Overview of the Nervous System

So we're going to start the nervous system today. I realize we did not get all the way through the skeletal system, but as I talked about on Thursday, what we got through on Thursday is the skeletal system for us. I think I had one more page, but we have to move on to our next body system. which is the nervous system for us. So we'll be spending some time talking about the nervous system in this module, sort of the background components of it. Really, in this class, I talk mostly about... the structural details of the nervous system and then in human body 2 we go into more of the electrophysiology like action potentials neurotransmitters a little bit of pharmacology but in this class really just focusing on the overall structure of the nervous system and its functions so we're going to get right into that starting with kind of just an overview or an introduction to the nervous system and some of this will look pretty familiar to things we've already done because we're going to start with a flow chart. If you have the notes, you know exactly what I'm talking about. A flow chart that looks pretty familiar to things and that's because that chart really explained a lot of different things that we talk about in this class. So nervous system introduction. We're going to do a couple things on this first part here. We're going to talk about sort of the main components of the nervous system and then I will diagram them out using that wire line diagram you see on the right hand side and then we'll see how all these components fit together to help coordinate the body. So when we think about the nervous system, really three main functions to the nervous system starting very broadly, we're going to see that certain neurons help sense or detect the environment and when I talk about the environment you think environment, you think like external environment, you know, temperature outside or amount of light being received, but it turns out the nervous system also detects the internal environment of the body. So the nervous system, as far as sensing and detecting here, would be responsible for understanding things like blood pH or blood pressure or the composition of gases in the blood, for example. So there's lots of different things we can detect. So when I put in parentheses internal... and external, I'm referring to the environments, and the nervous system has to detect all of that in real time, and that's a lot of information coming in in real time. If you think about all the different things that the body has to detect inside and out, that's a lot of stuff. So that's one thing. The second thing that we're going to talk about is that the nervous system will collect or amass, some people have not seen the word amass before. I like to use it on exams, but then I realized... People are not too sure about that. So amass just means collect sensory information. So all the sensory information, internal and external, that's a big load, comes in through the central nervous system. And that will have to be collected and accumulated. And then some sort of decision is going to have to be formulated. So the nervous system gets all this information and then decides, what do I do with it at this point? Do I do something or do I do nothing? In some cases that is true. And then we'll see that if a decision needs to be carried out, something needs to change or move, it is also the nervous system's job to activate effectors. Can you remind me what are the types of effectors we find in the human body? What type of effectors might we see? Effectors. These are anatomical structures that are activated by a neuron. That's right. Glands is one. Good. Glands, and there's a lot of different glands. What else? Muscle. Yeah, and so when we think muscle, I'm just going to write muscle. Remember, there's different types of muscle, so don't always assume skeletal muscle. There are other types, so smooth muscle, cardiac, and then, of course, skeletal. So you've seen this before but I think it's good to sort of remind you and help you apply it to a new situation which is really taking this information that we've already talked about in one context and then applying it to a different context. which is the nervous system. So as we move through sort of this overview, you will see me catching my paper up here. You will see me refer to really two main parts of the nervous system. One of them is the central nervous system. And the central nervous system is usually abbreviated CNS but abbreviations aren't too helpful if you're not quite sure what they mean so central nervous system or CNS we talk about two things with this location and in doing so also talk about function. So when we think about the nervous system, the central nervous system, anatomically it sounds really straightforward. So for example, the central nervous system anatomically only consist of the brain and spinal cord. That's it. Brain and spinal cord. If it is anything other than brain and spinal cord, it cannot be central nervous system. So that sounds pretty straightforward, but then we get into sort of this gray area, like cranial nerves. So cranial nerves, turns out, most of them are not central nervous system. They're peripheral nervous system. So we can sort of understand this at a very superficial level saying brain and spinal cord, but we need to be very comfortable about what that also entails. Do not be eluded or tempted by a word like cranial nerve and think, well, cranial head, that's got to be central nervous system. The vast majority of cranial nerves are actually peripheral nervous system, not central nervous system. So when we think about brain and spinal cord, their big function, if we had to wrap this up into just a small description. What do they do? They amass information like we saw in item two. They amass info, all that sensory info, and make a decision. That is the job of the brain and spinal cord. Take in all this information and then decide what are we going to do about it, if anything. And that's a big job. But we have other functions, right? We still have one and three. And that turns out to be the job of the peripheral nervous system. The peripheral nervous system, or PNS, a lot easier to write. Peripheral nervous system. Again, we could use sort of a straightforward definition, which is fine if we're just learning, and talk about what is this thing anatomically that we call the peripheral nervous system. And we could start with the location detail. Location, it is any nervous tissue outside of the CNS. In other words, if it's not brain and spinal cord, it's peripheral nervous system. But then again, you get into this, you know, like, well, but what about cranial nerves? And it turns out peripheral nervous system for the vast majority of us. So the peripheral nervous system does a lot of work, but what it does not do is collect information and make a decision. It cannot do that. It does not have the right cells for that. But it can sense and detect internal and external environmental information and take that to the central nervous system. And it carries out the decisions of the central nervous system by having neurons that go all the way out in the body and activate effectors, glands, or the three types of muscles. So the peripheral... nervous system does a lot. While we're here talking about sort of the overall anatomy of these two systems, let me make also a distinction that will be important probably later in your career when you're working like in a clinical setting. The central nervous system is encased in bone. And so maybe that helps you, you know, the cranial vault or the vertebrae, like I showed you last week when we were talking bones. This is why I like to have the nervous system follow the skeletal system. They have a pretty decent relationship. The peripheral nervous system is not well protected. It is not encased in bone. It is more vulnerable to damage. So we'll see some sort of like add-on features that we have gained over the millennia to help protect nerves which are a big part of the peripheral nervous system. So it's funny how these two systems work together and yet they are in many ways very different, very different as far as location, how they function, the type of cells they contain, what type of level of protection they get. So before we go really any farther, I want to come over here and talk about sort of an introduction to the central nervous system as well as a peripheral nervous system since I kind of mentioned how they work together but how they are also individual. So if you have this printed off, you see... A very simple line diagram wire. I'm going to add some details onto this, of course. You're looking at, and here's some terms that I want to add. You're going to hear me use them in this class as well as in human body. too. I want to talk about ventral versus dorsal. So we usually want to talk about anatomical terms for bipeds like humans. We use anterior and posterior. You're familiar with these, but for some reason in the nervous system, we don't really use that. We use instead dorsal and ventral. And my understanding is that the anatomical terms that apply during fetal development for some reason stick with the nervous system, even when you're an adult. So I'm going to introduce those terms to you and help you make the connection between our more familiar anterior. posterior, as well as dorsal and ventral. The reason I mention this is when we get to the bottom of this page, you will see a cross-section of a spinal cord, and it turns out the anatomical regions of a spinal cord also contain dorsal and ventral. So it's good to just get used to them now, so when you see them in smaller anatomical features or organs, you're like, oh, that's what that means. So dorsal and ventral are things you'll see. So if you'll notice at the top, I right, and I can't move this down a little bit, but I have dorsal aspect already written for you. And so dorsal refers to the back, the spine, for example. So dorsal in this case would be similar to your more familiar posterior, but we don't use posterior, anterior, we use dorsal and ventral. Ventral would be the belly aspect, the underside part, dorsal is the back. So on this um fairly simple diagram. What I just want to introduce to you is kind of the physical relationship between the central nervous system and the peripheral nervous system and how they are both required in order to get a person to be, you know, truly fully functional and how issues in one can really cause issues elsewhere. So first of all, let's talk about the things that I'm going to have you label and we talk about the central nervous system. So I'm going to zoom in just to that. Thank you. brain and spinal cord area here as much as I can. When we talk about the brain, there's a few anatomical regions here that you're going to hear me talk about. And so I thought if you haven't heard about them before, this would be a good time to sort of just mention them. So that way you're not kind of lost in the vocabulary. The nervous system is full of kind of complex vocabulary that a lot of people aren't too comfortable with. So first of all, when it comes to the brain, so I'm going to draw a line right here. The first three lines here are really about the brain. So first of all, we see a lot of wrinkles, and these wrinkles are due to these sort of protruding areas. They sort of look like not mountainous areas, but they are very sort of bubbly looking areas. These are called gyri. And in the middle here we have, I'll just sort of outline it here, we have little valleys and those are called sulci and these are plural, cingulus gyrus or sulcus. So gyri and sulci together, what they do, I'm going to put a little bracket here, is they increase surface area. So SA for surface area. And increasing the surface area here is super important because the more surface area you have, the more room you have for cortex. And cortex would just be the kind of the, if you cut this open, you would see the cortex is about about a half inch thick. And it contains what are called interneurons. And interneurons are the cells responsible for processing information and making a decision. So humans have this dilemma. And that is a similar dilemma that other mammals have, but the more cortex you have, the more processing you can do. I'm just going to call that intelligence, for lack of a better term. The problem is, we are constrained. You can't have the... a brain grow forever. You can't be, you know, like this big with a brain because that would require a really strange look to the person, which would probably make their head too heavy. But more importantly, you got to fit all this through the birth canal as a neonate. And so evolution had a problem. And that constraint was how do we have a brain that can become quite large with growth and not kill mom in the process. Does everybody understand what I'm talking about here? And all males faces. The animal that has live birth has this problem. You've got two evolutionary forces that are sort of opposing. Select for an organism that can process and make better decisions because that's called survival. At the same time, live birth requires your head not to be so big. that you take mom out of the gene pool and yourself. So what a conundrum. So to solve that, we have a lot of folding, a lot of folding, folding, folding. We see folding when the brain develops, it sort of like folds in on itself. And then we go one step farther. and have the gyri and sulci emerge. And the more wrinkles you have, the more surface area you have, which means more cortex, which means more interneurons. Long-standing idea has been smarter animals, those that can solve problems, have more wrinkles. And so far, of all the mammals we've investigated, they're really not so bright ones, like the grizzly bear, not very bright, don't tell that I said that, all the way up to dolphins and porpoises. We see this trend hold true and it turns out animals that can solve problems they usually work in groups they have complex social behaviors a lot more wrinkles like humans ones that don't really work in groups don't solve problems like grizzly bears not so many wrinkles now let me just put a little plug in for grizzly bears i'm not anti-bear i think they're great but they solve their problems differently they have brute force they have more motor neurons than we do you don't need to be necessarily great at solving problems when you're just the biggest badass on the planet right you just go like like well I'm gonna take it I don't need to think about how I'm gonna take that in a very savvy way I'm just gonna do it and so we think that some animals it's not that they couldn't be smart they just didn't need to be and so anyway take from that what you will but long story short more surface area more interneurons more process so having a lot of foals answered that problem when our brain was constrained to this cranium of ox we really couldn't make much better So a little bit about brain and that. We move down to the spinal cord. The spinal cord in humans is not that long, especially in adult humans. If you look at a skeleton like in lab, you're like, well, that's a pretty long vertebral column. That whole thing must be full of spinal cord. It is not. The spinal cord ends in most people at the level of L1, L2. Have you studied that nomenclature of vertebrae in lab? Lumbar number one, lumbar number two, the small of your back. That's where. where the spinal cord