Hello everyone and welcome to chapter 14 on nervous tissue. We are going to introduce the nervous system here and the components of the nervous system. As we've done with previous chapters, we're going to take a look at the nervous system from both a structural aspect, so looking at the anatomy, and also a functional aspect. And we're going to be able to categorize the nervous system based on anatomy and then based on function. It is really important that you understand the terms and the concepts in this chapter.
before moving on to the brain and to the senses. We build on what we learn here in the nervous system chapter in brain and in the senses lectures, so if you don't understand these concepts here, you're going to have a really hard time when we get to those lectures. So make sure you take the time and fill out your study guide as you go to help you organize your thoughts.
This very first image we're seeing here is of a neuron. You may remember looking at neurons and glial cells when we looked at the histology of the nervous tissue. In particular here, actually in addition to seeing a neuron, we actually also see a glial cell, and more specifically the type of cell that we're seeing is a neurolemocyte.
After I get through this lecture, you're actually going to know, based on this being a neurolemocyte here, whether this neuron exists in the CNS, the central nervous system, or the PNS, the peripheral nervous system. I can also tell you that this neuron is multipolar, which is one of the structural types of neurons that we can have. It's the most abundant structural type we have in the body.
And so we'll get to identify the different parts of these neurons and how they communicate with other neurons and how they exist within systems as well. So what is the nervous system responsible for? It really helps us to be able to understand the world around us, interpret the world around us, and be able to respond to the world around us. So how are we able to interpret?
sensations and that information. What we actually have available to us is receptors. And what are those receptors receiving? They're receiving stimuli.
Stimuli is plural for stimulus. And so for instance, if you take one finger and touch your arm on the opposite limb, that finger touching the skin is providing a stimulus. There's receptors within that skin that allow us to experience that touch. You already actually know about those receptors. You might remember talking about tactile receptors when we talked about the skin?
Well, those are receptors that are considered to be within the nervous system that allow us to experience sensation. The nervous system, in addition to sending information for sensations, will also allow us to control movements. And so in addition to having receptors, we also have what are called effector organs, which allow us to have a response to those sensations.
Now some sensations we're aware of, some we're not. We'll talk about the difference between those in a second. But our effectors are things like our muscles and our glands.
So for instance, if you are too hot, one of the sensations you would feel is an increase in temperature. What specific stimuli are we going to receive? That would be the temperature, and we have thermoreceptors to be able to experience that or to receive that stimulus. Well, how are you going to respond to that? You're going to respond to that with effectors, and specifically your sweat glands will allow you to sweat and to release some of that heat and that energy.
So just to give you an idea, again, we're able to interpret and control all the sensations and movements through the nervous system. It is also our body's primary communication and control system. So when we say communication, communication with organs within our body.
And when we say control system, a... a big, big part of that is going to be the brain. So we'll talk about the brain being the major control system, our ability to have conscious thought and be aware of things and to interpret them with our consciousness that's gonna come to the brain, and we'll cover that in the next lecture. Now coming back to the nervous tissue, we already know that tissue is made of cells, and the two main cell types that we're going to see in nervous tissue are neurons and glial cells. Neurons, again, are going to be the Major functional player when it comes to the nervous system, but the neurons can't perform their task without glial cells, which are the supporting cells of the nervous system.
We are going to learn in this lecture about the different types of glial cells and where we find them within the nervous system and the specific roles that they play. So let's first organize the nervous system by structure. So let's look at the anatomy of the nervous system first.
If we were to categorize the nervous system structurally, we have two major parts. I'm just going to do a mini chart here, and then we're going to have a larger chart for the functional divisions. So structural.
classifications or structural organization of the nervous system. The two major categories we have for the structural classification of the nervous system are the CNS, which stands for central nervous system, which you can see up here, and the PNS, which stands for peripheral nervous system. The CNS contains two things, the brain and the spinal cord. So those are the only two parts of the CNS.
Alternatively, our PNS, the peripheral nervous system, is going to contain essentially everything else. And what is everything else? That's going to be nerves and ganglia.
So I'm just gonna write nerves and ganglia. Now when it comes to nerves and ganglia, first these are actually definitions which I'm sure you've heard of nerves before but you maybe didn't really understand what it was and we're going to have a try and come up with a very thorough definition and understanding of nerves and ganglia now. So first you already know a neuron is a cell within the nervous system and specifically in the nervous tissue.
It is going to be the major player in communicating to whether it's from the PNS, the peripheral nervous system, up through the spinal cord to the brain, or maybe from the brain back out to the effector organs. Specifically, nerves are a bundle of axons. And when we talk about axons, we have started to talk about axons when we... Learned about the neuromuscular junction in muscle. So you know that there is something called an axon and that a nerve impulse can travel down it.
We focused on the neurons in relation to motor neurons that control skeletal muscle. But bundles of axons, we're gonna learn about axons a little bit more here in a second. These bundles of axons of neurons are within the PNS, so the peripheral nervous system, okay? So these are parts of neurons that are within the peripheral nervous system, meaning that these axons exist outside of the brain and outside of the spinal cord. Now, in addition to axons, this is something we'll come back to again, neurons also have a soma.
The soma is the main body of a neuron. So if we take a just step back here, right here underneath those cells, all of this would be the axon. And so when I have a...
bundle of axons in the PNS that is nerve. So imagine having a whole bunch of these guys together that I'm collecting. If these exist outside of the spinal cord or outside of the brain, this bundle right here of axons would be a nerve.
I can also have a bundle of soma. Soma is the cell body where the majority of the important organelles are going to be like the nucleus, the smooth endoplasmic reticulum, rough endoplasmic reticulum, all that good stuff. Majority of those organelles are going to be within the cell body, which we call the soma.
If we have a collection of these soma within the PNS, we call that a ganglion. So a ganglion is a bundle of soma in the PNS as well. So when we say nerves, we can have cranial nerves and spinal nerves.
What I want you to know is that for the cranial nerves, the soma of those cranial nerves may be within the brain, as you hear, cranium. But we're talking about the axons of those neurons that are extending out and away from the brain. And so those bundles of axons we call cranial nerves.
Same thing, we may have soma, so the body of the neuron might be within the spinal cord. But if we have those axons extend outside of the spinal cord, we call those spinal nerves. And if we have soma outside of the brain and the spinal cord, we are going to be calling those ganglia. Ganglia is plural, ganglion is singular.
We will come back to this idea, so if it's not quite clear yet, I promise that I will come back to this. But again, structurally, we can divide the nervous system into the CNS, the central nervous system, and the PNS, peripheral nervous system. The brain and the spinal cord are part of the central nervous system, and everything else is the PNS, or the peripheral nervous system. Now that we've identified the structural classifications of the nervous system, we're going to go into more depth and there's more here that we need to know when it comes to the functional organization of the nervous system. As I've already told you, we have the CNS and the PNS.
