The neuron and nervous system
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Anatomy of a neuron
We could have a debate about what the most interesting cell in the human body is, but I think easily the neuron would make the top five, and it's not just because the cell itself is interesting. The fact that it essentially makes up our brain and our nervous system and is responsible for the thoughts and our feelings and maybe for all of our sentience, I think, would easily make it the top one or two cells. So what I want to do is first to show you what a neuron looks like. And, of course, this is kind of the perfect example. This isn't what all neurons look like. And then we're going to talk a little bit about how it performs its function, which is essentially communication, essentially transmitting signals across its length, depending on the signals it receives. So if I were to draw a neuron-- let me pick a better color. So let's say I have a neuron. It looks something like this. So in the middle you have your soma and then from the soma-- let me draw the nucleus. This is a nucleus, just like any cell's nucleus. And then the soma's considered the body of the neuron and then the neuron has these little things sticking out from it that keep branching off. Maybe they look something like this. I don't want to spend too much time just drawing the neuron, but you've probably seen drawings like this before. And these branches off of the soma of the neuron, off of its body, these are called dendrites. They can keep splitting off like that. I want to do a fairly reasonable drawing so I'll spend a little time doing that. So these right here, these are dendrites. And these tend to be-- and nothing is always the case in biology. Sometimes different parts of different cells perform other functions, but these tend to be where the neuron receives its signal. And we'll talk more about what it means to receive and transmit a signal in this video and probably in the next few. So this is where it receives the signal. So this is the dendrite. This right here is the soma. Soma means body. This is the body of the neuron. And then we have kind of a-- you can almost view it as a tail of the neuron. It's called the axon. A neuron can be a reasonably normal sized cell, although there is a huge range, but the axons can be quite long. They could be short. Sometimes in the brain you might have very small axons, but you might have axons that go down the spinal column or that go along one of your limbs-- or if you're talking about one of a dinosaur's limbs. So the axon can actually stretch several feet. Not all neurons' axons are several feet, but they could be. And this is really where a lot of the distance of the signal gets traveled. Let me draw the axon. So the axon will look something like this. And at the end, it ends at the axon terminal where it can connect to other dendrites or maybe to other types of tissue or muscle if the point of this neuron is to tell a muscle to do something. So at the end of the axon, you have the axon terminal right there. I'll do my best to draw it like that. Let me label it. So this is the axon. This is the axon terminal. And you'll sometimes hear the word-- the point at which the soma or the body of the neuron connects to the axon is as often referred to as the axon hillock-- maybe you can kind of view it as kind of a lump. It starts to form the axon. And then we're going to talk about how the impulses travel. And a huge part in what allows them to travel efficiently are these insulating cells around the axon. We're going to talk about this in detail and how they actually work, but it's good just to have the anatomical structure first. So these are called Schwann cells and they're covering-- they make up the myelin sheath. So this covering, this insulation, at different intervals around the axon, this is called the myelin sheath. So Schwann cells make up the myelin sheath. I'll do one more just like that. And then these little spaces between the myelin sheath-- just so we have all of the terminology from-- so we know the entire anatomy of the neuron-- these are called the nodes of Ranvier. I guess they're named after Ranvier. Maybe he was the guy who looked and saw they had these little slots here where you don't have myelin sheath. So these are the nodes of Ranvier. So the general idea, as I mentioned, is that you get a signal here. We're going to talk more about what the signal means-- and then that signal gets-- actually, the signals can be summed, so you might have one little signal right there, another signal right there, and then you'll have maybe a larger signal there and there-- and that the combined effects of these signals get summed up and they travel to the hillock and if they're a large enough, they're going to trigger an action potential on the axon, which will cause a signal to travel down the balance of the axon and then over here it might be connected via synapses to other dendrites or muscles. And we'll talk more about synapses and those might help trigger other things. So you're saying, what's triggering these things here? Well, this could be the terminal end of other neurons' axons, like in the brain. This could be some type of sensory neuron. This could be on a taste bud someplace, so a salt molecule somehow can trigger it or a sugar molecule-- or this might be some type of sensor. It could be a whole bunch of different things and we'll talk more about the different types of neurons.
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