technically ends. In the adult, the spinal cord is only about 18 inches long. It's not very long at all. So 18 inches long. And so a lot of that vertebral canal in humans does not technically contain a spinal cord. So I'll talk about that here in a moment. But when we look at the connection between brain and spinal cord, it should be seamless. So the foramen magnum is the hole in the skull through which the spinal cord So the spinal cord actually begins a little bit before the vertebrae do. And 18 inches long, it ends in an area called the conus medullaris. That's about the level of L1, L2, the vertebrae. So it differs a little bit by person, so I usually like to give a range. So I'll say L1, L2, lumbar 1, lumbar 2, how vertebrae are numbered. So at lumbar vertebrae 1 and 2. somewhere in there we find the end of the spinal cord and then it splays out into this really cool anatomical formation called the cauda equina which is Latin for a horse's tail and that's what it looks like that's what the early anatomist thought it was was a horse's tail based on what they thought things look like and so you see this spinal cord in and these really large nerves sort of emanate out from that splaying out like a horse's tail And these become the large nerves that would innervate lower areas of the body, distal areas of the body, like the limbs, for example. So 18 inches, not that much. But everything I've shown you up here so far, up until this, is fully encased in bone. Should be pretty well protected. But it comes at a price. Everything I've showed you so far, brain and spinal cord, is fully dependent upon the peripheral nervous system to provide it information. The brain doesn't even have nociceptors of its own. Nociceptors are pain receptors. It does not have nociceptors of its own. And so it's interesting, anything that happens in the body, even though the brain is what interprets it, it's not coming from an area of the brain that actually got that information directly. It's coming in from a different source. And then it rattles around in the brain and comes back out. So let's talk about these peripheral nervous system structures and how this is going to work with sensory input using... sensory neurons. as well as motor output so we can activate these effectors that we talked about. Questions so far before I continue? So far so good? All right, let's talk about sensory neurons. I'm just going to be drawing on this side. This sounds like a very obvious statement, but in the past when I've only done diagrams on one side, for some reason some students think there are no neurons on the other side. Please don't think that. I'm just out of room. We are bilaterally symmetrical. So whatever I do on this side would exist on this side. So if you need to make a note of that, that's fine. I just sometimes forget if you've not had this, maybe it does look a little... little strange the way I draw it. So I'm going to talk about sensory neurons and we're going to be talking about sensory neurons that bring in information from somewhere in the periphery of the body. That could be the skin surface. That could be muscle. We don't know. We just know it's outside of the central nervous system. And to get information in from much of the periphery, we need these strange looking neurons. These are called primary. sensory neurons. Primary sensory neurons. They are usually abbreviated with a one degree primary and then sensory neurons. So one degree first degree neurons that's a lot easier to write than primary sensory neurons. These have a strange shape. not expecting something like this to be presented as a neuron. This is truly what most of the sensory neurons in the peripheral body look like. They have a strange shape, but there's a reason for that, of course. But for right now, we're just going to stand at this area here. is the area that receives the information. It has specialized receptors, and it can pick up different types of stimuli, and then it would conduct that information to the spinal cord and then to the brain for interpretation. So primary sensory neurons, their job is to conduct information from the periphery of the body to somewhere to the surface there. So let's see if I can... We have a contrasting color. So we'll say this one detects information from the skin. So we'll make this sort of the surface of the body. Call this the skin. So anything that you are feeling right now with your hands, your pencil. your desk, you're eating something, you're drinking something, whatever you're picking up, the tactile receptors, they're going to look more or less like this. And they're going to go from the skin all the way as an entire cell to the spinal cord. deliver that information. Or we could do something a little different and say let's say you ate too much lunch and your gut is all distended. You're gonna have a very similar setup. Sensory neuron coming, understanding stretch or pressure in the gut and sending that information back to the CNS. So it's kind of a similar setup just to detect different types of stimuli. The other type of neuron, main type of neuron, that we'll see in the peripheral nervous system are motor neurons. And there's different types of motor neurons. I'm going to be using an example for skeletal muscle since that's something that's common. People understand skeletal muscle a little bit more than, say, smooth muscle. So to get something like a skeletal muscle cell to contract, we have to use a motor neuron and tell it, hey, now is your time to contract. So for that, we're going to see a neuron. exist, part of it anyway, maybe in the spinal cord. Here's its little soma or cell body. It will stretch all the way out in the periphery and it will activate skeletal muscle cells, which are striated cells. Usually draw them sort of tube-like with little striations. Most people seem to understand what I'm trying to get at here, which is a skeletal muscle cell. So in order to activate any effectors, we have to sort of use a reverse route from the central nervous system back out to the periphery. So it's kind of a circle, sensory information in, maybe it goes up to the brain for interpretation, and then it's got to come back down the spinal cord. and go out to the periphery to activate anti-effector. So these two systems, take-home message, CNS and PNS, work together all the time. And for as amazing as the central nervous system is, it is... only as good as the information it receives. It cannot act appropriately or correctly if it receives verbal information. So if you have disruption system-free input, maybe you've got nerve damage, maybe the information didn't get to the brain correctly or on time. The brain has a hard time understanding, what am I supposed to do with this information? So we can see that kind of play out using different examples. For example, people who Who are, for whatever reason, placed in solitary confinement. They don't have a lot of sensory input. That's part of how that's believed to work. Their brain will start imagining things because they don't see the outside world. They don't hear the outside world. They don't communicate very well. And so their brain will say, you know, I used to get a lot of sensory information and now I'm not getting very much. And so that starts the process of hallucinations. And so when we think about things we do to people, we don't always recognize at first sort of the long-term implications of that. Another example is phantom lymph pain. That bottle likes to make an appearance. Phantom lymph pain, thank you very much. Have we heard of phantom lymph pain? So phantom lymph pain, if you haven't heard of it, is when... person fully developed human loses a limb and so people that serve in the military unfortunately have too much of this so the problem with phantom limb pain this irretractable pain they get is that serves an arm. There are many sensory neurons that serve that arm. If part of that arm is missing, it's possible these primary sensory neurons are still existing in a truncated form. And they still, for whatever reason, get the idea that they should tell the brain there's pain in this limb. But that limb isn't there anymore. And so how do you tell the brain, hmm, that sensation is not real? You should ignore it. And that, really, you can imagine how incredibly disruptive that would be for that person's day. You can't fix the burn. sensation in the limb that's not there. So for a long time they tried a lot of different drug therapies, how do we maybe it was almost a sedative which is not a good quality of life long term. Very brilliant neuroscientists came up with the most elegant way to fix it and it cost less than a dollar. So that's good science. He knew how the brain and the spinal cord and how the peripheral nervous system really work together. And he would take veterans who were having this problem with PTSD as well as pain from limb pain. And he just simply said, stand in front of the spooling mirror, and he would do this several times a day for several weeks, and look at the limb that used to be there, or where the limb used to be, and just look at this missing limb. And that sounds cruel, but what he was doing, he thought, and it turned out to be true, he was rewiring the brain. To understand this is not real. This is not real. There is no limb there that could be causing you pain. It turned out that alone was more effective than any chemical therapy they could have given those people. And it was something they could do on their own at home. science and so when we think about helping people sometimes you have to really get down to the weeds obviously understand something for free elegant solution for people really have your attractable pain and there's no side effects of that, right? There's no chemical imbalances caused by that. So it's just an interesting thing. And that's just kind of two examples I could come up with at the top of my head about why you really need to be very familiar with how this works. So many different examples. But anyway, that's kind of a primer to where we're going with the nervous system. Before we go any farther, I want to introduce the next part here. We've seen this before, but it's worth revisiting. And that is our old friend. I like to say old friend. Maybe it's a new friend. This flow chart that we've seen before, the nervous system had its fingerprints all over it. All over it. What I want to do is remind you how many fingerprints of the nervous system are on this flow chart. So before I let you go for a break, not permanently, of course, but for a break, take a couple minutes here in a moment. We're going to be doing this. And so some people just like to kind of. of get ahead or think about what we're doing. Maybe they don't want to break so I'll sort of give you a primer. I'm going to be using this flow chart and I'm going to be using a specific example to have the nervous system could get involved. And this flow chart, complete the flow chart for homeostasis focusing on the nervous system, we're going to be using a very familiar setup here. In this case core body temperature is too low and I think that's appropriate because I think this room is really cold. It's been cold in here all day. So I'm going to say the core body temperature is too low. And in this case, I'm going to make it pretty low. You know, it should be 37 degrees C. Let's say you've been in Acker way long today, and it's now 35 degrees C, and you're flirting with hypothermia. So I'm going to give you a three-minute break. Come back at 3.03, 3.04, something like that. If you want to stay and you want to sort of challenge yourself, see if you can fill this up. Otherwise, I'll do it with you in a couple minutes. Thank you. The conus medullaris, where would that be? It's just the con at the end. Okay, so right before it goes to the... Yeah, so the conus medullaris is just right here. Okay, is there a specific function or is it just kind of the last... It's just where the spinal cord tapers. Okay. Yeah, so it would have the same function as the rest of the spinal cord, it just has a different shape. Okay, thank you. Good question. Sure. Great. Alright, I'm going to go ahead and get your attention back up here. Start talking about the homeostasis flow chart. And in this case, as we've seen before, we're going to talk about the variable of interest. And in this case, the variable of interest Core body temperature is too cold, too low. So we're gonna see core body temperature approaching, must be really crazy, approaching 35 degrees C. Remember, it should be 37 degrees. so 35 degrees C is pretty low so we're definitely gonna be setting off some alarms as far as the nervous system is concerned and we'll start to see how the central nervous system and peripheral nervous system work together to fix this we We hope quite quickly. So as we go through this, just keep in mind this is our goal, to indicate where and how these are involved. So I'm going to sort of zoom in on this now that we kind of see our goal. When we talk about, this is a variable of interest, too low. Let's talk about the type of receptor that would interpret this or be triggered at first. Do you remember its name? It detects temperature type of receptor. Thermoreceptor. Yeah, I heard a couple people say it. Thermo. receptor and in this case thermoreceptors are located in multiple areas we've got the skin the vertebral canal which I'm going to abbreviate like that vertebral canal liver and the hypothalamus And if the core body temperature is 35 degrees C, I would say all of these thermoreceptors are probably lighting up as though we have a huge problem, because indeed we do. So this arrow here is the problem is being detected by these thermoreceptors. Thermoreceptors exist at the very end or tips of primary sensory neurons. So to connect this back to things we've talked about, the primary sensory neuron that reaches out into the body, this thing right here, It would have thermoreceptors. If this was a neuron responsible for detecting temperature, it would have thermoreceptors in its little dendrites here. Some of those dendrites would be reaching into the skin. If this was a different thermoreceptor, some of those neurons would be reaching into the liver or even into the hypothalamus or staying really close to the spinal cord in the vertebral canal. So we have all sorts of sensory input. This represents a directional term. Do you remember what it was called? An action potential, but we gave it a direction. directional term. Afferent, good job. So this is an afferent action potential, which means it goes to the brain. And in this case, the brain, obviously we're moving towards the spinal cord and the brain. So at this point, we've got the peripheral nervous system, the sensory part, anyway, got its job done. The information goes to brain and spinal cord. because that's what makes up the central nervous system. And here we will see a really fast response. So we will amass that information and definitely respond. So let's think about responses. So let's talk about the motor side of the peripheral nervous system. The sensory side of a peripheral nervous system got its job done. Central nervous system, partly done, saying, yeah, we have a problem. Now let's talk about what it does. Who does it activate to take care of the problem? So in this case, we're going to see action potentials involved, and we also gave them a directional term. Do you remember what that was? Efferent. Good job. So these are going to be efferent action