Collectively together, the CNS and the PNS perform three functions. So again, functionally we can do three major things. One is collecting information. I already mentioned to you that in order to interpret that information, the information being stimuli, Stimuli is plural, stimulus is singular.
We have to have receptors for that information. And so our receptors are going to be on the dendrites of the neurons. These dendrites are extensions from the soma that are going to again have receptors. Alternatively, we can have receptor cells, and receptor cells will allow us to send that information to neurons. So again, we collect information.
That information we're collecting is sensory input. And then we're going to take that information from the PNS and we're going to pass it along to the CNS. So we'll go from the PNS, the peripheral nervous system, to the spinal cord in the brain.
So let me just take a step back. When we think peripheral nervous system, think about where that might be. We have the ability to see.
Your eyes are not in your spinal cord or your brain. They are outside of that. So you're going to have receptors within your eyes for light.
Your skin is not inside of your spinal cord or your brain. You have receptors there to receive things like touch and pressure. You even have within your body. Think about your heart and your blood pressure.
Think about maybe your blood glucose levels. There are all kinds of different receptors throughout your body that are going to be able to send information to your central nervous system. So all of these different places within your body, around your body, that receive stimuli, they're within the peripheral nervous system.
And they will send that information to the central nervous system to be interpreted, the CNS and the PNS. The second functional... job that the nervous system has is processing and evaluating information. So once we send that information from the PNS to the CNS, we can then actually evaluate that information. You're not going to always be consciously aware of every single thing that you experience.
Let me give you an example. When you're driving down the freeway, you need to be very mindful and conscious of the car that's in front of you, the distance that they are in front of you, how many other cars, is there a car next to you coming into your lane. There's a lot to focus on right in front of you, but your brain is collecting all kinds of other information too. In your periphery, in your peripheral vision, you can see things like the buildings that are passing by, the names on the signs of the buildings passing by, the colors of those buildings. That's not going to come into your conscious awareness until it needs to because you need to focus on what's in front of you on the road.
So again, we receive all kinds of information, but we process it and evaluate it for its importance within the CNS. In the CNS, again, spinal cord and then specifically the brain, we determine if we need to respond to that stimulus. If we do need to respond to that stimulus, we are then going to create a response with effector cells. And effector cells are going to be within structures like glands.
and muscles. And we'll talk a little bit more about that when we put together our diagram for the functional organization of the nervous system. The third function of the nervous system is to respond to information. So again, if we decide to have a response, it's going to be the CNS that actually initiates that response.
And what is the response going to be? It's going to be some kind of motor output, and that's going to be to effectors, like I just said, either muscles or glands. When I say muscles, it could be skeletal muscle or smooth muscle.
Glands could be sweat glands, all kinds of different glands, endocrine glands as well, to react to the environment. So again, let's see, something is causing you pain. You put your hand on top of a fire.
The stimulus is going to be that heat and maybe the pain as well on your hand and you are going to send that information from your PNS to your brain, the CNS, to say, hey that is a stimulus we need to respond to, we need to do something about that. The CNS, the brain, is going to tell, specifically in this case your skeletal muscles, hey contract and move that hand away from that flame so that we can protect that part of the body. So that's kind of an example of the process. PNS to CNS. and then we make a decision of how to respond cns to pns so let's go ahead and go over the functional divisions of the nervous system i'm also going to chart this out so that we can make sure we pay attention to the really important information so first and foremost when we are talking about the again functional organization of the nervous system we have two major categories sensory and motor When it comes to sensory, again, this is going to be sending information from the PNS to the CNS.
When it comes to the motor division of the nervous system, this is going to be from the CNS to the PNS. So with our sensory nervous system, what do we need to have? We need to have receptors. And these receptors are going to respond to stimuli. This information is then sent through the neurons to the CNS where we can interpret that information that we receive and decide to respond to it.
The response again will come from the CNS and go to the PNS and the way that that information is going to be manifested or the way that we're going to be able to respond is through effector organs and those effector organs are muscles, or glance. So let's go ahead and chart this out. Even though we do have it on a slide, I think it helps to chart it out ourselves so that we can make sense of it and I can draw your attention to what I believe is important for you to know.
So again, this is the functional divisions of the nervous system. So if I were to ask you what are the structural divisions of the nervous system, that would be the CNS and the PNS. When it comes to the functional divisions of the nervous system, this is the chart that you want to reference or have in mind. So again, the two major categories of the nervous system functionally are the sensory division and the motor division.
The sensory division you can hear in the name sensory. That's going to be sensing information and how do we sense that with receptors. And that information is going to be sent from the PNS to the CNS.
Alternatively, the motor division is going to be sending information from the CNS to respond. with the PNS. And in order to have that response available to us, that is going to be with effectors, namely muscles and glands.
Now coming back to the sensory division of the nervous system, we have two major categories that fall underneath sensory. The first one is somatic and we want to be specific and say somatic sensory because unfortunately, so rude, there is also a somatic motor. So somatic sensory is the first subcategory of the sensory division. The second is visceral. The major difference between these two categories is whether or not we are aware of the sensations that we receive.
So when it comes to somatic, we are aware of these sensations. So what kind of sensations are we talking about? Well, the first one is probably the one you're least familiar with, which is called proprioception. Proprioception is your awareness in space. proprioception.
So that's awareness in space. In addition to that, other senses you're aware of, you are aware of touch, you are aware of smell, taste, sight, pain, These are just some of the things you're aware of. You're also aware of pressure, but it kind of goes along with touch. So all of these are going to be sensations you're aware of.
And we're going to learn about the special senses. Specifically, we'll learn about, for instance, smell and taste and sight, hearing. Let's go ahead and add that in there too.
Some of these special senses we're going to dive in specifically to the types of receptors that we have and the special mechanisms that we utilize to be able to receive that sensation or to be made aware of it. So what are we used to be able to have these sensations that we're aware of? Well, this is often going to be, if you think about your sight, that's going to be your eyes. Touch would be your skin. Smell would be your nose.
Taste would be your mouth. And you will also have, for proprioception, your skeletal muscle in your joints. Help with your interpretation of proprioception.
Now, somatic sensory information you are aware of, visceral information you are not consciously aware of. So not aware or unaware of. Examples of this would be your heart rate.
You can sometimes feel the effects of your heart rate, but not the heart rate itself. Gas levels. So think about carbon dioxide and oxygen levels in your body. Ions, hormones, and another one we'll list is sugar levels. All of these, for the most part, are related to homeostasis.
