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Astrocytes

This video describes the structure and function of astrocytes. By Matt Jensen.

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Created by Matthew Barry Jensen.

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  • female robot amelia style avatar for user Ambica
    at 2;08 he says a word about the astrocytes "pro....". something...whats that word?
    i got the meaning as he explained pretty well later...but i require the words
    (6 votes)
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    • leafers seedling style avatar for user Paige
      At he says, "Processes". According to my own Anatomy and Physiology textbook, these are arm like structures that extend from the cell body of all neurons. There are two types.
      1. Dendrites are short, tapering, and diffusely branching extensions. They convey incoming messages toward the cell body.
      2. Axons arise from the axon hillock, then narrow to form the slender process. They are the conducting region of a neuron.
      Hope that helps!
      (15 votes)
  • leaf green style avatar for user Quynh Tram
    at , how does neuron process lactate into ATP, is this process similar to how liver cells process lactate back to pyruvate for later oxidative phosphorylation (since lactate is a product of fermentation)?
    (5 votes)
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    • male robot hal style avatar for user chaddamitz
      I found an explanation concerning your question about neuron lactate process into ATP in an article entitled "Brain Energy Metabolism: Focus on Astrocyte-Neuron Metabolic Cooperation." Here is what the authors wrote. "Glucose is the energy substrate of the adult brain. Nevertheless, under particular circumstances the brain has the capacity to use other blood-derived energy substrates, such as ketone bodies during development and starvation (Nehlig, 2004, Magistretti, 2008) or lactate during periods of intense physical activity (van Hall et al., 2009). Glucose enters cells trough specific glucose transporters (GLUTs) and is phosphorylated by hexokinase (HK) to produce glucose-6-phosphate. As in other organs, glucose 6-phosphate can be processed via different metabolic pathways (Figure 1A ), the main ones being (1) glycolysis (leading to lactate production or mitochondrial metabolism), (2) the pentose phosphate pathway (PPP), and (3) glycogenesis (in astrocytes only, see below). Overall, glucose is almost entirely oxidized to CO2 and water in the brain (Clarke and Sokoloff, 1999). Nevertheless, as evidenced by the different metabolic routes that glucose can follow, each individual brain cell does not necessarily metabolize glucose to CO2 and water. Indeed, a wide range of metabolic intermediates formed from glucose in the brain can subsequently be oxidized for energy production (e.g., lactate, pyruvate, glutamate, or acetate) (Zielke et al., 2009)."
      (4 votes)
  • blobby green style avatar for user Kelly Shaffer
    I was told that the MCAT doesn't require much knowledge of these (astrocytes, microglia, etc), and to just know their main function. Is this still true or is my source outdated?
    (4 votes)
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  • leafers seedling style avatar for user Michael Fu
    If Astrocytes exists only in CNS, therefore for PNS, there will be only 3 ways to remove the neurotransmitters:
    - diffusion
    - re-uptake
    - enzymes
    is this right?
    (3 votes)
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  • blobby green style avatar for user Timothy J. Calcara Jr.
    Could you explain a little more about the blood brain barrier? I have heard Matt talk about it in other videos. Is it just like it sounds, the astocytes using their end feet to prevent blood from entering the CNS? Am I missing anything important? Thanks.
    (2 votes)
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    • piceratops ultimate style avatar for user ILoveToLearn
      The BBB is anatomically composed of the tight junctions of the capillary endothelial cells which provides a very efficient barrier to large molecules and/or molecules that are not coupled to a carrier. This presents a challenge to pharma aimed at the CNS by requiring carriers and other mechanisms to allow the drugs into the protected CNS.
      The astrocytic endfeet insulate synapses and cerebral vasculature. They form a ring around the perivascular pericytes (I will explain if you have not heard of them yet) which themselves form a ring around the endothelial cells of the BBB.
      It's an amazing system. If you have any more questions, please, ask away!
      (2 votes)
  • leaf green style avatar for user julie.vi.psy
    Glial scar function is function of microglial cells? isn't it?
    (1 vote)
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  • male robot hal style avatar for user Mahmoud Helal
    are all the glial cells are derived from neural stem cell?
    (2 votes)
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  • female robot amelia style avatar for user slay
    What exactly do you mean about the synapses needing to turn on and off for the neurotransmitters to work? If it's off, how exactly does it convert the information and send it to the target cells?
    (1 vote)
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    • blobby green style avatar for user Calvin Lee
      I believe the point he was trying to make is that synapses would be non-functional if neurotransmitters "lingered" between the neuron and target cell. In this way, the presynaptic neuron must have the ability to "turn on" the synapse through release of neurotransmitters and "turn off" by removing them. The astrocyte helps to remove these neurotransmitters thus effectively resetting the synapse for further use.
      (2 votes)
  • duskpin ultimate style avatar for user The Q
    I've heard reference to 'glial cell plaque' with regard to Alzheimers. Is the the same as or related to a glial scar?
    (1 vote)
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  • blobby green style avatar for user Rafid Khalid Nahi
    What things should I know before studying the potential stuffs of the neurons?
    By this I mean any pre-knowledge in chemistry or any membrane stuff?
    (1 vote)
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Video transcript