potentials, and efferent because they activate effectors. Efferent effectors. So we're going to see motor neurons, much like this one, be activated. They're going to come out. from the spinal cord and they're going to go out to effectors that can fix the job. So when we think about specific effectors that could fix this problem, what would you want to activate? Shivering. What effector is that going to require? Skeletal muscle. Yep, so definitely skeletal muscle. What else would we want to activate specific to this problem? Say what? Cardiac. Cardiac, yeah. You're going to see an increase in heart rate, so cardiac muscle. And cardiac muscle has to increase its heart rate because guess who feeds the blood to the shivering skeletal muscles? The heart. If you're going to feed more... blood to the skeletal muscles so they can shiver you're gonna have to increase that heart rate cardiac muscle we're also gonna see smooth muscle smooth muscles involved because we're gonna have to feed more blood to certain parts of the body body and restrict blood flow to other parts of the body that we deem not essential like the skin the ears the nose you notice those parts of your body get cold fastest that's because the brain says you know what I only have 5 liters of blood to deal with I can't perfuse or shunt blood to all those places at the same time so we'll see some places the smooth muscle cells around blood vessels leading to the skin or the ears the nose really gets tied off other places at smooth muscle relaxes and really allows a massive amount of blood flow to go to those areas because they're deemed more important. But you can only do that if you can constrict blood vessels and that requires smooth muscle. If everything goes well, we should return this back to, we're going to increase that temperature back to 37 degrees C. So we have a really familiar, I hope, example of how all this works together, but only if the central nervous system and peripheral nervous system are effective. Damage to any of this will drastically decrease your ability to respond to some sort of stimuli. And that's where it gets pretty dangerous pretty quickly. So kind of our introduction to the nervous system. Questions on this before I go a little bit more into the cell details of who's actually doing all the work here. Are the effectors part of the peripheral nervous system? So they are activated by the peripheral nervous system. So skeletal muscle, craniocerebral muscle, smooth muscle, parts of their own system that they rely on the peripheral nervous system to tell them, hey, do something. Good question. Other questions on this? All right. Well, let's move into a little bit of details about the cells. One's doing the work here. So we're going to talk about cells of the nervous system. And when we think about the nervous system, there are billions and trillions probably of neurons and glial cells involved in this. The number changes based on the paper you read. So I hate to give a formal number because people argue about the numbers all the time. seems kind of inconsequential, but anyway, there's a lot. There's a lot of neurons and glial cells in the nervous system. And together, they form a vast network and really their overall goal, whether it's sensing, determining, making a decision, making a motor output is super rapidly. between cells. That's what the nervous system is really known for. Exceptionally rapid communication. Remember B-slash-W is shorthand for between. Note-taking gets easier if you learn shorthand. This is an interesting thing I just want to highlight here for a moment because I hope we can get to the endocrine system before our next exam and this is a good time to sort of compare and contrast endocrine and nervous system. like a side note for a moment the endocrine system is also a system for communication that's what hormones do they're they're ligand the communicating chemical tells one cell do this because this other cell said to but it's slow it can take weeks if not months if not long for the endocrine system to do its job and that's by design sometimes we don't want a fast response we want a very carefully formulated regulated response but sometimes we need to be fast right we don't have weeks or months to wait for the nervous system to detect it environmental stimulus and fix it. That would be ridiculous. So we have two communication systems in the body, endocrine and nervous. They are very different, but at the end of the day, they're both for communication. She said this is meant to be exceptionally rapid. And so if you're going to be a rapid system, you better have a really tight communication network that allows you to tell cells what needs to be done. And in order to get that network of communication to work, work we really rely on two different types of cells that are part of the nervous system. One of them you probably expected me to talk about neurons. These are really the functional cells or you could almost call them the functional units of the nervous system. functional cells or units of the nervous system. And as we saw, neurons specialize in a unique role. They are either going to be great at detecting a stimulus of some sort, internal or external. external, they're either going to be good at that or integrating this information, amassing, collecting it, making a decision, or activating an effector. But they can't do it all. They have to be sort of one of these three categories. And they can do this, this detecting or integrating or activating, because they are capable of producing and conducting electrical impulses called action potentials. And that's the rapid form of communication that's required. From here on out, I abbreviate action potentials, APs. You probably already knew that, but again, ambiguous abbreviations are just not helpful. So this really sets neurons apart from the other type of cell we see as part of the nervous system, which are glial cells. Glial cells. Glial cells have a contentious history. First of all, we didn't even know they existed. We just thought it was neurons. And then the early anatomists found them and said, well, we don't really know what they do, but they're all kind of like this. ambiguous milky white color it must be some form of laugh if you want glue they thought glial cells glial means glue in latin helped glue these neurons in place sealing them in their location making sure they could communicate well glial cells the glue cells, but that's only because we didn't really know what they did, which is fine. I mean, imagine the mistakes we make now in science at a hundred years from now, people will be like, you fools, right? Like, how did you let that happen? Anyway, glial cells are the supportive cells. Glial cells sustain neurons in every possible way. Let me get that to focus. Glial cells sustain neurons in so many different ways. Without glial cells... Neurons do not live, they cannot communicate, they cannot heal themselves, they are pretty much done at that point. Glioblastos, the same neurons in multiple ways. However, glial cells are not capable of generating action potentials. Their job is not communication. They just support the cells that do the communication and that's a big enough job. In fact, it takes multiple types of glial cells supporting a neuron. Neurons require a support staff. They don't have a lot of patience. They need what they need and they need it now or they're not going to make it very long. So glial cells, absolutely crucial. Because of that, we believe, this is why, glial cells outnumber neurons in total. And the reason why I say likely... Because nobody really knows the ratio. It used to be 10 glial cells to neurons and then it was like no no that's way too much. Then it's five to one and now they're like one to one. So again these numbers depends on how you measure, who's looking at what organ. Anyway, let's go with the safe route and say glial cells likely outnumber neurons in total. That would be especially true in the brain. Less true in the peripheral nervous system. Glial cells, I could also say at least as numerous as neurons. As I mentioned, different glial cells exist, and I will talk about their details in a subsequent lecture. look ahead, not the next drawing, but the drawing after, I talk about the differences in glial cells. So the reason I talk about glial cells so much is because their importance in helping overcome traumatic brain injuries is becoming more and more important. So they used to think traumatic brain injuries like concussions, for example, so people get playing football, it's a great time to talk about this because it's football season, or any high-impact sport, riding bikes, riding horses, whatever is going to make you more likely to hit your head. They used to think traumatic brain injuries were a problem with neurons. They got jolted, they had a hard time sort of coming back online, and if we could fix the neurons, then the problem would likely not get too worse for the person. We now know it's really not the neurons at all that are having trouble, it's the glial cells. And so they've actually taken the information and started designing helmets a little bit differently with the end goal of protecting these in addition to protecting those. It turns out glial cells, if they are lost during a traumatic brain injury, the neurons, even if they are fine, they usually don't last either. So you have to support the supportive cells in order to get the neurons to live. So it's really changing how helmets are designed. And I think that's really important. I think in the next 10 or 15 years, you will see very different helmets. Probably won't look any different internally or externally, but the technology inside is really going to be aimed at preventing damage here to help preserve brain function and reduce traumatic brain injury. So a lot about real cells that we just didn't think about for the longest time. So kind of a question down here. I think I can fit it all in, but if you've got it printed, you'll have it. In the human body, fully formed adult body. we'll find quite a few intervertebral discs. So these are fibrocartilage discs. They don't compress. They're not meant to compress. They need to withstand the gravity and the concussive forces that the spine gets every day. So that's in between each vertebra, 25 discs. These discs permit flexibility. There we go. Flexibility of the spine and act like shock absorbers. And they prevent vertebrae from growing. grinding together. As you can imagine, that's bad. If you have a slipped disc, it allows the disc to sort of crush in on one side and pinch the nerves that come out. This is not an uncommon thing in people, especially in an aging population. You get arthritis of the spine, you get bulging discs, you get slipped discs, no shortage of problems there. But what happens is when those discs move, um, the vertebrae can grind together. Interestingly, why that happens, why these discs... are kind of fragile is because the center contains a special area called the nucleus propulsus, and that's really responsible for much of the shock-absorbing functions. This nucleus propulsus, this internal sort of jelly-like area of each disc, is a remnant of the notochord, and so the notochord formed really early during fetal development, and I'm not sure if I got to talk to you about the sort of trajectory of human development. I might have had to club that out. So I'm just going to go ahead and talk about this with you. This remnant of the notochord means it formed during week three of this right here is embryonic development. And so when we think about the spinal cord being fully encased in bone, That bone is also reliant upon other structures to keep the vertebrae aligned, stacked appropriately, not compressed on each other, and that allows the spinal cord and the neurons extending from that cord not to be compressed. because compressed neurons cause a lot of pain. And so when people think about these slipped discs, these ruptured discs, herniated discs, things you've probably heard about, either you know someone or heard about it, just think about the fact that that... structural integrity is reliant upon a structure that developed during week three of embryonic development. That means there was conception and three weeks later this form, this structure formed. That's a really long time for that to last. So it doesn't really come as much as a surprise for these things to fail. So when we think about the central nervous system, peripheral nervous system, we always have to think about the structural relationship between that spinal cord and all the vertebrae that support it. because remember we don't just look at the body system by system we have to give an eye towards they are all reliant upon each other which is why I like to follow the skeletal system that we talked about last week with the nervous system so you have some idea of all the osseous processes in the vertebrae that are helping or hurting this particular structure the spinal cord but we're going to move on to structure and function of neurons so this was sort of our introduction to the nervous system. We talked about central nervous system, peripheral nervous system, some is encased in bone, some is not. We talked about how they work together and then we came down here and sort of talked about broadly two types of cells in the nervous system. What we'll do in the next piece of paper is really just distill it down and function or focus solely on neurons, how they're structured and what they do. So the neuron you see here before we go, this is is called a multipolar neuron, and this is a very common, very common form. It's one that I use a lot of to make sort of points and notes here. This is a multipolar neuron. I'll talk about why it's called that, but many neurons that we talked about in the central nervous system and in the peripheral nervous system are structured like this. Neurons can be quite long, quite long, and so we'll see that they have different zones or regions that specialize in different functions. functions. So that's where we're headed. Questions on anything you see here that you'd like clarification on? Or if you'd like something repeated, let me know. Otherwise, we'll move on to our next set of notes. Any questions on this? Yes, ma'am. Yeah, so remember, CNS is only brain and spinal cord. So this was the whole function of the CNS right there. Otherwise, we saw, I can sort of like highlight it, and that might kind of answer this. You know, this whole part here, this is all. the PNS, the sensory division. So all of this relies on things like primary sensory neurons to bring that information in. And then when we look at the central nervous system in this particular set of of diagrams, we can sort of highlight this and say, well, this was an important role, but that's really the main role of the CNS. So, you know, we think about like how cool the brain is and the spinal cord, but realize its role here is just a part of something much broader. And then when we looked at sort of the end part here, we can kind of highlight this and say, all this over here, this was the job of the motor part, the motor division of the peripheral nervous system. So this is a good question. I'm glad that you asked. I bet you're not the only one with those questions. So I'm just going to say the role of the CNS, I highlighted that in pink. The role of the PNS, we talked about the sensory part, which I did in green. And then I talked about the motor