So we definitely need to have some sensation for them in our body. We need to be able to respond to these levels if they go outside of our normal levels that we need to maintain. But you're not going to be consciously aware of them.
And so where would we have receptors for these different sensations? These would be our internal organs. We would have receptors for these. All right, so that's sensory. That brings us over to motor.
Again, this is going to be our ability to respond, and specifically with effector organs. There are two categories of motor, two main categories, somatic motor, remember that's different than somatic sensory, so somatic motor, and autonomic. When it comes to somatic sensory, excuse me, somatic motor, this is something that you have voluntary control over.
So you are able to control this with conscious thought and say, hey, I want to move that. So voluntary control. Alternatively, when it comes to autonomic, this is involuntary control. You don't control this consciously. So for somatic motor, what are we going to utilize in terms of our effector organs?
Remember, these can be muscles or glands. You might remember of the muscles that we have, the one that is voluntary is our skeletal muscles. So our skeletal muscles are going to be utilized here. Let me just go ahead and add these. There we go.
So our effector organs here are going to be the skeletal muscle. Alternatively, for our involuntary autonomic nervous system, we're going to use muscles that we have involuntary control over. So you might remember that our cardiac muscle and our smooth muscle was involuntary.
And this will also include our glands. Okay. The autonomic nervous system can be abbreviated as ANS. The autonomic nervous system can be further divided into two subcategories.
Remember, this is involuntary control. And so when it comes to the two subdivisions, again, both of these will be involuntary. The first one is parasympathetic. Parasympathetic is known as rest, oops, sorry, rest and digest.
It doesn't want to stay. Okay. With parasympathetic, you are, again, resting and digesting.
So your heart rate, in this case, if you're resting, would it be up or down? Hopefully, you know that it would be down. Your breathing would be down because you are resting.
But your digestion would be up, right? So our digestion would be up. Alternatively, we have sympathetic. The way I remember it is, sorry, I'm running out of room. Sympathetic, I think I'm so stressed out, have sympathy for me.
So sympathetic is fight or flight. And the sympathetic division is exactly the opposite of the, sorry about that, is exactly the opposite of parasympathetic. So instead, your heart rate would be up, your breathing would be up, and your digestion would be down.
So let's think about that really quick. You spot a bear. You're out in the woods, you spot a bear, the bear is really close, and it looks like it's going to chase you. You need to be able to respond, and you need to be able to respond quickly.
So what is your nervous system going to do? It's going to kick into gear the sympathetic division of the nervous system. So specifically the subcategory of autonomic nervous system, the sympathetic division.
Your heart rate is going to go up, and in increasing your heart rate, and increasing your breathing, you're going to increase the oxygen supply available to your muscles so that you can make more ATP so you can move those muscles, specifically the skeletal muscles, and get them going. You will also probably start to overheat, so your sweat glands will go. But the last thing that you want to do is take a break and eat.
You don't have time to digest. Also, I don't know if you've ever been startled and kind of like, you'd like pee yourself a little bit. That's a reflex.
That is not... Not... the active parasympathetic during this process.
So just a little bit different, but again, your heart rate's going to be up, your breathing's going to be up, but you're not going to be digesting at this time. Alternatively, say you make it out of the woods, the bear is gone, you get to finally calm down and go to sleep. You now are going to bring your heart rate down, your breathing down.
This is where you can do digestion because now we can take the energy that we needed to outrun that bear. Now the energy that we have available to us, we can utilize that for other purposes that needed to take a need to go on the back burner earlier on when we're in sympathetic so again we have our autonomic motor system so um this is the ans that i was just talking about the two divisions are parasympathetic and sympathetic so again kind of like pump the brakes for parasympathetic this is our rest and digest And this allows the body to essentially regain energy. We do need to break down our food for more ATP in the future.
But it's going to allow us to take the energy that we have, put it towards those processes. It's almost like an investment in energy. Sympathetic is the fight or flight.
So we speed things up and this is preparing the body for really intense consuming activities. So again, fight or flight, rest and digest. With the sympathetic division, it actually takes a while. I don't know if you've ever had your adrenaline going.
It takes a while for you to calm down. And the reason for that is that while the nervous system is very quick to respond and quick to turn off, essentially it's just communication signals between neurons. What the sympathetic division also does though is it triggers different organs to release hormones, like adrenaline.
Adrenaline is... a protein that's actually made. And so when you think about proteins, it actually takes your body a significant amount of time to break them down. And so hormones stay in your body floating through your circulatory system for quite a long time, maybe after and for quite a while after that initial stimulus that made you go into fight or flight mode.
And so our hormones and the endocrine system has long lasting effects compared to the nervous system. However, the nervous system drives the endocrine system, and so we can't really control how long it takes us to calm down after we have a fight or flight experience. So this is just a comparison. You'll focus more on when it comes to physiology, but again, the parasympathetic and the sympathetic, they actually act on the same organs. but they act in opposition to one another.
And so for instance, if we're thinking about our eyes, if you are in parasympathetic and utilizing the parasympathetic division of the nervous system, you don't need to have your eyes wide open. You don't need to see your surroundings as well. You're in rest or digest. And so we will cause the pupils to constrict.
Alternatively, if you're in fight or flight, you're utilizing the sympathetic division of the nervous system. You're going to dilate your pupils because you need to get more light into your eyes and receive more information about your surroundings to determine how you need to respond. Let's just do one more example here, the salivary glands. Remember these are glands and we'll talk more about them when we get to the digestive system, but they're going to help us to break down food. In the case of the parasympathetic division, rest and digest, we want to activate those salivary glands to help us to break down the food and start that process of digestion.
Alternatively, if we're in the fight or flight mode, sympathetic division, we do not want to digest at this time. We don't want to put our efforts towards that, so we're going to inhibit saliva production by our salivary glands. So now we're going to switch gears back to our anatomy.
and specifically cytology, so our cells. Remember, cytology is the study of cells. So there are two major types of cells within the nervous system, the neurons or the nerve cells.
So remember, nerves are gonna be bundles of axons. Nerves are still made up of neurons. It's just specifically the axon component and it's a bundle of them within the PNS.
Neurons are the ones that are excitable. So what are we going to have? These nerve impulses that travel down the neuron. down the axon of the neuron, and allow us to secrete neurotransmitters from that neuron to be received by another neuron.
And so we're going to talk a little bit about that process later. But these nerve impulses can be received by what we would call a postsynaptic neuron, and trigger that neuron to then have nerve impulses. Glial cells are not excitable, okay? So they are not going to undergo any nerve impulses.
They act more like the other traditional cells in the body. They will have different proteins that they make and they're going to surround the neurons in different ways in order to support the neuron activity. So the neurons are the major players.
