it travels through the neuron just kind of generally.

So basically what's happening with our neuron is that, again, it's, you know, it'll start at rest -70 mV, and then as other neurons are trying to send us signals that is going to bump up the internal voltage in the cell body and then we're going to hit threshold right about here, kind of where the cell body and axon meet. So we're going to hit threshold and then that, of course, will trigger an action potential. The voltage, the change in voltage of the action potential, is going to travel down the axon, down the axon terminals, and to the terminal button, and when it gets to that terminal button it's going to trigger the release of chemicals, and then that is going to basically kickstart the chemical communication that happens between neurons, and we're going to talk about that in our next video, so I will see you there. Bye-bye.

Communication in the Nervous System Example 2

All right, so for this example we're going to go through this image and use these words to label our graph and I'm just going to go in order here. So first up we have this section where we can see we are going rapidly from a negative voltage to a positive voltage. So that is going to be our depolarization phase, right, which is b over here. So we'll label that as b. Following depolarization, after our neuron gets nice and positive, we are going to repolarize.

Right? We're going to get back down to where we were, and so over here repolarization is d, so we'll put that there. Remember, one way to remember those is that our neuron likes to be in the negatives. Think of it as like polar. It's like literally very cold.

Right? And if we are depolarizing, we are getting warmer up into the positive temperatures, and if we are repolarizing, we are like refreezing, right, getting back down to those negative temperatures. There's one way to remember that. Now, after that, we're going to have this little period, and that is our refractory period, right, which looks like it is a so we'll put an a there. That is the little period where our neuron can't fire, it's taken a little break, and then after that brief refractory period, our neuron is going to get back to about negative 70, which we know is resting potential so we will put a c there.

And there you have it, that is our sequence of an action potential. There you go.

Which of the follow statements about action potentials are true?

I) A neuron’s resting potential is -55 mV.

II) An all-or-nothing response means once the threshold is reached, the action potential will fire.

III) The refractory period means there is a time where the neuron can’t fire.

• A. I & II only.
• B. I & III only.
• C. II & III only.
• D. I, II, & III.

Chemical Communication – The Synapse

All right, so like we talked about, when an action potential reaches the terminal button, it's going to get converted into a chemical message. Okay, so we had the electrical communication happening within our neuron, and now we are going to be talking about the chemical communication that happens between neurons. So one important term to know is the term synapse, and the synapse is basically the gap between the presynaptic and postsynaptic neurons, and I'm going to break down what that means. So, one cool thing about neurons is that they never actually touch each other; there's always a little gap in between them, and that gap is called the synapse. Now, presynaptic just refers to the neuron that comes before the synapse or before the synapse.

So this is going to be the neuron that is sending the message. So here, we're thinking about the axon or, like, the terminal button of the neuron that is sending the signal, and then postsynaptic refers to the neuron that comes after, or post, the synapse. So here, we're thinking about the neuron that is receiving the signal, so we're thinking about the tip of the dendrite, basically, of the neuron that is receiving the signal. Okay? We're going to go over the process of what happens here.

So in our presynaptic neuron, the action potential is going to cause vesicles to open. So remember how those terminal buttons have little vesicles in them that are full of neurotransmitters. So when the action potential reaches that terminal button, it's going to cause those vesicles to basically open up, and that will release neurotransmitters into the synapse. You can see we have our little neurotransmitters drifting into the synapse here, and neurotransmitters are just little chemical messengers within our nervous system. Okay, so these are things like serotonin and dopamine; we're going to talk about those in more detail in a future video.

So now we have neurotransmitters drifting in that synapse, and then they are going to basically connect with that dendrite, our postsynaptic neuron. So in our postsynaptic neuron, our dendrite, it has little specialized receptors, and those receptors are basically, like, specially designed to receive different neurotransmitters. So our dendrite here has these little receptors along it, and that will basically allow the neurotransmitters to bind with that dendrite. Now, what is going to happen there is that when a neurotransmitter binds to a receptor, ion channels are going to open. So you can see we have some little sodium ion channels here.

So basically, this is how I mentioned how, like, when other neurons are trying to talk to a neuron, it kind of bumps up the internal voltage just a little bit. That is what these ion channels are doing, basically. And so when neurotransmitters are binding to our dendrite, that is going to open up ion channels, which then changes the voltage of our postsynaptic neuron, which will then trigger an action potential, and then that, of course, will just keep traveling throughout all of the neurons that are connected. Alright, so that is kind of the basics of what is happening in the synapse. That's how chemical communication is taking place between neurons, and I will see you in our next one to talk about some neurotransmitters in more detail.

Bye bye.