In this video, we're going to talk about astrocytes. And in their name comes from the Greek words for "star cell." Astrocytes are glial cells of the central nervous system, which are derived from neural stem cells. Astrocytes have a soma of variable number and branches of processes. But they often have a lot. They often have quite a few processes that are highly branched, which is how they got their name of star cells. Because some people thought they look like stars when they looked at them under the microscope. And at the end of their processes, they have special structures called end-feet. Let me draw in a few of these end-feet that are at terminus of the astrocyte processes. So that's end-feet. And that's all of these structures at the end the astrocyte processes. And astrocytes are work horses. They have arguably more functions than any other cell type of the nervous system. So first, we can talk about how the astrocytes really form the scaffold for the entire central nervous system. These cells really occupy a huge amount of the space of the central nervous system and form the majority of the structure that actually makes up the brain and the spinal cord. So they provide the structural support and the place for all the other cells to be, like the neurons and all the other glia. The second function we can talk about is called the glial scar. And that refers to what astrocytes do if there's injury somewhere in the central nervous system. So if there is some type of injury somewhere in the brain or the spinal cord, what we see is that astrocytes proliferate. They divide and form more astrocytes. And they migrate over to the area of injury. They then surround that area of injury. And their processes hypertrophy. They grow these much larger, thicker, longer processes that actually serve to wall off that area of injury. And between all the processes, they form a thick tissue, kind of like scar tissue that we see elsewhere in the body. But that scar tissue elsewhere in the body is formed by different cell types because the astrocytes are only present in the central nervous system. So this whole process of astrocytes reacting to injury has multiple names. It's called gliosis, or it's called astrogliosis, or it's called astrocytosis, or it's called reactive astrocytosis. And the actual scar tissue that's produced is called the glial scar. And the grill scar probably performs kind of a similar structural role to the astrocyte's role as the general scaffolding for the central nervous system. Because probably what they're trying to do is wall off an injured area. And particularly if there's a cavity, if a hole has formed, and they're trying to shore that area of structural support from this wall. Another function of astrocytes is homeostasis. And homeostasis just means trying to keep everything in optimal conditions in homeostasis of the interstitial fluid, that is the fluid between all the cells of the central nervous system. And this is very important because the neurons require a very fine-tuned, a very even keel environment for them to function properly. And if certain things like the concentrations of certain ions, and particularly potassium ions, if those concentrations are abnormal, the neurons can't function properly. So one thing the astrocytes do is that they're constantly monitoring the interstitial fluid. And they're either taking in ions or they're releasing ions to try to keep those ion concentrations exactly the same all the time, in homeostasis. Another thing they do for neuron sometimes is they release lactate into the interstitial fluid. And the reason they do this is because neurons have very little internal energy stores in their cells. Neurons are completely dependent on a continuous supply of oxygen and glucose to have all the adenosine triphosphate they need to perform their functions. Now, astrocytes do have some internal energy stores in the form of glycogen. And they can convert some of that to lactate and secrete that, so that the neurons can use lactate in a pinch if they have lost access to continuous oxygen and glucose. Another function of astrocytes is contributing to something we call the blood-brain barrier. And this is a barrier that prevents large molecules in the bloodstream-- so I'll just draw a little blood vessel passing through the central nervous system. And this prevents large molecules from leaving the blood to enter the central nervous system unless the cells actually want that large molecule to enter. Components of the blood vessels themselves play a role in this blood-brain barrier. But astrocyte processes also play a role. And, in particular, it's the end-feet. These end-feet, on the end of their processes, are plastered all over the blood vessels that are passing through the central nervous system. And they play a role in preventing certain large molecules from leaving the bloodstream and entering the brain. So that between the astrocyte end-feet and components of the blood vessels of the central nervous system, there's a quite effective barrier function between the blood and the central nervous system. And this includes a spinal cord. But we traditionally call this the blood-brain barrier. And one more very important function that the astrocytes perform is that they help to clear out synapses between neurons. And so synapses are the connections between neurons and their target cells. So if we drew an axon of one neuron coming down here. And this will be the terminal of the axon. And then this axon is forming a synapse with say the dendrite of another neuron here. Just like the astrocytes are using their end-feet to surround the blood vessels that pass through the central nervous system, the astrocytes are also extending their processes and placing their end-feet all over synapses so that the end-feet are plastered all over the synapses and they're actually helping to clear out those synapses. And we'll cover this more when we cover synapses. But basically they're clearing out the molecules that communicate between neurons and their target cell, called neurotransmitters. And this is very important to reset the synapse so that it can be used again for communication between the neuron and its target cell. Because if neurotransmitters just lingered in the synapse, then that synapse wouldn't be functional any more. It would just constantly be turned on. When instead, the neurons need to be able to rapidly turn on and off the synapses to be able to communicate information effectively. In addition to these functions, astrocytes appear to influence neurons and other glia, and vice versa, through exchange of a variety of other substances. So as you can see, astrocytes are very hard working cells in the central nervous system. No other cell of the nervous system appears to do such a huge variety of functions like the astrocytes do.