If we have Batman and Robin here, neurons are definitely Batman, glial cells are definitely Robin. But we have a whole bunch of different Robins that help us with different ways in which to support the neurons. So focusing on the neurons, you might imagine to have these nerve impulses, we have a very high metabolic rate.
Coming back to the metabolic rate, that's our metabolism. And metabolism is our different processes that we have in the body, and specifically in the cells, to allow us to make ATP. And so we are going to have a lot of mitochondria in neurons, and we're going to have a lot of ATP production because we need a lot of energy to allow the neurons to conduct these nerve impulses. impulses.
Neurons are extremely long-lived because unfortunately we can't have them go through mitosis or they do not go through mitosis. So they're non-mitotic, meaning they don't divide. If we lose a neuron, we lose it for good.
And what that means then, you might remember I've talked before about how the body, we don't like to have any empty space. Say that a neuron dies and we now have some empty space as a result of that neuron being degraded. Well, what's going to replace that empty space?
It's going to be glial cells. So it's not neurons. Neurons don't go through mitosis.
We can't replace them. But they're extremely long-lived, and they have a high metabolic rate to produce a lot of ATP. Now let's talk about the structure of a neuron.
And we're actually going to draw this out. A neuron has a cell body, which is called a soma. It has dendrites, which are projections coming off of the soma. to receive signals and send information to the soma. So imagine you can receive little, for instance, neurotransmitters with receptors here on the dendrites.
And you have the axon. The axon is the long projection from the soma that is going to send information to the effector, or it can be to another neuron that would be down here. Specifically within the cell body, this again is going to contain our typical organelles and most of the organelles of the neuron.
So our nucleus will be here. Remember the nucleus contains our DNA, the genetic information, and also the nucleolus, nucleoli being plural. The nucleolus is important because remember we need that for protein synthesis, excuse me ribosome synthesis.
We need to make ribosomes to make a bunch of proteins, so we have the nucleoli there. We also have mitochondria, lots and lots of mitochondria within the soma, and we have a special structure that is composed of other structures that we're aware of. These are called nissl bodies. Nissl is the person that discovered these nissl bodies, so he named it after himself, so nice of him. But essentially what these are, this is raffiar.
It is ribosomes and raffiar. In the case of the neuron, these ribosomes and raffiar have a gray color. This gray color is going to lend to the color that we have for gray matter. You might know or have heard of gray matter and white matter in the brain.
Well, we also have gray matter and white matter in the spinal cord too. We're going to talk later about how that color comes from these nasal bodies. The nasal bodies, again, exist in the soma.
Let me just tell you now, if I have a whole bunch of soma, what color is that tissue going to be? It's going to be gray. So we'll talk about these collections of either soma, and we'll talk about what gives white matter its color as well. The axon, again, this is going to be the projection from the soma.
This is where the action potential travels and the nerve impulse. The axon helix, if you think of the soma like a head, The axon hillock is like the neck. So it's specifically this region right here, the connection point between the soma and the axon.
The axon hillock plays a special role in that this is where the action potential first takes place. And we're going to talk lots about action potentials later, and you spend a lot of time on action potentials in physiology. Where the action potential begins. So we don't actually have the nerve impulse start in the soma.
We collect information in the soma. And more specifically, what's that information going to be that we're collecting? It's going to be neurotransmitters are going to be received either on the dendrites or the soma itself.
They're going to be received by receptors on the dendrites or the soma. And that's going to lead to ion channels opening. The ion channels let in positive and negative charges to the soma. And the collection of those positive or negative charges eventually will reach the axon hillock. And if we reach a very specific charge at the axon hillock, we start the process of the action potential or the nerve impulse.
So again, action potentials or the nerve impulse begin at the axon hillock. The nerve impulse and the series of action potentials that are going to take place down the axon, they always go away from the soma. or the cell body.
And they're always going to go away towards the farther end so that we can eventually cause the release of neurotransmitters. Along with the axons, we have some associated structures. Axon collaterals are essentially branches coming off of the axon. So you can have multiple kind of branches to communicate with different neurons.
We also have the teledendria. These are the end branches of an axon and the teledendria are going to finally send that electrical impulse to the synaptic knobs. So the synaptic knobs are the final place and where the signal will be received, so the nerve impulse will be received.
And more specifically what's important about the synaptic knobs is they contain the neurotransmitters. So We learned about the motor neurons last unit, and the motor neurons had that nerve impulse travel down the axon, and then that triggered neurotransmitters, specifically acetylcholine, to be released from the synaptic knobs. So all neurons, not just motor neurons, but all neurons are going to have these synaptic knobs, these little feet-like structures, and again, they contain the neurotransmitters, just waiting to be released. And how do we trigger release? via a nerve impulse.
So let's go ahead and just redraw this for ourselves. Again, drawing through it helps us to remember. So just as a reminder, what we're actually drawing here, there are three different anatomical types of neurons.
We're actually drawing a multipolar neuron, which is our most common type of neuron that we see within the nervous system. And I'm not the best drawer, but we will get this done. Okay, so again what we have here is we have the soma.
The soma is also known as the cell body, and that is going to contain our nucleus, most of our organelles, and what's really important here is our nasal bodies. because our nasal bodies, which is made up of ribosomes and the rough ER, gives the gray color. So it's not the whole neuron that's gray. It's specifically the soma. Coming off of the soma, we have our dendrites.
And again, our dendrites are going to be projections that are going to have receptors. And so these are our dendrites. And they receive stimulus. In the case of our neurons, it is usually going to be neurotransmitters that are going to be received, and that's going to lead to ion channels opening.
going to receive a stimulus could be, it could be neurotransmitters and usually is unless it is a modified neuron that directly receives stimulus through the peripheral nervous system or from the peripheral nervous system. So we could have receptors for stimuli or we could have receptors Or we could have receptors for neurotransmitters. And I'm just drawing the nucleus here. We also have our axon hillock. Our axon hillock is where the nerve impulse begins, or the action potential begins.
So I'm going to write AP begins nerve impulse. We have our axon. the action potential travels down the axon. And then that brings us to the teledendria. I think teledendria and I think the T and I think the end.
And then extensions, expanded extensions of the teledendria, those are our synaptic knobs. And our synaptic knobs. contain the neurotransmitters that will be released. So we would release those to our neighboring neuron.
While we're here, say that there's another neuron here that's on the other side of that. We don't have an action potential travel through this synaptic knob and directly into the next neuron. Again, it is action potential, nerve impulse travels through one neuron. And then what happens is these neurotransmitters are going to be released. and recede by receptors on the next neuron, which will then cause eventually, if we can get to the axon hillock of this next neuron, will cause another action potential and nerve impulse to be created.