part, which I did in red. So sometimes you can just sort of circle and highlight and say this was the role. Good question. Thank you for asking that. Other questions before I continue? Alright, we're going to talk about neurons and I'm going to use this sort of as my example for many neurons. It's not the only type of neuron that you have, but it's a really common one, so it makes sense. And we're going to talk about the structural features of these very complicated cells. And not only are they complicated in function, they have some other strange features about them that we don't see in other cells. For example, they are amyototic. What does that mean, amyototic? They can't divide. Just think about that challenge for a moment. You have some of the most crucial cells in your body, neurons, they can't divide. So if you lose them, they don't come back. And that's a problem for a cell that started developing during the embryonic stage of fetal development and has to last, some people, 100 years. That's a big ask for a cell. Most cells in the body can be replaced if necessary, but not neurons. And I'll sort of explain why that is as we go along here. They are big and they are complicated and they have to weave through a lot of soft tissue as well as bone to get to their target. And if that's the... damaged, you can't really convince another cell to push its way through tissue that already exists. It just doesn't work. So we have to, hopefully, if damaged, because we cannot rely on them to divide and produce more, we hope to repair them. But naturally, biologically, that is limited, which is why permanent nerve damage due to some sort of trauma or accident or virus is a problem because we can't fix it because of how these neurons are. kind of designed. So kind of right off the bat, hopefully you're starting to think about sort of applications to your broader career field when it comes to this information. We're going to do a couple things on this. I'm going to introduce the structural features of a typical multipolar neuron. Again, I kind of reminded you of the structure I'm talking about here. I think most people have seen this. This is probably not too unfamiliar to you. We'll talk about the basic structure and features of each component, and then I'll sort of draw it out. sort of sketch what I mean by this. So we're going to first start with what is truly kind of the reference point of the cell and that's called a soma and soma is the cell body. Soma is the cell body. It contains the nucleus in most of the organelles that you would expect to find in any cell. I mean at the end of the day neurons are still living cells. They have all the needs that any other living cell would have. They need nutrients, they need waste removal, they need to produce proteins and lipids, so there's still a cell and it's easy to forget that because they have so many capabilities. But the soma or cell body is where we'll find most organelles. Some are distributed throughout other parts of the neuron. The soma also serves as a reference point. So that is true when I'm talking about projection that come out of the soma, as well as directional terms. So if it's going away from the soma and through the axon towards the axon terminal, we call that anterograde. If it's coming from the axon terminal back up the axon to the soma, we call that retrograde. So I'll explain all that more in a little bit, but I just want to be clear that the reason the soma is so important is because it really is the anatomical and physiological reference point. So before I sketch... that out I want to introduce another very closely associated structure or set of structures. These are called dendrites and these are fibrous. They look in many neurons like hair coming out of the soma. I mean they're just so numerous and usually referred to as a dendritic tree because they have a lot of branches. So fibrous process or dendritic tree. I think I have enough room to write the word church. tree in there. And dendrites are really great. Again, they extend out of the soma. Numerous, but they have a lot of surface area. And that's great because they contain, the dendrites contain, the specialized receptor. that detect all that environmental stimuli. So when we talked about thermoreceptors, those would be part of the dendrites that are located on a neuron. So we're really talking about parts among parts among cells. So dendrites and soma usually go together. Dendrites and soma usually go together. So I'm going to take a moment to kind of sketch this out. I'll sketch it in a little more detail than what I provided in some of those line diagrams because I think it's important to really understand what are we dealing with here. So what is it about soma and dendrites that are so important? So I'm going to take the top half of this and kind of explain what I mean. I'll be sort of going through these bulleted points. So the soma, I'm going to draw fairly large because we have to put some things in there that's pretty important. So here's our soma. It has a phospholipid bilayer like any other cell. And extending out from this cell body are numerous dendrites and I'm going to draw them fairly exaggerated, sort of broad and flat so we could talk about what they do and what would be on their surface. Again, surface. area is really a big function of the dendrites and the more dendrites you have you can even have dendrites upon dendrites the more sensitive this neuron would be to incoming stimuli. So however you want to draw it, it does not at all need to look just like these. I'm just trying to draw room here where we could add some additional features to this very important cell structure. So flat and broad. So we have a decent dendritic tree. Again, all of these extensions here are called dendrites. Collectively dendritic tree. In the soma is where we find a lot of really important structures. I'm going to draw in a nucleus. Because we have to have a lot of proteins made, neurons make a lot of proteins. So when we think about nucleus and protein production, we're going to see also extending from that nucleus a few things we've already talked about in this class when it comes to organelles. What do you think this is? Extends from the nuclear membrane, has ribosomes attached. RefER, good job. RefER. Now, in a neuron, the RefER is so extensive, it looks like what are described as rosettes. So it doesn't just extend from one surface of the nucleus. It's going to extend from all parts of that nucleus. So this entire soma sometimes looks like it is just absolutely chock full of rough ER. And in neurons, Neurons, and as far as I know, only neurons, we give this very extensive rough ER a new name. It's called a nistle body. And nistle, after the scientists that discovered how extensive these are, N-I-S-S-L, nistle body. That is the name of the rough ER in a neuron. Neurons have some of the most extensive, well-developed rosettes of rough ER found in any cell. and that's really an example of a cell that makes a lot of protein and secretes that protein. Some of that protein would be neurotransmitters, which have to be produced and leave the cell. So, nissl body, n-i-s-s-l. We also find in this Golgi, when you think about cell structures we've already talked about, Golgi bodies, sort of these flattened stacks called cisternae, and that's important for protein production, so we'll find Golgi. We'll find a lot of mitochondria, mitochondria, of course, producing ATP, and we find mitochondria, this is one of those cells that's not just in the soma. We can find mitochondria in the axon and axon terminals. Reason being, ATP is not a very long-lived molecule. It's used... It's estimated within about 20 seconds of production, and that's pretty liberal. Usually it's used the minute it's produced. And so because these cells are really long, think about a motor neuron whose soma lives in the spinal cord and whose axoness extends all the way out to your foot. That's a neuron, a cell that could be two feet long. And so you need to produce ATP in multiple locations. So the mitochondria are one of the organelles that we find. not just in the soma, but also the axon and axon terminal. So just a few things about the neuron. I'm going to sort of stop with my description there when it comes to soma. Let's talk about dendrites. Dendrites are really important for detecting specific environmental stimuli. Some dendrites, though, are really great at detecting neurotransmitters or chemicals. So the way they do that, whether it's chemicals, pressure, anything that is a stimulus, they have some specialized... features on the very surface of these dendrites and these dots are just areas or locations of where we may find specialized receptors. And receptors are there to detect stimuli and neurons specialize in the detection of certain stimuli. So you can't have, for example, a neuron that understands both temperature and pressure. That would be ridiculous because how does the brain know what that neuron means? The brain says, did you mean it was too hot or you got crushed? The neuron is like, yes. So you can't have that. You have to be very specific in your communication. So receptors are very specific for a certain type of stimulus. So that way the brain knows, because remember, the brain can't detect it for itself. It's reliant upon very clear communication for getting that information and knowing what to do about it. So that's a little bit about our neuron. We have two more components to talk about. Questions before I continue. Questions about axon or not axon, soma and dendrites. We're going to the axon, but we haven't got there yet. Soma or dendrites. So far, so good then. Let's talk about other components we will find as part of a neuron. We're going to see extending from the soma something called an axon. This is a very long fairly thick diameter, and I mean thick, I mean the diameter is fairly big, it's usually called a fiber because it is pretty substantial as far as structure. So a long, thick fiber extending from the soma, again the soma serves as our reference point, there is only ever one axon per neuron. Only ever one axon per neuron. While we may have many dendrites, there is only one axon. In all neurons ever investigated, this is proven to be true. The function of an axon is to conduct action potentials. And then we'll get to the axon terminals. So from this soma, we see... In most neurons, an axon coming straight out. And this axon, I'm not going to draw it all right now, but it's long, it's fibrous, it leaves the soma, and it will be conducting or carrying going into... internally right inside the membrane, action potentials. So action potentials move along the inside of that axon's membrane. And then we will encounter the ending of this neuron, which is called axon terminals. Axon terminals look like swellings that branch out from this axon. They are sometimes called terminal boutons. It's French, very fancy. You can impress your friends and family. Or we could just call them terminals, which is what I use. They look like swellings or knobs extending from the end of this axon. So they branch directly from the axon and there could be numerous terminals that exist. So while we only have one axon, at the very end it could really splay out and form multiple axon terminals. Their function is to release neurotransmitters, or abbreviated NT, a lot easier to write. So what you'll see me do is I'm going to extend this axon a little bit and we're going to see it branch into numerous terminals and I'll sort of make a note about how these are released and where they go. So I'm coming back over here to the drawing. I'm going to extend end this. Axons can be pretty wavy, they can take fairly torturous routes, and then at the end they will eventually terminate into axon terminals, which I am drawing a little bit broad so you can sort of see their detail. So here's our axon terminal. swellings at the end of the axon. So again this would be the axon. Long, fibrous, thick diameter. And the axon terminals are an interesting area. Here we see the release of neurotransmitters. So these dots are neurotransmitters being released from these axon terminals. NT are released here. And these neurotransmitters, when they're released, they have to diffuse across a space called a synapse. synapse or synaptic cleft, both of those are correct, and they will hit their target cell, or we sure hope. And that target cell could be another neuron or it could be an effector like skeletal muscle. So I'm going to draw the proverbial skeletal muscle cell. Looks sort of long and fibrous, striated. So here's an example of an effector. I'm gonna make this skeletal muscle and that skeletal muscle has receptors. for those particular neurotransmitters. And so when those neurotransmitters hit that receptor, and yes, it usually takes two, but it's nice because it makes a smiley face. And who doesn't like that? Looking at you at science. get that target cell to receive those neurotransmitters, it will cause contraction of the skeletal muscle. So this would be an example of a motor neuron coming out of the spinal cord, going into the periphery to get an effect on my skeletal muscle. So in this case, we've kind of come full circle from what I talked about on my previous drawing. Just kind of adding details as we go along. So when we look at an axon and an axon terminal, they do not look as detailed structurally. They look like this. They don't look super impressive, but really, honestly, this is where so much of that physiology happens. I made a mention at the very beginning up here that repair is possible but limited. And this is a good time and sort of area to talk about what I mean by possible but limited. We know from experiments on neurons that if a neuron is severed, let me see, I'll just draw like a line. on is cut or severed really near the soma let's say for whatever reason the trauma happened here and the entire axon and axon terminals were lost this will likely not live it's too it's missing too much however if the damage occurred down here and you lost a little bit of the axon and some axon terminals this has a pretty good chance of doing some re-sprouting it will regenerate a little bit of that that axon. It has the ability to regenerate axon terminals and if that can happen soon, fast, with medical help of course, you can preserve or re-establish the connection between that neuron and its effector and that is crucial for maintaining function and quality of life. But that's a big if. So you know when someone has neuron damage you have to understand how this repairs. if it repairs, depends on a lot of factors. One being, where did that damage happen? And two, how quickly were you able to stabilize the synapse and maintain support of the effectors? Because it turns out, effectors like skeletal muscle, they don't just rely on the neuron for information about when they contract. It turns out skeletal muscles get something interesting from their neurons that actually helps keep them alive. And that's not something that we typically thought of for a long time. But. If you've ever seen someone with neuron damage and they had skeletal muscle atrophy, part of that is due to the fact that the skeletal muscle itself is still alive, but without this constant communication from its neuron, it won't survive. And so we're not sure what they're getting from that neuron. They're getting this. Anyway, I'll stop there for today. I'll see you on Thursday. Let me know if you have any questions.

