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Studying for a test? Prepare with these 2 lessons on Nervous system diseases.
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- [Voiceover] Let's say you're a clinician and you want to tell if someone has multiple sclerosis. One of the first things you might do is you might actually look into their brain. The way to do this is to use a technique called Magnetic Resonance Imaging, otherwise known as MRI. What you do is you have someone lay down on this bed and they enter this kind of cramped and really loud machine over here. That machine is actually gonna take pictures of their brain. Interestingly enough, it's actually gonna take pictures from three different perspectives. On the left, we kinda have a picture of someone who looks like they're looking towards the left. This slice of the brain is called a sagittal slice. In the middle, it looks like we have a slice of someone who's looking in this direction. So it's kinda as if we're looking down from the top of their head. We call this a horizontal slice. On the far right, we have a slice of someone who looks as though they're looking towards or away from me. We call this a coronal slice. In all three slices you'll see one thing that's common. They have this bright spots over here. These bright spots that they actually point out. These bright spots are referred to as lesions. A lesion is just a piece of damaged tissue. If you think someone has multiple sclerosis and you have all these brain scans, the first thing you're gonna look for are these bright spots here. These bright spots are actually located wherever there is a white matter. Remember that the white matter is really just the axons and the myelin of your neurons. So this informative, right? You see that you have damage in the brain, but that damage could result from anything. So we haven't really narrowed down specifically. We can still do a few more tests to really confirm that this is multiple sclerosis. We can actually do something that is a little bit more invasive. Here I have a picture of someone's lower back. In purple here is the person's spinal cord. You can see this kind of blue stuff that's surrounding the spinal cord and that stuff is just called your cerebrospinal fluid. The cerebrospinal fluid just bathes both the spine and your brain. It contains all sorts of good things; proteins and ions and all sorts of things that you need for the brain and spinal cord to work. What we can do is we can actually extract that cerebrospinal fluid using this needle, and we can collect it into this vial. Now we have a vial and that vial has a whole bunch of cerebrospinal fluid, but the question is, what is in that cerebrospinal fluid? There's going to be something in here that's characteristic of multiple sclerosis. What is that thing? Remember that multiple sclerosis is an autoimmune disease so the immune system is really crossing paths with the nervous system. We want to see something that's characteristic of the immune system. So maybe if we kinda zoom in here. What's characteristic of the immune system? Well, antibodies. If you have a lot of antibodies in your cerebrospinal fluid then that may indicate that your immune system is in the central nervous system. How do we determine if there are antibodies in here? We can use a technique known as gel electrophoresis. I'll spare you the details of how exactly this technique works. But really what it does is first it separates the contents in your sample and it allows us to visualize those contents. Basically, we're gonna send this vial off to a lab and they're going to perform gel electrophoresis. When you separate the contents of this cerebrospinal fluid and when you visualize it, it looks something like this. Here, I kinda have two different strips for you. The left strip corresponds to a person who is healthy. The right strip corresponds to someone who has multiple sclerosis. Immediately, you'll notice that there are these white bands over here. These bands, they're gonna correspond to different proteins. In the healthy sample, you'll notice that these bands are separated uniformly. But in the multiple sclerosis sample, you'll see that there are these thicker bands over here. These thicker bands are referred to as oligoclonal bands. When you see these oligoclonal bands, that's actually pretty indicative that there are a lot of antibodies in this sample. Because you have a lot of antibodies in this sample, that means you have a lot of antibodies in your cerebrospinal fluid. That tells us that the immune system is really really active in the central nervous system. So we have two pieces of evidence. We have pictures of the brain that showed the lesion and we have evidence that shows that the immune system is involved in the central nervous system. But there's still one more test that we can do to definitively say that this person has multiple sclerosis. We can actually measure the electrical activity in the person's brain. Let's say you have a person here and he's just sitting in a dark room, and you put these electrodes here onto their scalp. Those electrodes are gonna measure their brain activity. More specifically, it's gonna measure the brain's response to some kind of visual stimulus. In this case, it's a computer screen. On your computer screen, you have a checker board. You have all these black tiles and you have all these white tiles. These tiles are actually going to alternate every half second or so. So maybe every half second so, all the black tiles are now in pure white, and all the white tiles will now be pure black. It's just gonna alternate back and forth. We're basically measuring the brain's response to this visual stimulus. That response is gonna be called a visual evoked potential. What are we measuring here? Well, we're measuring an electrical response. We're gonna measure it graphically. Let's say we have a graph over here. On the x-axis is time, usually on the scale of milliseconds. And on the y-axis, we have voltage, usually on the scale of microvolts. Let's say we're kinda measuring the person's brain activity and at some point the checker board alternates, and we're gonna see this kind of peak in the voltage. It's gonna go back down and continue onwards. We're gonna be looking at these peaks in response to the alternating checker board. Usually, you collect data from a lot of peaks, but this is just one such example. Now, given that this is a visual evoked potential, you're gonna be measuring activity from visual areas in the brain. Let's say you have your eyes over here, so this is the person's eyes. At the back of the eyes are the optic nerves. The optic nerves will carry visual information in the form of electrical activity to the rest of the brain. Let's just say for the sake of argument that this person has a lesion in their left optic nerve. The right optic nerve, however, is perfectly healthy. So when we measure the electrical activity from both of the optic nerves, we can actually compare them. We can compare activity from the healthy optic nerve to activity from the damaged optic nerve. Let's say that the graph that I've drawn here, as it is now, is just the optic nerve that's healthy, so it's the right optic nerve. Then you look at the electrical activity from the left optic nerve, it might look like something like this. Immediately, you could tell that there's actually a delay between the two. It took a longer period of time for the left optic nerve to respond to the alternating checker board. This actually makes a lot of sense because if we think back to it, let's say we have a neuron over here, ordinarily your neurons have a bunch of myelin that insulate their axons and this myelin helps the electrical signals travel then a lot more quickly on the axon. But in multiple sclerosis, that myelin gets degraded. When that myelin is degraded, the signal that moves down the axon is a lot slower, so when you have all these degraded myelin over here on the left optic nerve, it makes a lot of sense for the signal to be slightly delayed. I wanna make it clear that these signals that I've drawn here aren't actually action potentials. They're signals that you get from the overall nerve. But in general, the principle is still the same. You see there's a delay in the response. So the left optic nerve is now slower than the right optic nerve because you have this lesion over here. So all in all, I've shown you three very common tests that people use when diagnosing multiple sclerosis.