So what we're seeing here, if this is a space of communication between two neurons, this would be what we call a synapse. And more specifically, this space is called the synaptic cleft. The neuron that is before the synapse, which is sending the communication signals, this would be then the presynaptic neuron, so before the synapse. And the one that comes after the synapse is called the postsynaptic neuron. So in order to have communication between these two, you would have an action potential travel down the presynaptic neuron into the teledendria, into the synaptic knobs, which would trigger the synaptic knobs to release neurotransmitters into the synaptic cleft.
The neurotransmitters would be received by receptors on the postsynaptic neuron, which would then cause ion channels to open. So positive and negative signals, depending on what neurotransmitters received and which ion channel opens, would then go into the soma of the postsynaptic neuron, potentially triggering... Another nerve impulse or action potential at the axon hillock of the postsynaptic neuron. Now, another thing that I want to draw your attention to, since we have this image, is I want to come back to our definitions of a ganglion and a nerve. So imagine we have a whole bunch of neurons here.
side by side. I'm just going to represent a whole bunch of the axons. These represent the axons of different neurons. And imagine we have a whole bunch of soma here. If we have a bundle of axons in the PNS, so let's put bundle of axons.
If we have a bundle of axons in the PNS, We call this a nerve. But if we actually have a bundle of axons in the CNS, we call it something different. And this is going to help us when we get to the brain. A bundle of axons in the CNS is not called a nerve. A bundle of axons in the CNS is called a tract.
And so we're going to learn about different tracts in the brain. And when we say tracts, what we're talking about is a bundle of axons, specifically within the brain in that case, but we're also going to have tracts within the spinal cord as well. Now, in addition to bundles of axons, we can also have bundles of soma. And so I'm just going to put down here.
Again, it's going to depend on whether we're in the CNS or the PNS what we name these bundles of soma. If we're talking about a bundle of soma, so cell bodies, in the PNS, we already gave that a name. That's a ganglion.
One ganglion or one bundle is one ganglion. If we have multiple ganglions, we call it ganglia. That's the plural. but we actually call it something different if we have a bundle of soma within the brain or the spinal cord so in the CNS we call it a nucleus and I know that's confusing because you think nucleus that's within a cell a bundle of so is also called a nucleus. If we have multiple nucleuses, because that's wrong, we have a nuclei.
So we'd call them nuclei. This again will come back because we're going to learn there are nuclei that we need to be aware of in the brain, and there's tracts that we need to be aware of in the brain. If we know when we hear tract that that's a bundle of axons, we know that that's a bundle of parts of a neuron. Same thing with a nucleus.
It's going to be a bundle of soma. We're going to find that tracts and nuclei have different colors and it's all about what those bundles are made of. So for instance, let me just give you a little hint now. You might remember there's nasal bodies within the soma and those nasal bodies give the soma a gray color.
So I'm going to tell you right now when it comes to the nuclei within the brain. we're going to find that they're going to be part of the gray matter of the brain. And why is that? Because we have a whole bunch of soma, and those soma contain nasal bodies, and the nasal bodies are gray.
Hopefully that's starting to make sense. So when it comes to neurons, we can classify them both structurally and functionally. I keep telling you that the most abundant type of neuron structurally is multipolar.
However, there are two other types including unipolar and bipolar, and we're going to find that they play roles in specific parts of the functional divisions of the nervous system as well. So unipolar is going to have one pole, bipolar has two poles, and then multipolar many poles. Unipolar, I want you to think S, and that's because the unipolar neurons are going to be involved in the sensory division of the nervous system. So sensory division, single sensory.
It looks like there's two parts here, but in fact, it's this little process right here that gives us its unipole name. We're going from the dendrites, and we'll receive the signal there, and will then travel down the axon in this direction. So all of this is the axon. That's unipolar.
It looks kind of like if I were to draw it like this. Okay so that's unipolar. And again we specifically find unipolar neurons in the sensor. sensory division of the nervous system. Our bipolar, it actually to me, I have the I here because it looks like a capital I on its side.
And so our little bipolar neurons are actually going to be the least common. And we also find them playing a role in sensory, but only for special senses and only two special senses. I will bring them back up and draw your attention to them when we get to the senses lecture. So again, we have our dendrite.
We're moving in this direction. But notice instead of having this process, we're in between with our soma. This one is bipolar. Finally, we have our multipolar. The multipolar, I am italicizing the M there because the multipolar is going to be involved specifically in the motor division of the nervous system.
So multipolar motor. And again, this is the one that we most commonly draw. It has a single axon, but it has dendrites on one side, and then it has all kinds of dendrites, and then it has the teledendria as well, or maybe even it has these collateral axons like we talked about previously. Again, the multipolar is going to be the most common. So, functionally, we can also associate our neurons, and I already started to do that for you.
The functional classifications of neurons is based on which direction the nerve impulse goes. Does the nerve impulse go from the PNS to the CNS? or does it go from the CNS to the PNS? So when we say afferent for neurons or the afferent division, that goes hand in hand with the sensory division of the nervous system. Efferent goes with motor, okay?
So they're often used interchangeably. Sensory neurons are sending information from the PNS to the CNS. And our motor are sending nerve impulses and information from the CNS to the PNS.
We're going to see that our interneurons actually communicate between the sensory and motor neurons. So they facilitate communication, which you'll see on the next slide, or one of the next slides, facilitate communication between sensory and motor neurons. So first, sensory, which we've already covered, this is a functional division of the nervous system, this is going to be afferent.
I think afferent at, so we're going to the CNS from the PNS. We are detecting stimuli. Again, I've already mentioned to you those stimuli can be touch, light, pain, smell, sight, all of those kinds of different things. The light would be the sight part. And the majority of the neurons that we're going to be using for these are unipolar.
Okay, so the majority are going to be unipolar. Let's look at an example of this. Here is some skin receptors.
Those receptors are going to send information to the peripheral nervous system and this neuron within the peripheral nervous system. And it is then going to send information to the CNS. This is the spinal cord here.
We're seeing a cross-section of the spinal cord. So here, we're now in the CNS. Everything outside of that, we're here in the PNS.
So we're sending that sensory information to the CNS. Let's look at some of the information we have here. This neuron is sending information to the CNS, so this is a sensory neuron. We also have interneurons, which actually help us to communicate between the sensory neurons and the neurons of the central nervous system. So we have an interneuron here.
This little area right here, we're only seeing one, but there's actually multiple neurons here. This region or structure is called a posterior root ganglion. We're going to come back to this later, but I want you to notice ganglion. Ganglion, how do we define that? That is a collection of soma within the PNS.