So we'll be spending some time talking about the nervous system in this module, sort of the background components of it. Really, in this class, I talk mostly about... the structural details of the nervous system and then in human body 2 we go into more of the electrophysiology like action potentials neurotransmitters a little bit of pharmacology but in this class really just focusing on the overall structure of the nervous system and its functions so we're going to get right into that starting with kind of just an overview or an introduction to the nervous system and some of this will look pretty familiar to things we've already done because we're going to start with a flow chart. If you have the notes, you know exactly what I'm talking about.

A flow chart that looks pretty familiar to things and that's because that chart really explained a lot of different things that we talk about in this class. So nervous system introduction. We're going to do a couple things on this first part here.

We're going to talk about sort of the main components of the nervous system and then I will diagram them out using that wire line diagram you see on the right hand side and then we'll see how all these components fit together to help coordinate the body. So when we think about the nervous system, really three main functions to the nervous system starting very broadly, we're going to see that certain neurons help sense or detect the environment and when I talk about the environment you think environment, you think like external environment, you know, temperature outside or amount of light being received, but it turns out the nervous system also detects the internal environment of the body. So the nervous system, as far as sensing and detecting here, would be responsible for understanding things like blood pH or blood pressure or the composition of gases in the blood, for example.

So there's lots of different things we can detect. So when I put in parentheses internal... and external, I'm referring to the environments, and the nervous system has to detect all of that in real time, and that's a lot of information coming in in real time. If you think about all the different things that the body has to detect inside and out, that's a lot of stuff.

So that's one thing. The second thing that we're going to talk about is that the nervous system will collect or amass, some people have not seen the word amass before. I like to use it on exams, but then I realized... People are not too sure about that. So amass just means collect sensory information.

So all the sensory information, internal and external, that's a big load, comes in through the central nervous system. And that will have to be collected and accumulated. And then some sort of decision is going to have to be formulated.

So the nervous system gets all this information and then decides, what do I do with it at this point? Do I do something or do I do nothing? In some cases that is true. And then we'll see that if a decision needs to be carried out, something needs to change or move, it is also the nervous system's job to activate effectors.

Can you remind me what are the types of effectors we find in the human body? What type of effectors might we see? Effectors.

These are anatomical structures that are activated by a neuron. That's right. Glands is one. Good. Glands, and there's a lot of different glands.

What else? Muscle. Yeah, and so when we think muscle, I'm just going to write muscle.

Remember, there's different types of muscle, so don't always assume skeletal muscle. There are other types, so smooth muscle, cardiac, and then, of course, skeletal. So you've seen this before but I think it's good to sort of remind you and help you apply it to a new situation which is really taking this information that we've already talked about in one context and then applying it to a different context. which is the nervous system.

So as we move through sort of this overview, you will see me catching my paper up here. You will see me refer to really two main parts of the nervous system. One of them is the central nervous system. And the central nervous system is usually abbreviated CNS but abbreviations aren't too helpful if you're not quite sure what they mean so central nervous system or CNS we talk about two things with this location and in doing so also talk about function. So when we think about the nervous system, the central nervous system, anatomically it sounds really straightforward.

So for example, the central nervous system anatomically only consist of the brain and spinal cord. That's it. Brain and spinal cord.

If it is anything other than brain and spinal cord, it cannot be central nervous system. So that sounds pretty straightforward, but then we get into sort of this gray area, like cranial nerves. So cranial nerves, turns out, most of them are not central nervous system. They're peripheral nervous system. So we can sort of understand this at a very superficial level saying brain and spinal cord, but we need to be very comfortable about what that also entails.

Do not be eluded or tempted by a word like cranial nerve and think, well, cranial head, that's got to be central nervous system. The vast majority of cranial nerves are actually peripheral nervous system, not central nervous system. So when we think about brain and spinal cord, their big function, if we had to wrap this up into just a small description.

What do they do? They amass information like we saw in item two. They amass info, all that sensory info, and make a decision.

That is the job of the brain and spinal cord. Take in all this information and then decide what are we going to do about it, if anything. And that's a big job.

But we have other functions, right? We still have one and three. And that turns out to be the job of the peripheral nervous system. The peripheral nervous system, or PNS, a lot easier to write.

Peripheral nervous system. Again, we could use sort of a straightforward definition, which is fine if we're just learning, and talk about what is this thing anatomically that we call the peripheral nervous system. And we could start with the location detail. Location, it is any nervous tissue outside of the CNS. In other words, if it's not brain and spinal cord, it's peripheral nervous system.

But then again, you get into this, you know, like, well, but what about cranial nerves? And it turns out peripheral nervous system for the vast majority of us. So the peripheral nervous system does a lot of work, but what it does not do is collect information and make a decision.

It cannot do that. It does not have the right cells for that. But it can sense and detect internal and external environmental information and take that to the central nervous system.

And it carries out the decisions of the central nervous system by having neurons that go all the way out in the body and activate effectors, glands, or the three types of muscles. So the peripheral... nervous system does a lot.

While we're here talking about sort of the overall anatomy of these two systems, let me make also a distinction that will be important probably later in your career when you're working like in a clinical setting. The central nervous system is encased in bone. And so maybe that helps you, you know, the cranial vault or the vertebrae, like I showed you last week when we were talking bones.

This is why I like to have the nervous system follow the skeletal system. They have a pretty decent relationship. The peripheral nervous system is not well protected.

It is not encased in bone. It is more vulnerable to damage. So we'll see some sort of like add-on features that we have gained over the millennia to help protect nerves which are a big part of the peripheral nervous system. So it's funny how these two systems work together and yet they are in many ways very different, very different as far as location, how they function, the type of cells they contain, what type of level of protection they get. So before we go really any farther, I want to come over here and talk about sort of an introduction to the central nervous system as well as a peripheral nervous system since I kind of mentioned how they work together but how they are also individual.

So if you have this printed off, you see... A very simple line diagram wire. I'm going to add some details onto this, of course.

You're looking at, and here's some terms that I want to add. You're going to hear me use them in this class as well as in human body. too. I want to talk about ventral versus dorsal.

So we usually want to talk about anatomical terms for bipeds like humans. We use anterior and posterior. You're familiar with these, but for some reason in the nervous system, we don't really use that.

We use instead dorsal and ventral. And my understanding is that the anatomical terms that apply during fetal development for some reason stick with the nervous system, even when you're an adult. So I'm going to introduce those terms to you and help you make the connection between our more familiar anterior. posterior, as well as dorsal and ventral.

The reason I mention this is when we get to the bottom of this page, you will see a cross-section of a spinal cord, and it turns out the anatomical regions of a spinal cord also contain dorsal and ventral. So it's good to just get used to them now, so when you see them in smaller anatomical features or organs, you're like, oh, that's what that means. So dorsal and ventral are things you'll see. So if you'll notice at the top, I right, and I can't move this down a little bit, but I have dorsal aspect already written for you. And so dorsal refers to the back, the spine, for example.

So dorsal in this case would be similar to your more familiar posterior, but we don't use posterior, anterior, we use dorsal and ventral. Ventral would be the belly aspect, the underside part, dorsal is the back. So on this um fairly simple diagram. What I just want to introduce to you is kind of the physical relationship between the central nervous system and the peripheral nervous system and how they are both required in order to get a person to be, you know, truly fully functional and how issues in one can really cause issues elsewhere. So first of all, let's talk about the things that I'm going to have you label and we talk about the central nervous system.

So I'm going to zoom in just to that. Thank you. brain and spinal cord area here as much as I can. When we talk about the brain, there's a few anatomical regions here that you're going to hear me talk about.

And so I thought if you haven't heard about them before, this would be a good time to sort of just mention them. So that way you're not kind of lost in the vocabulary. The nervous system is full of kind of complex vocabulary that a lot of people aren't too comfortable with.

So first of all, when it comes to the brain, so I'm going to draw a line right here. The first three lines here are really about the brain. So first of all, we see a lot of wrinkles, and these wrinkles are due to these sort of protruding areas. They sort of look like not mountainous areas, but they are very sort of bubbly looking areas. These are called gyri.

And in the middle here we have, I'll just sort of outline it here, we have little valleys and those are called sulci and these are plural, cingulus gyrus or sulcus. So gyri and sulci together, what they do, I'm going to put a little bracket here, is they increase surface area. So SA for surface area.

And increasing the surface area here is super important because the more surface area you have, the more room you have for cortex. And cortex would just be the kind of the, if you cut this open, you would see the cortex is about about a half inch thick. And it contains what are called interneurons.

And interneurons are the cells responsible for processing information and making a decision. So humans have this dilemma. And that is a similar dilemma that other mammals have, but the more cortex you have, the more processing you can do.

I'm just going to call that intelligence, for lack of a better term. The problem is, we are constrained. You can't have the... a brain grow forever. You can't be, you know, like this big with a brain because that would require a really strange look to the person, which would probably make their head too heavy.

But more importantly, you got to fit all this through the birth canal as a neonate. And so evolution had a problem. And that constraint was how do we have a brain that can become quite large with growth and not kill mom in the process.

Does everybody understand what I'm talking about here? And all males faces. The animal that has live birth has this problem. You've got two evolutionary forces that are sort of opposing.

Select for an organism that can process and make better decisions because that's called survival. At the same time, live birth requires your head not to be so big. that you take mom out of the gene pool and yourself.

So what a conundrum. So to solve that, we have a lot of folding, a lot of folding, folding, folding. We see folding when the brain develops, it sort of like folds in on itself. And then we go one step farther. and have the gyri and sulci emerge.

And the more wrinkles you have, the more surface area you have, which means more cortex, which means more interneurons. Long-standing idea has been smarter animals, those that can solve problems, have more wrinkles. And so far, of all the mammals we've investigated, they're really not so bright ones, like the grizzly bear, not very bright, don't tell that I said that, all the way up to dolphins and porpoises. We see this trend hold true and it turns out animals that can solve problems they usually work in groups they have complex social behaviors a lot more wrinkles like humans ones that don't really work in groups don't solve problems like grizzly bears not so many wrinkles now let me just put a little plug in for grizzly bears i'm not anti-bear i think they're great but they solve their problems differently they have brute force they have more motor neurons than we do you don't need to be necessarily great at solving problems when you're just the biggest badass on the planet right you just go like like well I'm gonna take it I don't need to think about how I'm gonna take that in a very savvy way I'm just gonna do it and so we think that some animals it's not that they couldn't be smart they just didn't need to be and so anyway take from that what you will but long story short more surface area more interneurons more process so having a lot of foals answered that problem when our brain was constrained to this cranium of ox we really couldn't make much better So a little bit about brain and that. We move down to the spinal cord.

The spinal cord in humans is not that long, especially in adult humans. If you look at a skeleton like in lab, you're like, well, that's a pretty long vertebral column. That whole thing must be full of spinal cord. It is not.

The spinal cord ends in most people at the level of L1, L2. Have you studied that nomenclature of vertebrae in lab? Lumbar number one, lumbar number two, the small of your back.

That's where. where the spinal cord technically ends. In the adult, the spinal cord is only about 18 inches long. It's not very long at all.

So 18 inches long. And so a lot of that vertebral canal in humans does not technically contain a spinal cord. So I'll talk about that here in a moment. But when we look at the connection between brain and spinal cord, it should be seamless.

So the foramen magnum is the hole in the skull through which the spinal cord So the spinal cord actually begins a little bit before the vertebrae do. And 18 inches long, it ends in an area called the conus medullaris. That's about the level of L1, L2, the vertebrae.

