If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:5:03

Video transcript

- [Voiceover] If we take a look at the brain tissue that's been affected by Alzheimer's Disease under a microscope, there's almost always two common factors that we'll see, and these are called plaques and tangles. Let's just look at these plaques for a minute. Well, actually, they're called beta-amyloid plaques. Amyloid is the name we use for fragments of proteins, so it's like not the whole protein, just little bits of it. And these are produced normally in the body. Beta-amyloids are protein fragments that have been snipped off of the larger protein by specific enzymes. So, you have your neuron with this protein called amyloid precursor protein sort of sticking out through the cell's membrane. And so, how beta-amyloid forms is when these enzymes swing by and chop off our beta-amyloid protein fragment. These little beta-amyloid snippets are somewhat sticky, and they'll start to clump together. And as they clump, they mix with other molecules and even other neurons, forming what's eventually called beta-amyloid plaques. Now, it's thought that these clumps of plaques, especially the smaller ones, may interfere with the cell-to-cell communication between the neurons, essentially blocking the signal at the synapse. It's also thought that they might activate the immune response as well, which triggers inflammation and also tells your immune cells to root out and destroy the disabled cells. Now, it's super, super important to note that this process is still not very well understood. It's especially unclear whether beta-amyloid plaques themselves contribute to causing Alzheimer's Disease or if they're essentially just a result of the progression of Alzheimer's Disease. So, those are the plaques. Now, let's talk about these tangles. Another more fancy name you might hear is neurofibrillary tangles. And these, unlike the beta-amyloid plaques, actually show up inside the neurons. So, usually, healthy neurons are internally supported by and partly made up of these structures called microtubules. These microtubules are like tracks that guide nutrients and other molecules from the cell's body down to the cell's axon, which is usually connected to another neuron. And this is basically how the neighboring neurons communicate. So, you could think of these microtubules like rails that carry a mining cart with a bunch of sweet ores and minerals, which are like the nutrients and the messages, to the other miners, which are like the other neurons. Now, a special protein simply called "tau" binds and stabilizes our rails. Kind of like how railroad spikes keep our rails in place, these tau proteins keep our microtubules in place. In Alzheimer's Disease, these railroad spikes, the tau proteins, are chemically changed and basically aren't the right type of spikes. And instead of holding the tracks together, they pair with other tau proteins. And when they do that, they get all tangled up with each other. And since the tracks don't have anything holding them together anymore, what do they do? Well, they start to fall apart, and so our microtubules fall apart, and these tau proteins become tangled. And since our track is all messed up, this disrupts our whole transport system. And when this happens, we call them neurofibrillary tangles. Since the tangles are sort of like these thread-like fibers or fibrils, we say "fibrillary." And since they're in the brain, we say that they're "neurofibrillary." And it's thought that the disruption of our rail system causes some communication issues between our neurons. And since they can't communicate or transport our materials, they eventually are decomissioned and undergo nerve cell death. Now, these plaques and tangles can be found in brains of many elder people, but they're found to a much, much greater extent in the brains of those with Alzheimer's Disease. And for Alzheimer's, they tend to start forming and spread from the cortex in a pretty predictable way, as Alzheimer's progresses. The earliest areas affected are usually those associated with learning and memory, here in the temporal lobe. And as it spreads and more neurons are affected, it usually goes up to the frontal lobe, where thinking and planning become affected. And then, more of the temporal lobe, as speaking and communicating become affected as well. And then, it kind of moves up into your parietal lobe, where your sense of where your body is in relation to objects around you becomes affected. In severe and late-stage Alzheimer's Disease, these plaques and tangles have spread throughout most of the cortex. And as this happens, the cortex gets seriously, seriously damaged. And due to this widespread cell death, the brain actually shrinks dramatically. But since Alzheimer's progresses at different rates for every patient, the rate of spreading of these plaques and tangles varies a lot from patient to patient.