And what do we have? A collection of soma within the PNS. So that is why it's called a ganglion. So again, the majority of the neuron types that we're going to see within the sensory division are going to be unipolar. I will tell you right now, if we go back, we also have bipolar.
Bipolar are very special. So bipolar are very special neurons that we only find in certain special senses. So very special. only in special senses.
So let me take you back to our functional divisions and let's add some of these different neurons based on their structure. So just to tie back with what we've already focused on, where we are right now is we just talked about sensory, which is also known as afferent. So when we're sending information from the PNS to the CNS, that's afferent. The types of neurons that we're going to use for the The afferent division are going to be unipolar. That's the majority of them.
So we have unipolar, put it up here, and those, remember, look like this. And we also have bipolar neurons. But remember those bipolar neurons, those are going to be very special and only for very specific special senses.
I will draw your attention back to that when we get to the senses lecture. We also have interneurons, and interneurons are between the afferent and the efferent. They're going to help us to connect our sensory neurons and the neurons of the CNS. So we have interneurons as well.
Let's actually add that in red so we don't forget. Interneurons. And I don't know if you noticed on the slide, but interneurons, they're multipolar.
So those are going to be our traditional looking neurons. So again, we have our sensory or afferent division, where we send information from the PNS to the CNS. The majority of the neurons that are going to make up this connection point are going to be unipolar neurons, but we also do have some bipolar neurons, and we're again sending that information to the CNS. We often have interneurons, which are multipolar, which are going to be sharing that information or communicating from the sensory neurons to the neurons of the CNS.
The other one is the motor, also known as efferent, and I think efferent exit. So this is when we're taking information and we're sending it back out from the CNS to the PNS via neurons. So CNS to the effectors, which is the PNS.
What are we doing here? We have that interneuron, and then in this case we're utilizing a motor neuron. You know of a motor neuron in that it is going to connect or meet the skeletal muscle at the neuromuscular junction, and specifically at the muscle fiber level.
But this is a motor neuron. It's sending information from, we have the soma, the action potential is going to travel down the axon, to the teledendria, to the synaptic knobs. So the types of neurons that we're going to see here are going to be multipolar neurons. So structurally, they're multipolar.
So let's go back to our chart and add this information. Again, we can also consider our motor division as efferent, also known as efferent. This is where we're sending information from the CNS to the PNS. And this division is going to structurally have multipolar neurons.
So again, the motor division has multipolar neurons. Interneurons are multipolar as well. And then for sensory or afferent division, we have structurally unipolar neurons, which make a majority of sensory, and bipolar neurons. Already off the bat, I want you to realize that all of the neurons structurally within the motor division, or efferent division, are multipolar. Also, we're going to learn that interneurons are the most abundant.
And they are multipolar. So if I asked you structurally, what is the most abundant type of neuron, anatomically or structurally, the most abundant type is gonna be multipolar. Because again, interneurons are the most abundant and they're multipolar. And the motor division is made of multipolar neurons.
So again, bringing us to our third kind of component of the functional division, I drew the neurons on our chart in between. And they again help to communicate between the sensory neurons and the motor neurons. So the sensory again sending information from the PNS to the CNS. We'll have some interneurons there that help to facilitate the communication.
And then sending information out from the CNS to the PNS or the efferent division, we have our motor neurons. As I mentioned, interneurons make up the large majority, 99% in fact, of our neurons are interneurons, and they're multipolar. So because of that, the majority of our neurons structurally are multipolar. So let's go ahead and put it all together with the neurons. So we are going to receive stimuli, and that stimulus is going to be received by sensory neurons in the peripheral nervous system, and travel via the...
travel via the sensory neurons to the interneurons, which communicate between the sensory neurons and the neurons of the CNS. This pathway going from receiving stimuli with receptors to the CNS is called the afferent division of the nervous system. And we have sensory neurons here, which the majority are going to be unipolar, and then with some exceptions, bipolar neurons.
The interneurons that communicate between the sensory and motor neurons are called interneurons. They are structurally going to be multipolar neurons. And then we have the efferent division of the nervous system, where we go from the CNS, in this case the spinal cord, to the PNS, to an effector organ.
In this case here, we're seeing a skeletal muscle. The motor neurons that are going to carry out this function of the efferent division are structurally multipolar as well. And again, they're going to communicate with our effector organ, in this case the skeletal muscle.
At the very end here, if we were to zoom in, we would be able to see a neuromuscular junction. In the afferent division, we also have posterior root ganglions because the soma of these sensory neurons are outside of the CNS. They're in the PNS and we have a collection of them, so that's a ganglion.
We're going to find that there is an anterior root and a posterior root. Specifically, the ganglion exists in the posterior root, and we'll talk about that later. Here's our communication point between the interneurons, which again are multipolar, and the motor neurons, which are multipolar. And there you go.
All right, we're moving on from our neurons, and now we're going to focus our attention on the glial cells. Right now, again, as you know, glial cells, we only know them as glial cells. They're supporting cells.
But we want to know the specifics about each glial cell. and what function it carries out. So here, all of these other, here you can see the neurons, all around them are different types of other glial cells that are supporting. You can see these teeny tiny ones, and in their name, they are microglial, super tiny cells. Here you can see astrocytes wrapping themselves around capillaries.
The astrocytes, and specifically the podocyte, the little feet of these astrocytes, are going to work together with the capillary and abasement membrane to make up the blood-brain barrier. We also have ependymal cells. We'll talk about them a little bit more. We have different cells that allow for insulation, in this case because we're in the CNS.
We have oligodendrocytes. We're going to talk about all of these here in a second. Now when it comes to the glial cells, we actually have different glial cells in the CNS versus the PNS.
And so I just want to draw your attention to that now. You do want to be able to categorize them based on which ones are in the CNS and which ones are in the PNS. What's interesting about glial cells, as you know from looking at the histology slides of them, they're much much smaller than neurons.
However, they are actually capable of mitosis. So like I mentioned, if we were to have a neuron die and then have some space left over as a result of that neuron's death, what would take place is the surrounding glial cells would go through mitosis to fill in the gaps of where that neuron was. Glial cells and neuroglia, same thing. So just as a reminder, I talked about that before when we talked about histology, but just to remind you, glial cells and neuroglia, same thing.
So the glial cells offer support, they provide protection, and they provide nourishment for the neurons. So we're going to find that the cerebrospinal fluid that surrounds the neurons is actually created by the glial cells. They are not excitable, they do not have nerve impulses, so they do not receive neurotransmitters, and they do not... as a result of receiving neurotransmitters open ion channels. So completely different process.
These are more like again traditional cells. We do have more of these in neurons. Just think neurons are much bigger in size but the glial cells we have more of.