So it differs a little bit by person, so I usually like to give a range. So I'll say L1, L2, lumbar 1, lumbar 2, how vertebrae are numbered. So at lumbar vertebrae 1 and 2. somewhere in there we find the end of the spinal cord and then it splays out into this really cool anatomical formation called the cauda equina which is Latin for a horse's tail and that's what it looks like that's what the early anatomist thought it was was a horse's tail based on what they thought things look like and so you see this spinal cord in and these really large nerves sort of emanate out from that splaying out like a horse's tail And these become the large nerves that would innervate lower areas of the body, distal areas of the body, like the limbs, for example. So 18 inches, not that much.

But everything I've shown you up here so far, up until this, is fully encased in bone. Should be pretty well protected. But it comes at a price.

Everything I've showed you so far, brain and spinal cord, is fully dependent upon the peripheral nervous system to provide it information. The brain doesn't even have nociceptors of its own. Nociceptors are pain receptors. It does not have nociceptors of its own.

And so it's interesting, anything that happens in the body, even though the brain is what interprets it, it's not coming from an area of the brain that actually got that information directly. It's coming in from a different source. And then it rattles around in the brain and comes back out. So let's talk about these peripheral nervous system structures and how this is going to work with sensory input using...

sensory neurons. as well as motor output so we can activate these effectors that we talked about. Questions so far before I continue?

So far so good? All right, let's talk about sensory neurons. I'm just going to be drawing on this side. This sounds like a very obvious statement, but in the past when I've only done diagrams on one side, for some reason some students think there are no neurons on the other side.

Please don't think that. I'm just out of room. We are bilaterally symmetrical. So whatever I do on this side would exist on this side. So if you need to make a note of that, that's fine.

I just sometimes forget if you've not had this, maybe it does look a little... little strange the way I draw it. So I'm going to talk about sensory neurons and we're going to be talking about sensory neurons that bring in information from somewhere in the periphery of the body.

That could be the skin surface. That could be muscle. We don't know. We just know it's outside of the central nervous system.

And to get information in from much of the periphery, we need these strange looking neurons. These are called primary. sensory neurons.

Primary sensory neurons. They are usually abbreviated with a one degree primary and then sensory neurons. So one degree first degree neurons that's a lot easier to write than primary sensory neurons.

These have a strange shape. not expecting something like this to be presented as a neuron. This is truly what most of the sensory neurons in the peripheral body look like.

They have a strange shape, but there's a reason for that, of course. But for right now, we're just going to stand at this area here. is the area that receives the information.

It has specialized receptors, and it can pick up different types of stimuli, and then it would conduct that information to the spinal cord and then to the brain for interpretation. So primary sensory neurons, their job is to conduct information from the periphery of the body to somewhere to the surface there. So let's see if I can...

We have a contrasting color. So we'll say this one detects information from the skin. So we'll make this sort of the surface of the body.

Call this the skin. So anything that you are feeling right now with your hands, your pencil. your desk, you're eating something, you're drinking something, whatever you're picking up, the tactile receptors, they're going to look more or less like this. And they're going to go from the skin all the way as an entire cell to the spinal cord. deliver that information.

Or we could do something a little different and say let's say you ate too much lunch and your gut is all distended. You're gonna have a very similar setup. Sensory neuron coming, understanding stretch or pressure in the gut and sending that information back to the CNS. So it's kind of a similar setup just to detect different types of stimuli.

The other type of neuron, main type of neuron, that we'll see in the peripheral nervous system are motor neurons. And there's different types of motor neurons. I'm going to be using an example for skeletal muscle since that's something that's common. People understand skeletal muscle a little bit more than, say, smooth muscle.

So to get something like a skeletal muscle cell to contract, we have to use a motor neuron and tell it, hey, now is your time to contract. So for that, we're going to see a neuron. exist, part of it anyway, maybe in the spinal cord. Here's its little soma or cell body. It will stretch all the way out in the periphery and it will activate skeletal muscle cells, which are striated cells.

Usually draw them sort of tube-like with little striations. Most people seem to understand what I'm trying to get at here, which is a skeletal muscle cell. So in order to activate any effectors, we have to sort of use a reverse route from the central nervous system back out to the periphery. So it's kind of a circle, sensory information in, maybe it goes up to the brain for interpretation, and then it's got to come back down the spinal cord.

and go out to the periphery to activate anti-effector. So these two systems, take-home message, CNS and PNS, work together all the time. And for as amazing as the central nervous system is, it is... only as good as the information it receives. It cannot act appropriately or correctly if it receives verbal information.

So if you have disruption system-free input, maybe you've got nerve damage, maybe the information didn't get to the brain correctly or on time. The brain has a hard time understanding, what am I supposed to do with this information? So we can see that kind of play out using different examples.

For example, people who Who are, for whatever reason, placed in solitary confinement. They don't have a lot of sensory input. That's part of how that's believed to work. Their brain will start imagining things because they don't see the outside world.

They don't hear the outside world. They don't communicate very well. And so their brain will say, you know, I used to get a lot of sensory information and now I'm not getting very much. And so that starts the process of hallucinations.

And so when we think about things we do to people, we don't always recognize at first sort of the long-term implications of that. Another example is phantom lymph pain. That bottle likes to make an appearance.

Phantom lymph pain, thank you very much. Have we heard of phantom lymph pain? So phantom lymph pain, if you haven't heard of it, is when...

person fully developed human loses a limb and so people that serve in the military unfortunately have too much of this so the problem with phantom limb pain this irretractable pain they get is that serves an arm. There are many sensory neurons that serve that arm. If part of that arm is missing, it's possible these primary sensory neurons are still existing in a truncated form. And they still, for whatever reason, get the idea that they should tell the brain there's pain in this limb.

But that limb isn't there anymore. And so how do you tell the brain, hmm, that sensation is not real? You should ignore it. And that, really, you can imagine how incredibly disruptive that would be for that person's day. You can't fix the burn.

sensation in the limb that's not there. So for a long time they tried a lot of different drug therapies, how do we maybe it was almost a sedative which is not a good quality of life long term. Very brilliant neuroscientists came up with the most elegant way to fix it and it cost less than a dollar.

So that's good science. He knew how the brain and the spinal cord and how the peripheral nervous system really work together. And he would take veterans who were having this problem with PTSD as well as pain from limb pain. And he just simply said, stand in front of the spooling mirror, and he would do this several times a day for several weeks, and look at the limb that used to be there, or where the limb used to be, and just look at this missing limb.

And that sounds cruel, but what he was doing, he thought, and it turned out to be true, he was rewiring the brain. To understand this is not real. This is not real. There is no limb there that could be causing you pain. It turned out that alone was more effective than any chemical therapy they could have given those people.

And it was something they could do on their own at home. science and so when we think about helping people sometimes you have to really get down to the weeds obviously understand something for free elegant solution for people really have your attractable pain and there's no side effects of that, right? There's no chemical imbalances caused by that.

So it's just an interesting thing. And that's just kind of two examples I could come up with at the top of my head about why you really need to be very familiar with how this works. So many different examples.

But anyway, that's kind of a primer to where we're going with the nervous system. Before we go any farther, I want to introduce the next part here. We've seen this before, but it's worth revisiting.

And that is our old friend. I like to say old friend. Maybe it's a new friend. This flow chart that we've seen before, the nervous system had its fingerprints all over it.

All over it. What I want to do is remind you how many fingerprints of the nervous system are on this flow chart. So before I let you go for a break, not permanently, of course, but for a break, take a couple minutes here in a moment.

We're going to be doing this. And so some people just like to kind of. of get ahead or think about what we're doing. Maybe they don't want to break so I'll sort of give you a primer.

I'm going to be using this flow chart and I'm going to be using a specific example to have the nervous system could get involved. And this flow chart, complete the flow chart for homeostasis focusing on the nervous system, we're going to be using a very familiar setup here. In this case core body temperature is too low and I think that's appropriate because I think this room is really cold. It's been cold in here all day.

So I'm going to say the core body temperature is too low. And in this case, I'm going to make it pretty low. You know, it should be 37 degrees C.

Let's say you've been in Acker way long today, and it's now 35 degrees C, and you're flirting with hypothermia. So I'm going to give you a three-minute break. Come back at 3.03, 3.04, something like that. If you want to stay and you want to sort of challenge yourself, see if you can fill this up. Otherwise, I'll do it with you in a couple minutes.

Thank you. The conus medullaris, where would that be? It's just the con at the end.

Okay, so right before it goes to the... Yeah, so the conus medullaris is just right here. Okay, is there a specific function or is it just kind of the last...

It's just where the spinal cord tapers. Okay. Yeah, so it would have the same function as the rest of the spinal cord, it just has a different shape.

Okay, thank you. Good question. Sure.

Great. Alright, I'm going to go ahead and get your attention back up here. Start talking about the homeostasis flow chart.

And in this case, as we've seen before, we're going to talk about the variable of interest. And in this case, the variable of interest Core body temperature is too cold, too low. So we're gonna see core body temperature approaching, must be really crazy, approaching 35 degrees C.

Remember, it should be 37 degrees. so 35 degrees C is pretty low so we're definitely gonna be setting off some alarms as far as the nervous system is concerned and we'll start to see how the central nervous system and peripheral nervous system work together to fix this we We hope quite quickly. So as we go through this, just keep in mind this is our goal, to indicate where and how these are involved. So I'm going to sort of zoom in on this now that we kind of see our goal.

When we talk about, this is a variable of interest, too low. Let's talk about the type of receptor that would interpret this or be triggered at first. Do you remember its name?

It detects temperature type of receptor. Thermoreceptor. Yeah, I heard a couple people say it.

Thermo. receptor and in this case thermoreceptors are located in multiple areas we've got the skin the vertebral canal which I'm going to abbreviate like that vertebral canal liver and the hypothalamus And if the core body temperature is 35 degrees C, I would say all of these thermoreceptors are probably lighting up as though we have a huge problem, because indeed we do. So this arrow here is the problem is being detected by these thermoreceptors.

Thermoreceptors exist at the very end or tips of primary sensory neurons. So to connect this back to things we've talked about, the primary sensory neuron that reaches out into the body, this thing right here, It would have thermoreceptors. If this was a neuron responsible for detecting temperature, it would have thermoreceptors in its little dendrites here. Some of those dendrites would be reaching into the skin.

If this was a different thermoreceptor, some of those neurons would be reaching into the liver or even into the hypothalamus or staying really close to the spinal cord in the vertebral canal. So we have all sorts of sensory input. This represents a directional term.

Do you remember what it was called? An action potential, but we gave it a direction. directional term.

Afferent, good job. So this is an afferent action potential, which means it goes to the brain. And in this case, the brain, obviously we're moving towards the spinal cord and the brain. So at this point, we've got the peripheral nervous system, the sensory part, anyway, got its job done.

The information goes to brain and spinal cord. because that's what makes up the central nervous system. And here we will see a really fast response.

So we will amass that information and definitely respond. So let's think about responses. So let's talk about the motor side of the peripheral nervous system.

The sensory side of a peripheral nervous system got its job done. Central nervous system, partly done, saying, yeah, we have a problem. Now let's talk about what it does. Who does it activate to take care of the problem?

So in this case, we're going to see action potentials involved, and we also gave them a directional term. Do you remember what that was? Efferent.

Good job. So these are going to be efferent action potentials, and efferent because they activate effectors. Efferent effectors. So we're going to see motor neurons, much like this one, be activated.

They're going to come out. from the spinal cord and they're going to go out to effectors that can fix the job. So when we think about specific effectors that could fix this problem, what would you want to activate?

Shivering. What effector is that going to require? Skeletal muscle.