Communication in the Nervous System Example 3

Alright. So we're going to be filling in this pathway here with the steps necessary for a signal coming from an axon to be passed to a dendrite. So our first step here is the action potential is moving down the axon of our presynaptic neuron and then our final step here is that specialized receptors on the dendrite of our postsynaptic neuron receive that neurotransmitter. So what three steps have to happen in between that? So if our action potential is moving down the axon, the next step is that it's going to go down the axon axon terminal to the terminal button and it is going to open up those vesicles in the terminal button, right?

So based on that it looks like that would be answer choice b. So our terminal vesicles open in response to that incoming action potential, and once those vesicles open we have to have neurotransmitters come out of them, right, and so that is going to be c here. So neurotransmitters leave the vesicle of our presynaptic neuron and then they have to actually cross the synapse, right, which would be a, and then that allows the neurotransmitters to bind with the receptors on the dendrite of our postsynaptic neuron. Alright. There you go, and I'll see you in the next one.

Bye bye.

The ____________ is the gap between two neurons.

• A. Synapse.
• B. Neurochemical.
• C. Gap of Ranvier.
• D. Neuronal chasm.

Chemical Communication – Neurotransmitters Part 1

This video, we're going to be going over neurotransmitters in a bit more detail. So neurotransmitters can either be excitatory or inhibitory. An excitatory neurotransmitter essentially increases the probability of our neuron becoming active and firing an action potential. This is kind of the equivalent of, like, if our presynaptic neuron was like hey, we have a message, we have to send it along, and then those neurotransmitters would make our postsynaptic neuron much more likely to fire that action potential and send that signal along. Now, in contrast, an inhibitory neurotransmitter is going to decrease the probability of the neuron firing an action potential.

So this is kind of the equivalent of like if our presynaptic neuron was like, hey, let's not send that one, and then that's going to basically decrease the chance of our postsynaptic neuron firing an action potential. Okay? So neurotransmitters can be excitatory or inhibitory. Now, we are going to go over some of the major neurotransmitters as well as their known effects. Now, this list is by no means exhaustive but this is typically what students are expected to know at this level.

So we're going to begin with glutamate, and glutamate is the major excitatory neurotransmitter in the brain. And one way that you can or maybe remember this is that if you've ever heard of MSG, it's what people use to often, like, make their food taste better, well the g in MSG is actually glutamate, it's monosodium glutamate. So I always think about how MSG kind of excites our taste buds to help me remember that glutamate is the major excitatory neurotransmitter in the brain. Now, next up we have GABA which stands for gamma-aminobutyric acid. Luckily, nobody calls it that, we all just call it GABA.

GABA is basically the opposite of glutamate, so it is going to be the major inhibitory neurotransmitter in the brain. All right? So, GABA will basically kind of slow things down and make it less likely for neurons to have action potentials. Okay? Next up, we have some that you've probably heard of.

We have serotonin. Serotonin has been heavily implicated in mood, which you may have heard about. We often talk about serotonin in the context of, like, depression. So serotonin is very important for mood. It is also important for sleep and appetite, and I know in my mind these things kind of group together very naturally.

Quite often if my mood isn't so great it's because my sleep or my food intake has been impacted in some way. So serotonin is important for mood, sleep, and appetite. Next up, we have dopamine. And dopamine, you may know it as kind of like the happy neurotransmitter or the happy chemical. It's very important for reward processing.

So, you know, dopamine essentially makes us feel good. It makes us feel happy. And so when we are doing an activity that makes us feel good, we get a little hit of dopamine in our brain, which our brain kind of processes as a reward. So dopamine is important for reward processing. It's also very important for movement.

So things like Parkinson's disease are very implicated in dopamine. Dopamine basically is not being produced in adequate quantities with Parkinson's and so that can lead to unwanted movements like tremors. So if we have something going wrong with dopamine we end up with some kind of dysfunction of movement. Dopamine has also been implicated in things like attention as well as cognitive function, so dopamine is doing a lot in our brains. I would say at this level the most important for you to probably know is that it's important for reward processing.

So just kind of think dopamine makes us feel happy, our brain interprets that as a reward. And then finally, another really important neurotransmitter is norepinephrine, and norepinephrine is involved in our fight or flight response, and one easy way to remember that is just that epinephrine is basically just the scientific name for adrenaline. Of course, when we think fight or flight we think about like adrenaline and that kind of rush that we get to help us engage with a threatening stimulus, right? Alright. So those are some of the major neurotransmitters in our brain and I will see you guys in our next video to talk a little bit more about neurotransmitters and how we can get them out of that synapse.

Bye bye.

Communication in the Nervous System Example 4

Okay. For this one, we're going to be matching our neurotransmitters with some of their known effects. So our first effect here is reward processing, and reward processing is associated with dopamine. Right? When we think dopamine, we think about how it makes us happy, makes us feel good, and then our brain processes that as a reward.

So we'll put an 'a' for dopamine. Next up, we have an excitatory neurotransmitter in the brain, and if we're thinking excitatory it's going to be glutamate. Right? Remember we can always think about MSG and how it excites our taste buds. Glutamate is our major excitatory neurotransmitter.

