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Studying for a test? Prepare with these 14 lessons on Human anatomy and physiology.
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When we're at a dance party, and we've determined that this is an appropriate time to dougie, how does our brain tell our body to do so? Well, in this video, we're going to be talking about motor neurons. Motor neurons. These are the nerve cells that come from our brain, go to our bodies, and tell our muscles that it's time to contract . So let's start from the top. In the brain, we have what's called an upper motor neuron. I'll just write "upper motor neuron" right here. An upper motor neuron sends a signal over to a lower motor neuron, that I'll abbreviate right here-- lower motor neuron. And I promise I'm going to draw this out in greater detail in a moment, but just to kind of get an idea of what they do, the lower motor neuron is the direct messenger to muscle to tell it that it's time to start contracting. The upper motor neuron, because it's sending a signal to the lower motor neuron, has two jobs. One, it'll also have the job to tell muscle eventually that it's time to start contracting. The other job it has is that it has to tell lower motor neurons when it's time to stop. It'll tell them that there's no more signal coming in so you should stop telling muscles to start contracting. So that's why upper motor neurons have two functions. Let's take a look at a lower motor neuron in better detail now. The first thing I like to draw here is called the soma. The soma is just the body of the lower motor neuron. The part of the lower motor neuron that receives the signal from the upper motor neuron is kind of branched like this. And there are multiple projections that come off right here. These guys are called dendrites, and they're going to receive the signal. It's where a signal is going to dock on our motor neuron. And after it docks the dendrite, passes through this station, the soma, the cell body, it's cast away along an axon that I'm drawing right here. And I've drawn this extra long because there's going to be a lot going on here before this slab of meat here, or muscle, receives a message. So this is our muscle, the motor end plate, or the muscle that's going to receive information from our axon right here. So one more time, when we're at a dance party and we decide that it's time to shake a leg, how do we tell our leg it's time to shake? Well, we have this signal that's generated from the brain, travels down the upper motor neuron, and then it goes to this lower motor neuron right here. And the way I like to think about a signal, it's kind of like how I think about ships in the Navy. So if this little ship right here is our signal, and it comes from our upper motor neuron, it needs to have somewhere it can dock on this lower motor neuron. That's what dendrites are for. This is where a signal is going to dock. Once it docks there, it has to pass through some type of station, this naval station, which is what the soma is. And after it passes through the station, it's going to move down the axon where it's cast away. The axon is where a signal is cast away from this motor neuron before it ends up going to muscle. But what could happen here? What problem could probably arise, considering how long this axon actually is? Well, the signal could die off. It may not be able to make it all the way to the end. And that's kind of what would happen if you had a lower motor neuron injury or a lesion. If something were to happen to our lower motor neuron, we wouldn't be able to tell a muscle it's time to start contracting. Instead, you would have weakness because your muscle would not be able to contract. The same thing happens when you have an upper motor neuron injury. You're also going to see weakness because you're not able to tell the lower motor neuron it's time to tell the muscle it's supposed to start contracting. But that's not the key characteristic we see with upper motor neuron injuries. The key thing we look for is that we're not able to tell lower motor neurons to stop what they're doing. And so because of that, the lower motor neuron just keeps on telling this muscle, hey, go ahead and contract. I'm not receiving any signal from here to tell me to stop. And so because of that, this muscle is just going to continuously, spastically contract. And so a key symptom or a key characteristic of an upper motor neuron injury that you don't see with lower motor neuron injuries is spasticity. And so that's what could happen if you have dissipation of a signal at the upper motor neuron. But nature has planned for this. What does she do to make sure that we don't have our signals dissipated? Well, she insulates our neurons. So I'm going to draw three insulating cells right here that wrap the round our axonal fiber right here, or this axon. And these three cells-- I'll label one right here. This is a single cell. It's known as a myelin sheath. That's an I. Myelin sheath. What does that mean? Well, myelin just means "fatty." So, yo mama's so myelin. They mean the same thing. Myelin sheath. And so we cover up the axon fiber to help insulate it, so instead of a signal dying off, it's able to go all the way to the end over here. And depending on where you have this myelin sheath cell in the body, there's a different name. In our central nervous system, which is strictly just the brain and spinal cord, we call them oligodendrocytes. And that's only in the brain and the spinal cord. In the peripheral nervous system, which is literally everything else, any other nerve in our body that's not in the brain or the spinal cord, we call these myelin sheath cells Schwann cells. So I've drawn three up here, and what's going to happen is that you're not going to have dissipation of your signal. No. Instead it's going to continue on to this myelin cell and then jump and land on this node right here, and it's going to jump again. And it keeps doing this from node to node, or this empty space to empty space, until it finally makes it to the end of this axon, or what we call the axon terminal right here. And so this space right here, this open, empty area that has nothing in it, has a specific name. We call this the node of Ranvier, named after probably the smartest scientist ever because he got his name on a scientific structure that literally has nothing. This is just empty space right here. And so as our signal is going to propagate down this axon here and jump from node to node, eventually it'll finally make it to our axon terminal right here, where we can then relay a message to our muscle that it's time to contract. And that's how our motor neurons work.