Yep, so definitely skeletal muscle. What else would we want to activate specific to this problem? Say what? Cardiac.

Cardiac, yeah. You're going to see an increase in heart rate, so cardiac muscle. And cardiac muscle has to increase its heart rate because guess who feeds the blood to the shivering skeletal muscles? The heart. If you're going to feed more...

blood to the skeletal muscles so they can shiver you're gonna have to increase that heart rate cardiac muscle we're also gonna see smooth muscle smooth muscles involved because we're gonna have to feed more blood to certain parts of the body body and restrict blood flow to other parts of the body that we deem not essential like the skin the ears the nose you notice those parts of your body get cold fastest that's because the brain says you know what I only have 5 liters of blood to deal with I can't perfuse or shunt blood to all those places at the same time so we'll see some places the smooth muscle cells around blood vessels leading to the skin or the ears the nose really gets tied off other places at smooth muscle relaxes and really allows a massive amount of blood flow to go to those areas because they're deemed more important. But you can only do that if you can constrict blood vessels and that requires smooth muscle. If everything goes well, we should return this back to, we're going to increase that temperature back to 37 degrees C.

So we have a really familiar, I hope, example of how all this works together, but only if the central nervous system and peripheral nervous system are effective. Damage to any of this will drastically decrease your ability to respond to some sort of stimuli. And that's where it gets pretty dangerous pretty quickly.

So kind of our introduction to the nervous system. Questions on this before I go a little bit more into the cell details of who's actually doing all the work here. Are the effectors part of the peripheral nervous system?

So they are activated by the peripheral nervous system. So skeletal muscle, craniocerebral muscle, smooth muscle, parts of their own system that they rely on the peripheral nervous system to tell them, hey, do something. Good question.

Other questions on this? All right. Well, let's move into a little bit of details about the cells. One's doing the work here.

So we're going to talk about cells of the nervous system. And when we think about the nervous system, there are billions and trillions probably of neurons and glial cells involved in this. The number changes based on the paper you read.

So I hate to give a formal number because people argue about the numbers all the time. seems kind of inconsequential, but anyway, there's a lot. There's a lot of neurons and glial cells in the nervous system. And together, they form a vast network and really their overall goal, whether it's sensing, determining, making a decision, making a motor output is super rapidly.

between cells. That's what the nervous system is really known for. Exceptionally rapid communication.

Remember B-slash-W is shorthand for between. Note-taking gets easier if you learn shorthand. This is an interesting thing I just want to highlight here for a moment because I hope we can get to the endocrine system before our next exam and this is a good time to sort of compare and contrast endocrine and nervous system. like a side note for a moment the endocrine system is also a system for communication that's what hormones do they're they're ligand the communicating chemical tells one cell do this because this other cell said to but it's slow it can take weeks if not months if not long for the endocrine system to do its job and that's by design sometimes we don't want a fast response we want a very carefully formulated regulated response but sometimes we need to be fast right we don't have weeks or months to wait for the nervous system to detect it environmental stimulus and fix it. That would be ridiculous.

So we have two communication systems in the body, endocrine and nervous. They are very different, but at the end of the day, they're both for communication. She said this is meant to be exceptionally rapid. And so if you're going to be a rapid system, you better have a really tight communication network that allows you to tell cells what needs to be done.

And in order to get that network of communication to work, work we really rely on two different types of cells that are part of the nervous system. One of them you probably expected me to talk about neurons. These are really the functional cells or you could almost call them the functional units of the nervous system. functional cells or units of the nervous system. And as we saw, neurons specialize in a unique role.

They are either going to be great at detecting a stimulus of some sort, internal or external. external, they're either going to be good at that or integrating this information, amassing, collecting it, making a decision, or activating an effector. But they can't do it all.

They have to be sort of one of these three categories. And they can do this, this detecting or integrating or activating, because they are capable of producing and conducting electrical impulses called action potentials. And that's the rapid form of communication that's required. From here on out, I abbreviate action potentials, APs.

You probably already knew that, but again, ambiguous abbreviations are just not helpful. So this really sets neurons apart from the other type of cell we see as part of the nervous system, which are glial cells. Glial cells. Glial cells have a contentious history. First of all, we didn't even know they existed.

We just thought it was neurons. And then the early anatomists found them and said, well, we don't really know what they do, but they're all kind of like this. ambiguous milky white color it must be some form of laugh if you want glue they thought glial cells glial means glue in latin helped glue these neurons in place sealing them in their location making sure they could communicate well glial cells the glue cells, but that's only because we didn't really know what they did, which is fine. I mean, imagine the mistakes we make now in science at a hundred years from now, people will be like, you fools, right?

Like, how did you let that happen? Anyway, glial cells are the supportive cells. Glial cells sustain neurons in every possible way.

Let me get that to focus. Glial cells sustain neurons in so many different ways. Without glial cells... Neurons do not live, they cannot communicate, they cannot heal themselves, they are pretty much done at that point.

Glioblastos, the same neurons in multiple ways. However, glial cells are not capable of generating action potentials. Their job is not communication.

They just support the cells that do the communication and that's a big enough job. In fact, it takes multiple types of glial cells supporting a neuron. Neurons require a support staff. They don't have a lot of patience. They need what they need and they need it now or they're not going to make it very long.

So glial cells, absolutely crucial. Because of that, we believe, this is why, glial cells outnumber neurons in total. And the reason why I say likely... Because nobody really knows the ratio. It used to be 10 glial cells to neurons and then it was like no no that's way too much.

Then it's five to one and now they're like one to one. So again these numbers depends on how you measure, who's looking at what organ. Anyway, let's go with the safe route and say glial cells likely outnumber neurons in total.

That would be especially true in the brain. Less true in the peripheral nervous system. Glial cells, I could also say at least as numerous as neurons.

As I mentioned, different glial cells exist, and I will talk about their details in a subsequent lecture. look ahead, not the next drawing, but the drawing after, I talk about the differences in glial cells. So the reason I talk about glial cells so much is because their importance in helping overcome traumatic brain injuries is becoming more and more important. So they used to think traumatic brain injuries like concussions, for example, so people get playing football, it's a great time to talk about this because it's football season, or any high-impact sport, riding bikes, riding horses, whatever is going to make you more likely to hit your head.

They used to think traumatic brain injuries were a problem with neurons. They got jolted, they had a hard time sort of coming back online, and if we could fix the neurons, then the problem would likely not get too worse for the person. We now know it's really not the neurons at all that are having trouble, it's the glial cells.

And so they've actually taken the information and started designing helmets a little bit differently with the end goal of protecting these in addition to protecting those. It turns out glial cells, if they are lost during a traumatic brain injury, the neurons, even if they are fine, they usually don't last either. So you have to support the supportive cells in order to get the neurons to live.

So it's really changing how helmets are designed. And I think that's really important. I think in the next 10 or 15 years, you will see very different helmets. Probably won't look any different internally or externally, but the technology inside is really going to be aimed at preventing damage here to help preserve brain function and reduce traumatic brain injury.

So a lot about real cells that we just didn't think about for the longest time. So kind of a question down here. I think I can fit it all in, but if you've got it printed, you'll have it.

In the human body, fully formed adult body. we'll find quite a few intervertebral discs. So these are fibrocartilage discs. They don't compress.

They're not meant to compress. They need to withstand the gravity and the concussive forces that the spine gets every day. So that's in between each vertebra, 25 discs.

These discs permit flexibility. There we go. Flexibility of the spine and act like shock absorbers.

And they prevent vertebrae from growing. grinding together. As you can imagine, that's bad.

If you have a slipped disc, it allows the disc to sort of crush in on one side and pinch the nerves that come out. This is not an uncommon thing in people, especially in an aging population. You get arthritis of the spine, you get bulging discs, you get slipped discs, no shortage of problems there.

But what happens is when those discs move, um, the vertebrae can grind together. Interestingly, why that happens, why these discs... are kind of fragile is because the center contains a special area called the nucleus propulsus, and that's really responsible for much of the shock-absorbing functions.

This nucleus propulsus, this internal sort of jelly-like area of each disc, is a remnant of the notochord, and so the notochord formed really early during fetal development, and I'm not sure if I got to talk to you about the sort of trajectory of human development. I might have had to club that out. So I'm just going to go ahead and talk about this with you. This remnant of the notochord means it formed during week three of this right here is embryonic development. And so when we think about the spinal cord being fully encased in bone, That bone is also reliant upon other structures to keep the vertebrae aligned, stacked appropriately, not compressed on each other, and that allows the spinal cord and the neurons extending from that cord not to be compressed.

because compressed neurons cause a lot of pain. And so when people think about these slipped discs, these ruptured discs, herniated discs, things you've probably heard about, either you know someone or heard about it, just think about the fact that that... structural integrity is reliant upon a structure that developed during week three of embryonic development. That means there was conception and three weeks later this form, this structure formed.

That's a really long time for that to last. So it doesn't really come as much as a surprise for these things to fail. So when we think about the central nervous system, peripheral nervous system, we always have to think about the structural relationship between that spinal cord and all the vertebrae that support it.

because remember we don't just look at the body system by system we have to give an eye towards they are all reliant upon each other which is why I like to follow the skeletal system that we talked about last week with the nervous system so you have some idea of all the osseous processes in the vertebrae that are helping or hurting this particular structure the spinal cord but we're going to move on to structure and function of neurons so this was sort of our introduction to the nervous system. We talked about central nervous system, peripheral nervous system, some is encased in bone, some is not. We talked about how they work together and then we came down here and sort of talked about broadly two types of cells in the nervous system.

What we'll do in the next piece of paper is really just distill it down and function or focus solely on neurons, how they're structured and what they do. So the neuron you see here before we go, this is is called a multipolar neuron, and this is a very common, very common form. It's one that I use a lot of to make sort of points and notes here.

This is a multipolar neuron. I'll talk about why it's called that, but many neurons that we talked about in the central nervous system and in the peripheral nervous system are structured like this. Neurons can be quite long, quite long, and so we'll see that they have different zones or regions that specialize in different functions.

functions. So that's where we're headed. Questions on anything you see here that you'd like clarification on? Or if you'd like something repeated, let me know.

Otherwise, we'll move on to our next set of notes. Any questions on this? Yes, ma'am.

Yeah, so remember, CNS is only brain and spinal cord. So this was the whole function of the CNS right there. Otherwise, we saw, I can sort of like highlight it, and that might kind of answer this. You know, this whole part here, this is all.

the PNS, the sensory division. So all of this relies on things like primary sensory neurons to bring that information in. And then when we look at the central nervous system in this particular set of of diagrams, we can sort of highlight this and say, well, this was an important role, but that's really the main role of the CNS.

So, you know, we think about like how cool the brain is and the spinal cord, but realize its role here is just a part of something much broader. And then when we looked at sort of the end part here, we can kind of highlight this and say, all this over here, this was the job of the motor part, the motor division of the peripheral nervous system. So this is a good question. I'm glad that you asked. I bet you're not the only one with those questions.

So I'm just going to say the role of the CNS, I highlighted that in pink. The role of the PNS, we talked about the sensory part, which I did in green. And then I talked about the motor part, which I did in red.

So sometimes you can just sort of circle and highlight and say this was the role. Good question. Thank you for asking that.

Other questions before I continue? Alright, we're going to talk about neurons and I'm going to use this sort of as my example for many neurons. It's not the only type of neuron that you have, but it's a really common one, so it makes sense. And we're going to talk about the structural features of these very complicated cells.