Next up we have appetite and mood, and if we're thinking about mood it's probably going to be serotonin. Right? We know that serotonin is very implicated in mood disorders like depression. So we'll put 'c' for serotonin. Next, we have our fight or flight response, and if we're thinking about fight or flight we can think about adrenaline, and the scientific name for adrenaline is epinephrine, and norepinephrine works in a very similar way.

So we'll put 'd' there. And then finally, we have the major inhibitory neurotransmitter in the brain and that is going to be GABA. We can always just think about how GABA is the opposite of glutamate. Alright, there you have it. Bye bye.

Benzodiazepines are a class of drugs that enhance the activity of GABA. Which of the following could you conclude based on this information?

• A. Benzodiazepines have an excitatory effect on the brain.
• B. Benzodiazepines have an inhibitory effect on the brain.
• C. Benzodiazepines will be most active on muscle tissues.
• D. Benzodiazepines will affect the reward processing system in the brain.

Chemical Communication – Neurotransmitters Part 2

Alright, so one thing to note about neurotransmitters is that they're not all necessarily going to bind to receptors on the postsynaptic neuron. Sometimes we end up with basically just leftover neurotransmitters kind of hanging out in that synapse, and we want to keep that synapse nice and clean for two main reasons. First, we don't want to sustain that signal for longer than necessary, and second, we want to keep that synapse nice and clean so that our next message can come through uninterrupted. So, leftover neurotransmitters are going to be removed from the synapse through a few different mechanisms, and we are going to go over each of them just very briefly for you. So the first mechanism is quite simple; it is called diffusion, and this is when neurotransmitters literally just drift or diffuse out of the synapse and into the extracellular space.

So, they basically just kind of float away, so it looks something like this. They are just diffusing out of that synapse. So, that one is nice and easy. All right. So, our next one is called degradation, and this is when a chemical reaction basically breaks neurotransmitters down.

So basically, some little enzymes come and literally just kind of break the neurotransmitters into tinier pieces in the synapse. So that looks a little bit something like this. Some little purple enzymes are coming and just breaking apart those neurotransmitters. You don't have to know what type of enzymes they are or anything like that, just know that it involves breaking the neurotransmitters down. And then finally, we have reuptake, and this is arguably the most important one for you to remember because this is a mechanism that some psychopharmacological treatments are going to be using, and you'll learn about those a little bit later in the course.

But reuptake is when neurotransmitters are basically reabsorbed back into the presynaptic terminal. So they basically get sucked back up by the neuron that originally released them, kind of like recycling. So it looks a little something like this. You can see how they're kind of going back up into that presynaptic neuron and then they can get used again. So again, it's very efficient, basically recycling.

Alright. So those are some of the mechanisms that we use to get neurotransmitters out of that synapse, and I will see you guys in our next video. Bye-bye.

Selective Serotonin Reuptake Inhibitors (SSRIs) are a class of drugs that prevent the reuptake of serotonin. Based on this, which of the following statements is true regarding SSRIs?

• A. SSRIs stop neurotransmitters from drifting out of the synapse.
• B. SSRIs close the receptors on the postsynaptic neuron.
• C. SSRIs decrease the amount of time that serotonin is in the synapse.
• D. SSRIs increase the amount of time that serotonin is in the synapse.

Putting it All Together

Okay, so we just learned a ton about neural communication but we're going to take a second to step back and think about the actual scale of what is happening here because we always teach neural communication as if one neuron is talking to one neuron, but it's of course much more complicated than that. In reality, neurons can be receiving signals from 100, if not 1,000 of other neurons simultaneously, and those signals that are coming in can be excitatory, inhibitory, or both. We could be getting any combination of excitatory and inhibitory messages. So, what's going to happen is if the excitatory messages outweigh the inhibitory ones, that is when we're going to have an action potential and we're going to get neural communication. We obviously could not draw thousands of neurons, it would look like a mess, but we have 3 neurons here communicating with our neuron and as you can see, 2 of them are sending excitatory messages and one of them is sending an inhibitory message.

And so what's going to happen here is our neuron is hanging out, sitting pretty at negative 70 millivolts; it's resting, but then these incoming excitatory messages are going to bump our voltage up to the negative 55 threshold. Boom, that is going to fire an action potential that will travel down our neurons sped up by that myelin down to the axon terminal into the terminal button. When it hits the terminal button, we're going to be releasing neurotransmitters across the synapse in order to pass this excitatory message on to our neighboring neurons. This entire process is taking milliseconds, and it is happening on a scale of millions, if not billions, of neurons simultaneously. Very, very cool to actually think about that.

Right? Alright. So great job with these lessons, you guys, and I will see you in our next video. Bye-bye.