Also as a reminder, glial cells can do mitosis. You may or may not know that tumors are the result of misregulation of the cell cycle and a lot of times that ends up in mitosis happening aberrantly or misregulated mitosis, so lots and lots of division without control. That is related to mitosis and division.
Remember, neurons don't divide. They don't go through mitosis. So really, your brain tumors, or when you hear about a brain tumor, it's usually going to be the result of one of the glial cell types, not neurons.
So first, we're going to focus on the glial cells of the CNS. There are four different types that we find within the central nervous system. astrocytes, which have kind of a star shape, ependymal cells, microglial cells, you hear teeny tiny in that, and then one that plays a very functional role in insulation of the axon of neurons, oligodendrocytes. Oligodendrocytes have a sister type of cell type of neuron, or excuse me, glial cell within the PNS that does the same thing.
However, they're structurally different, and again, where you find them is different. So the oligodendrocytes I want you to associate with the CNS. Make sure you keep that in your mind. There's a really nice table, 14.4 in your e-textbook, that helps you organize the, it has a nice organization of the glial cells and their function.
So the first one of the four that we find within the CNS is the astrocytes. Astro, maybe think of asteroid or astrophysics. That's going to be star, so they are star-shaped. It's this green cell right here. As I mentioned, the major function of the astrocytes is going to be informing the blood-brain barrier.
One of the things that you need to know is what makes up the blood-brain barrier. There are three things that I want you to associate with the blood-brain barrier. There are...
endothelial cells of the capillary. So capillaries are made up of endothelial cells. You might remember epithelial cells and specifically those simple squamous cells that can make up blood vessels. So endothelial cells is the first component. Then in between you have a basement membrane.
And the final component is going to be these little feet of the astrocytes. So the perivascular feet of the astrocytes. So those are the three components I want you to know of the blood-brain barrier. As you know, some things can get in that are really small, but a lot of things can't.
So this helps us to protect our neurons. Notice that you have to be able to get all the way through this astrocyte to be able to eventually get to the neurons. So the astrocyte is kind of like the bouncer at a club. It's very, very specific about who it lets in.
When it comes to the astrocytes, it does play also other roles. Again, what I really want you to associate with the astrocyte is the blood-brain barrier, but it does also help to regulate the tissue fluid, so it's going to secrete some of the surrounding fluid that we would find within the brain. and it's going to help to replace damaged neurons. So it actually helps as kind of a healer in a way too. It also helps to guide development.
So you can see these feet can potentially push and pull different other cells into different places. So it really is kind of the jack-of-all-trades of cells. It helps to repair damage, maintain the environment.
It helps to provide a barrier. for things coming into the central nervous system and it guides neuronal development. And the neuronal development, as you know, because we don't go through mitosis actively, that's when neurons are first created.
Ependymal cells have this really interesting shape. You kind of see this projection coming off of it, but if you look at the majority of it, it kind of has that square shape. Also what you're going to see on the top are cilia. So these are ciliated cuboidal epithelial cells. So hopefully ciliated still sounds familiar to you.
Cuboidal still sounds familiar. I did tell you unit one would come back for you. So epidermal cells are ciliated cuboidal cells.
What they play the largest role in is forming the choroid plexus. We're going to learn about the choroid plexus when we get to the brain, but the choroid plexus is what is going to help us to produce the cerebral spinal fluid, CSF. This is the fluid that is going to circulate, bringing fresh nutrients to the brain.
removing waste products. So this cerebral spinal fluid is incredibly important. All of the fluid within the body is essentially like blood.
It doesn't necessarily have the cells of blood and won't unless it's blood, but the fluid that makes up blood, a lot of that is very similar to what we find for the fluids in the rest of the body. So cerebral spinal fluid, very similar to blood, but made by the choroid plexus, which is made up of epidemal cells. Microclio cells are teeny tiny.
We have very few of these. So not only are they small, but we have the smallest percentage of them. They're going to help and be phagocytes. So think of these as our immune protectors.
Phagocytes, you might remember phagocytosis, so cellular eating. If they find anything that is not supposed to be there, they'll go ahead and phagocytize it. They'll also remove any cellular debris.
So they kind of act like, I don't know if you've seen those sucker fish that are inside of tanks and help to clean off the algae on the outside of the tank. tanks. That's what I think of when I think of microglial cells. They go ahead and clean up any dead or dying debris.
They kind of just are the waste management cell of the brain and the spinal cord. And then we have our oligodendrocytes. Mentioned, CNS only.
Now what are they going to be covering? They are covering the axons of neurons. What I want you to notice, this is one single oligodendrocyte, and this single oligodendrocyte has multiple projections. So it's coming around the axon of this neuron, and with another projection it's coming around the axon of this neuron here, same axon but different part. Here's another projection onto a different neuron.
So with these projections of the oligodendrocytes, it can attach and insulate the axon of multiple neurons at the same time. So in multiple places, multiple neurons. What is this insulation, what is this all about? Well, remember that action potentials are essentially ion charges.
They are an electrical impulse. And so in order to insulate that electrical impulse that travels on the axon, we utilize these projections of the oligodendrocytes. What's inside of that wrap?
Well first, what is that wrap called? It's called a myelin sheath. Myelin is essentially fat.
So what we're doing is we're wrapping adipose, a whole bunch of fat, around these axons. And it kind of acts like electrical tape. So I don't know if you've ever used electrical tape before.
It helps to insulate the electrical wire so that you don't have leakiness, that you have a better electrical signal. Same thing with our neurons. We can insulate them with these oligodendrocytes, specifically of the oligodendrocytes, the myelin sheath, and that allows for better communication and action potential charge down the axon of the neuron.
That brings us to the PNS. We only have two different cell types in the PNS. The two types are satellite cells and neurolemocytes.
Neurolemocytes are the PNS version of... oligodendrocytes. So they're going to do the same thing, we're just going to find that they have a different structure and a different name, and there's another thing we can call them too.
So we'll look at that in a second. Same thing if you want to check out your e-text, there's a good table here for this. First for our satellite cells, let me show you where we are. We are in the posterior root ganglion. So we looked at this when we put together the afferent and the efferent divisions of the nervous system.
We're in the posterior root ganglion, so we're in the PNS here. This is a collection of soma in the PNS, so that's the ganglion. And this is actually the soma of a neuron here. There's another soma, another soma.
What the satellite cells are actually these pink cells that are on top of it. All of these are individual satellite cells. So I think of satellite outside.
They're not in the central nervous system. They're only in the peripheral nervous system. What they do is they wrap themselves around the soma of the neurons. And that helps to separate the signals that one neuron soma might receive compared to another.