And not only are they complicated in function, they have some other strange features about them that we don't see in other cells. For example, they are amyototic. What does that mean, amyototic?

They can't divide. Just think about that challenge for a moment. You have some of the most crucial cells in your body, neurons, they can't divide. So if you lose them, they don't come back. And that's a problem for a cell that started developing during the embryonic stage of fetal development and has to last, some people, 100 years.

That's a big ask for a cell. Most cells in the body can be replaced if necessary, but not neurons. And I'll sort of explain why that is as we go along here. They are big and they are complicated and they have to weave through a lot of soft tissue as well as bone to get to their target. And if that's the...

damaged, you can't really convince another cell to push its way through tissue that already exists. It just doesn't work. So we have to, hopefully, if damaged, because we cannot rely on them to divide and produce more, we hope to repair them.

But naturally, biologically, that is limited, which is why permanent nerve damage due to some sort of trauma or accident or virus is a problem because we can't fix it because of how these neurons are. kind of designed. So kind of right off the bat, hopefully you're starting to think about sort of applications to your broader career field when it comes to this information.

We're going to do a couple things on this. I'm going to introduce the structural features of a typical multipolar neuron. Again, I kind of reminded you of the structure I'm talking about here. I think most people have seen this.

This is probably not too unfamiliar to you. We'll talk about the basic structure and features of each component, and then I'll sort of draw it out. sort of sketch what I mean by this.

So we're going to first start with what is truly kind of the reference point of the cell and that's called a soma and soma is the cell body. Soma is the cell body. It contains the nucleus in most of the organelles that you would expect to find in any cell.

I mean at the end of the day neurons are still living cells. They have all the needs that any other living cell would have. They need nutrients, they need waste removal, they need to produce proteins and lipids, so there's still a cell and it's easy to forget that because they have so many capabilities. But the soma or cell body is where we'll find most organelles.

Some are distributed throughout other parts of the neuron. The soma also serves as a reference point. So that is true when I'm talking about projection that come out of the soma, as well as directional terms.

So if it's going away from the soma and through the axon towards the axon terminal, we call that anterograde. If it's coming from the axon terminal back up the axon to the soma, we call that retrograde. So I'll explain all that more in a little bit, but I just want to be clear that the reason the soma is so important is because it really is the anatomical and physiological reference point.

So before I sketch... that out I want to introduce another very closely associated structure or set of structures. These are called dendrites and these are fibrous. They look in many neurons like hair coming out of the soma.

I mean they're just so numerous and usually referred to as a dendritic tree because they have a lot of branches. So fibrous process or dendritic tree. I think I have enough room to write the word church.

tree in there. And dendrites are really great. Again, they extend out of the soma. Numerous, but they have a lot of surface area.

And that's great because they contain, the dendrites contain, the specialized receptor. that detect all that environmental stimuli. So when we talked about thermoreceptors, those would be part of the dendrites that are located on a neuron.

So we're really talking about parts among parts among cells. So dendrites and soma usually go together. Dendrites and soma usually go together. So I'm going to take a moment to kind of sketch this out.

I'll sketch it in a little more detail than what I provided in some of those line diagrams because I think it's important to really understand what are we dealing with here. So what is it about soma and dendrites that are so important? So I'm going to take the top half of this and kind of explain what I mean.

I'll be sort of going through these bulleted points. So the soma, I'm going to draw fairly large because we have to put some things in there that's pretty important. So here's our soma.

It has a phospholipid bilayer like any other cell. And extending out from this cell body are numerous dendrites and I'm going to draw them fairly exaggerated, sort of broad and flat so we could talk about what they do and what would be on their surface. Again, surface. area is really a big function of the dendrites and the more dendrites you have you can even have dendrites upon dendrites the more sensitive this neuron would be to incoming stimuli. So however you want to draw it, it does not at all need to look just like these.

I'm just trying to draw room here where we could add some additional features to this very important cell structure. So flat and broad. So we have a decent dendritic tree.

Again, all of these extensions here are called dendrites. Collectively dendritic tree. In the soma is where we find a lot of really important structures. I'm going to draw in a nucleus.

Because we have to have a lot of proteins made, neurons make a lot of proteins. So when we think about nucleus and protein production, we're going to see also extending from that nucleus a few things we've already talked about in this class when it comes to organelles. What do you think this is? Extends from the nuclear membrane, has ribosomes attached.

RefER, good job. RefER. Now, in a neuron, the RefER is so extensive, it looks like what are described as rosettes. So it doesn't just extend from one surface of the nucleus.

It's going to extend from all parts of that nucleus. So this entire soma sometimes looks like it is just absolutely chock full of rough ER. And in neurons, Neurons, and as far as I know, only neurons, we give this very extensive rough ER a new name.

It's called a nistle body. And nistle, after the scientists that discovered how extensive these are, N-I-S-S-L, nistle body. That is the name of the rough ER in a neuron.

Neurons have some of the most extensive, well-developed rosettes of rough ER found in any cell. and that's really an example of a cell that makes a lot of protein and secretes that protein. Some of that protein would be neurotransmitters, which have to be produced and leave the cell. So, nissl body, n-i-s-s-l.

We also find in this Golgi, when you think about cell structures we've already talked about, Golgi bodies, sort of these flattened stacks called cisternae, and that's important for protein production, so we'll find Golgi. We'll find a lot of mitochondria, mitochondria, of course, producing ATP, and we find mitochondria, this is one of those cells that's not just in the soma. We can find mitochondria in the axon and axon terminals. Reason being, ATP is not a very long-lived molecule. It's used...

It's estimated within about 20 seconds of production, and that's pretty liberal. Usually it's used the minute it's produced. And so because these cells are really long, think about a motor neuron whose soma lives in the spinal cord and whose axoness extends all the way out to your foot.

That's a neuron, a cell that could be two feet long. And so you need to produce ATP in multiple locations. So the mitochondria are one of the organelles that we find. not just in the soma, but also the axon and axon terminal.

So just a few things about the neuron. I'm going to sort of stop with my description there when it comes to soma. Let's talk about dendrites.

Dendrites are really important for detecting specific environmental stimuli. Some dendrites, though, are really great at detecting neurotransmitters or chemicals. So the way they do that, whether it's chemicals, pressure, anything that is a stimulus, they have some specialized... features on the very surface of these dendrites and these dots are just areas or locations of where we may find specialized receptors. And receptors are there to detect stimuli and neurons specialize in the detection of certain stimuli.

So you can't have, for example, a neuron that understands both temperature and pressure. That would be ridiculous because how does the brain know what that neuron means? The brain says, did you mean it was too hot or you got crushed? The neuron is like, yes. So you can't have that.

You have to be very specific in your communication. So receptors are very specific for a certain type of stimulus. So that way the brain knows, because remember, the brain can't detect it for itself. It's reliant upon very clear communication for getting that information and knowing what to do about it. So that's a little bit about our neuron.

We have two more components to talk about. Questions before I continue. Questions about axon or not axon, soma and dendrites. We're going to the axon, but we haven't got there yet.

Soma or dendrites. So far, so good then. Let's talk about other components we will find as part of a neuron.

We're going to see extending from the soma something called an axon. This is a very long fairly thick diameter, and I mean thick, I mean the diameter is fairly big, it's usually called a fiber because it is pretty substantial as far as structure. So a long, thick fiber extending from the soma, again the soma serves as our reference point, there is only ever one axon per neuron.

Only ever one axon per neuron. While we may have many dendrites, there is only one axon. In all neurons ever investigated, this is proven to be true.

The function of an axon is to conduct action potentials. And then we'll get to the axon terminals. So from this soma, we see...

In most neurons, an axon coming straight out. And this axon, I'm not going to draw it all right now, but it's long, it's fibrous, it leaves the soma, and it will be conducting or carrying going into... internally right inside the membrane, action potentials.

So action potentials move along the inside of that axon's membrane. And then we will encounter the ending of this neuron, which is called axon terminals. Axon terminals look like swellings that branch out from this axon. They are sometimes called terminal boutons.

It's French, very fancy. You can impress your friends and family. Or we could just call them terminals, which is what I use.

They look like swellings or knobs extending from the end of this axon. So they branch directly from the axon and there could be numerous terminals that exist. So while we only have one axon, at the very end it could really splay out and form multiple axon terminals. Their function is to release neurotransmitters, or abbreviated NT, a lot easier to write. So what you'll see me do is I'm going to extend this axon a little bit and we're going to see it branch into numerous terminals and I'll sort of make a note about how these are released and where they go.

So I'm coming back over here to the drawing. I'm going to extend end this. Axons can be pretty wavy, they can take fairly torturous routes, and then at the end they will eventually terminate into axon terminals, which I am drawing a little bit broad so you can sort of see their detail.

So here's our axon terminal. swellings at the end of the axon. So again this would be the axon. Long, fibrous, thick diameter. And the axon terminals are an interesting area.

Here we see the release of neurotransmitters. So these dots are neurotransmitters being released from these axon terminals. NT are released here.

And these neurotransmitters, when they're released, they have to diffuse across a space called a synapse. synapse or synaptic cleft, both of those are correct, and they will hit their target cell, or we sure hope. And that target cell could be another neuron or it could be an effector like skeletal muscle. So I'm going to draw the proverbial skeletal muscle cell.

Looks sort of long and fibrous, striated. So here's an example of an effector. I'm gonna make this skeletal muscle and that skeletal muscle has receptors.

for those particular neurotransmitters. And so when those neurotransmitters hit that receptor, and yes, it usually takes two, but it's nice because it makes a smiley face. And who doesn't like that?

Looking at you at science. get that target cell to receive those neurotransmitters, it will cause contraction of the skeletal muscle. So this would be an example of a motor neuron coming out of the spinal cord, going into the periphery to get an effect on my skeletal muscle. So in this case, we've kind of come full circle from what I talked about on my previous drawing. Just kind of adding details as we go along.

So when we look at an axon and an axon terminal, they do not look as detailed structurally. They look like this. They don't look super impressive, but really, honestly, this is where so much of that physiology happens. I made a mention at the very beginning up here that repair is possible but limited.

And this is a good time and sort of area to talk about what I mean by possible but limited. We know from experiments on neurons that if a neuron is severed, let me see, I'll just draw like a line. on is cut or severed really near the soma let's say for whatever reason the trauma happened here and the entire axon and axon terminals were lost this will likely not live it's too it's missing too much however if the damage occurred down here and you lost a little bit of the axon and some axon terminals this has a pretty good chance of doing some re-sprouting it will regenerate a little bit of that that axon. It has the ability to regenerate axon terminals and if that can happen soon, fast, with medical help of course, you can preserve or re-establish the connection between that neuron and its effector and that is crucial for maintaining function and quality of life. But that's a big if.

So you know when someone has neuron damage you have to understand how this repairs. if it repairs, depends on a lot of factors. One being, where did that damage happen?

And two, how quickly were you able to stabilize the synapse and maintain support of the effectors? Because it turns out, effectors like skeletal muscle, they don't just rely on the neuron for information about when they contract. It turns out skeletal muscles get something interesting from their neurons that actually helps keep them alive. And that's not something that we typically thought of for a long time. But.

If you've ever seen someone with neuron damage and they had skeletal muscle atrophy, part of that is due to the fact that the skeletal muscle itself is still alive, but without this constant communication from its neuron, it won't survive. And so we're not sure what they're getting from that neuron. They're getting this.

Anyway, I'll stop there for today. I'll see you on Thursday. Let me know if you have any questions.

Transcript for:Overview of the Nervous System

Transcript for:
Overview of the Nervous System