Biology Nervous Systems and the Neuron

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When we talk about structure follows function, that's an exciting topic, but until you get to nerve cells, you haven't even been there yet. Nerve cells are unbelievable. When you think of a cell - sadly, I've trained you guys to think of a cell to look like that, because that's pretty much the only way I've ever drawn them. And every once in awhile, I'll draw them like an amoeba. Nerve cells are incredible when it comes to structure follows function. Look at these things. Look at these nerve cells. Look at the cytoplasmic extensions. Look at the network. Look at the structure. And tell me that cell-cell communication doesn't happen between these cells. Tell me that homeostasis doesn't happen between these cells. Tell me that these cells are not involved. If I didn't tell you these were nerve cells, you could still tell me that these were big-time homeostatic cells. That they were communicative cells. These were the great communicator cells. These are nerve cells. Which, by the way, brings us to the nervous system.
The nervous system involves nerve cells. And when we take a look at the big picture of the nervous system, it really is unbelievably complex. So complex that volumes and volumes could be written on like that little spot right there. You can't see that because I made a tiny little spot, and that's my point. The whole intricacy of the nervous system is unbelievable. So we'll start out general, okay? What I want you to understand is that there are really two nervous systems within our body when you look at the big picture. There is one called the central nervous system, which obviously is at the center of the body. And that includes the brain and the spinal cord. And in many ways, the nervous system, particularly the brain, is the last great frontier in our bodies. When you think about the fact that, you know, what is memory? It's biochemical because your brain are cells, but why can you look at the person sitting next to you and remember their name and remember something like them? Why can you think back to your childhood? Why, when you hear a song, does it remind you of something that happened last summer? Or when you smell a smell, it reminds you of something that happened ten years ago? Why does that happen? It's biological, but we don't know. What is self? When you think of yourself, we've all had these like metaphysical conversations with ourselves. Why am I me? I mean, this whole idea of self, and the concept of self, that's biochemical. The brain is a frontier still to be explored. And it all comes down to DNA and RNA?
And there's the peripheral nervous system. The peripheral nervous system is on the side. It's the periphery. And the peripheral nervous system, are these nerves that come off to the side. And the cranial nerves, the nerves that feed your face. The nerves that hook directly into the brain. And the peripheral nervous system is divided into two parts, too. One part that is sensory, which imports material into the central nervous system, and one part that is motor, which outputs direction that sys, "Okay, pick up your hand. Write this. Take a step." And all that needs intact peripheral nerves as well as an intact central nervous system.
So that's the overview of the nervous system. But to really understand, we're not going to scratch the surface of the nervous system until you and I get down to the level of the cell and the level of the molecules within those cells. We have to talk about nerve cells and how they work. Let's take a look at - remember I told you about the complexity of the nerve cell? That's one cell. That's one cell. That looks nothing like the cells you're used to seeing. But yet, it's very much the same.
Let's start out with the cell parts that it has in common. First of all, the cell has a body. And we call it the cell body or cyton. Cell body is fine. And what you see in that cell body is structures that you see in every cell. We see a nucleus. We see DNA. We see RNA. We see ER, we see [gold dew], we see mitochondria. This is a cell, just like every other cell except its structure is perfectly linked to its function. Picture the nerve cell. You know that nerves kind of somehow are involved in electrical currents. You knew that. Did you ever smash a nerve? Like, accidentally bang your elbow and it feels like you just got a shock? Well, that's because you did, in essence. That's because there's a nerve plexus there. But that's another story. But one of the key things is, since they are nerves, and since they have electrical impulses to conduct, they are very much like wires. And they have places to receive a stimulus and receive and start out an electrical impulse. And places to end an electrical impulse. So let's label some of these.
At the top, or by the cell body, are structures called dendrites. Functionally, the role of the dendrite could be sensory. If the dendrites are right in the ends of your eyes, where the light photoreceptors are, where the vibration sensors in your ears are, or the touch sensors in your hands are. They may pick up a stimulus, or they may pick up a stimulus from another neuron right next door. Now some of these neurons, by the way, are very long. You have neurons that one neuron will run in a nerve bundle, so many of these neurons running concurrently right next to each other, will run the entire length of your leg. So one neuron can be the entire length of your leg. One cell. Why? Because of this thing right here. So we have the dendrites and we have this thing right here.
This thing right here is called an axon. And the axon is kind of like a cable. And if you use that analogy of the cable, well then cables can have insulation, and it is covered by a sheath of something called myelin. And myelin is a fatty substance and at first, when we discovered this myelin, we said "Oh, look, look, it's a fatty substance wrapped around the actual axon." But you know what it is? It's a cell. It's actually a Schwann cell discovered by Schwann, and there are two different ones. In the peripheral nervous system, it's a Schwann cell. And a Schwann cell is a cell that is wrapped like a jelly-roll around the axon. So, it's wrapped and wrapped and wrapped. So what you're really seeing when you see the myelin is the cell membrane of the Schwann cell. It even has a nucleus. And here it is right here. So here's the Schwann cell wrapped around, wrapped around the axon. There's the nucleus of the Schwann cell. And there's the axon right there. That's very cool stuff. Well, you get the idea then that we must be propagating an impulse down through the nerve, down through the axon, down somehow through the axon and eventually to the end. And in the end, is where we find the terminus of the nerve, what is called the terminal branches, or might otherwise be known as the synaptic terminals. Because all nerves end there. Nerves do not connect to other nerves. Did you hear that? Nerves do not connect to other nerves. Now there's a puzzle for you. But I'm not going to tell you how to solve that puzzle until another lesson.
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