So imagine, remember, these soma are receiving all kinds of signals and neurotransmitters, right? Well, we want to make sure that we separate those signals from one another. And so these almost act like buoys or separators. I almost imagine them like, I don't know, like, I don't know if you've seen the packing. Like the little packing balls that come in boxes, like a styrofoam packing...
Oh, I think it's called packing popcorn. Maybe. I don't know.
Anyways, I think of it like that. So it's like packing popcorn that helps to separate the soma from one another and then makes it so that they each get separate signals. It also helps them to each get their own nutrients, because if they were all on top of each other, they'd have to be sharing the nutrients a little bit more as well.
Alright, the second type of cell that we find within the PNS are called the neurolemocytes. Another name for neurolemocytes is Schwann cells. So if you hear Schwann cells, that's another person's name. Often Schwann cell is used in physiology over neurolemocytes.
Please note these are only in the PNS, but they do the same thing. They're going to insulate the axon just in the PNS though. I also want you to notice you don't see any multiple projections coming off of these like the oligodendrocytes. These can only wrap around one part of one neuron, so they are discrete.
They do the same thing though in providing insulation with myelin. Again, fat wrapping around. If you remember, adipose is white, has kind of a white appearance. So you might remember the soma and collections of soma are going to be gray. Where we have collections of axons, especially those that are insulated, they are going to be white.
So where we have bundles of axons, we're going to have white, maybe white matter. And when I say white matter, that's usually in relation to the CNS. So those would be like our tracks. Where we have gray is collections of soma because of the nasal bodies. Here's just a comparison side by side.
Oligodendrocytes here at the top. Remember, those are only in the CNS. Neurolemocytes, also known as our Schwann cells, only in the PNS.
Oligodendrocytes can cover with their projections multiple axons. However, neurolemocytes, also known as Schwann cells, can only cover one part of one axon. Notice that there's little spaces in between. Those are called nodes of Ranvier, which will become important when you talk about them in physiology.
So there are spaces, little discrete spaces, in between each of these cells, whether it's oligodendrocytes or neurolemocytes. But both of them collectively allow for electrical insulation. so that we can have that signal, that electrical impulse, travel down as fast as possible through these cells. Now, not all neurons have myelination.
Not all neurons are going to have oligodendrocytes wrapped around them or neurolemocytes. But the ones that do tend to be very long, and it helps with that process of having the communication signal travel down the axon. Again, as I mentioned, these have a white appearance.
Wherever we have myelinated axons, they tend to look white, which adds to the white color that we will see in the spinal cord and in the brain. All right, so that brings us to our nerves. As a reminder, these are a bundle of axons and we know that they are a bundle of axons in the PNS. Just because the soma is in the CNS, that's fine, but if the axons that we're talking about are in the PNS, those are nerves. If you remember the different connective tissue wrappings that we have around muscle, the nerve connective tissue wrappings are going to come much easier.
You might remember endomysium when we talked about the muscle, perimysium, epimysium. These names are the same except now we're talking about around nerves, so we're going to use nurium. So let's go from the inside out and I'm going to show you on a picture.
The endonurium is going to be made of areolar connective tissue. Hopefully that sounds familiar. Excuse me.
And it's going to be around the Schwann cell. Okay. We'll look at that in just a second, but endoneurium areolar connective tissue.
This is going to help us to isolate the axons from one another. Then around our fascicles. Does that sound familiar? We also have the tissue level here.
This is going to be the perineurium. and the perineurium is made of dense irregular connective tissue. Does that sound familiar? I hope it does because the paramecium, same thing, dense irregular connective tissue. Finally on the very outside we have the epineurium.
This is going to bundle those fascicles together. It's going to surround the whole nerve, and this is also made of dense irregular connective tissue. So that is what this looks like. If you remember the muscles, tubes within tubes within tubes, it's pretty much the same thing when it comes to a nerve.
So this whole thing here is a nerve. It is surrounded by a connective tissue layer, the epineurium, which we know has dense irregular connective tissue. Within the epineurium, and the whole nerve we have bundles of fascicles. These bundles of fascicles are going to be surrounded by the perineurium, which is also dense irregular connective tissue. Within the fascicles, we have individual axons that are myelinated, and around those myelinated axons we have the endoneurium, which has areolar connective tissue.
We do have blood supply in between these different fascicles, which is necessary for providing oxygen, and we know that Neurons are highly metabolic and they need lots of oxygen to make lots of ATP. All right, that brings us to our synapses. I actually already started to introduce this when we were drawing the neurons, but let's go ahead and take a look at this signal here.
First, what we see is a nerve impulse or an action potential traveling down this neuron into the synaptic knob. This synaptic knob is specifically the synaptic knob of what we call the presynaptic neuron. Why?
Because it's before the synapse. The presynaptic neuron is always going to send the signal to the postsynaptic neuron. So the postsynaptic neuron is here and you can see that right in the name.
So this is going to look very similar to what we saw for the neuromuscular junction. with the exception that instead of having our receptors for the neurotransmitters on a muscle fiber, we instead have our receptors for the neurotransmitter on a postsynaptic neuron. So the nerve impulse travels down the presynaptic neuron axon to the synaptic knob, where there are vesicles containing neurotransmitters, in this case specifically acetylcholine. When that action potential reaches the synaptic knob, it triggers these the um vesicles to release the neurotransmitters, specifically acetylcholine, into the synaptic cleft, the space in between. That acetylcholine can then be received by receptors on the postsynaptic neuron, which triggers these ion channels to open, changing the charge within the postsynaptic neuron.
You might remember sodium is Na+, so now we're flooding the inside of this postsynaptic neuron with positive charges. In a separate video, I will talk about the action potential and how that takes place. We are going to make, essentially, when we make it more positive inside of the postsynaptic neuron, we can also think of that as less negative.
That will come into play when we talk about action potentials. One thing before we move on, electrical versus chemical. What is taking place within the neurons themselves, those changes in charges, that's electrical.
It's all about ions traveling through the different neurons and how those concentrations of ions change is due to ion channels and we can reset those ion charges or that the concentration of ions as well or those charges with our sodium potassium pumps. Again I'm going to bring this back in our action potential video. But alternatively what's actually received and secreted by the neurons is chemical. Acetylcholine, Cholesteroline isn't a charged molecule.
It is a molecule, though, that's going to bind to the receptor and open ion channels. So the electrical part is within the neurons, but what we are using as a communication signal between neurons is chemical. It is a molecule. Again, in this case, acetylcholine. I hope that helps.
That is our nervous tissue lecture. If anything is unclear after you watch the Action Potential video, let me know. Again, you want to make sure you get this down really well so that you can apply some of this information to the brain and to the senses.
As always, reach out if you have any questions.