Intro to Cells of the Nervous System (Neurons Series)

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Introduction to the Cells of the Nervous System. We are in 3A, which is right here. So in this lesson we're going to be looking mostly at the structure of the neuron and a little bit at glial cells as well. You're probably used to seeing neurons drawn something like this image here on the screen.

And I do encourage you to just take a minute and try and match up the parts of the neuron with the words here in the box. Maybe you know this stuff really well, maybe you haven't seen it in years, but go ahead and just take a second and try that out. Okay, great, so let's go through them. So up here we have the soma.

It's also called the cell body and it has all the things you would expect to find in a human cell. So, many different organelles and it definitely has a nucleus. And the cell body, it performs most of the functions needed just to keep the neuron alive. And it's also the place where the neurotransmitters are produced.

So these branches coming off of the soma are called the dendrites. This long part right here is the axon. And the axon is how messages get sent from one neuron to the other over distance. F is the axon hillock and the axon hillock is actually part of the soma. It serves as a tight junction and it generates action potential based on whatever else is happening in the dendrites or in the soma.

In the next lesson we'll be looking at that a little bit more. It's full of voltage gated ion channels. When voltage gated sodium channels open, an action potential can propagate down the axon. And when an action potential reaches the base here, which is sometimes called the axon terminal, in h we have the synaptic buttons.

The action potential itself stimulates vesicles to release their cargo. And that basically shoots the messages, neurotransmitters, into the synapse, and then this can start an action potential in the next neuron. And so basically that's how the neural impulses get transmitted from one to the other, it happens very quickly, as you would imagine. So two elements that really kind of speed up this whole process are e, the Schwann cells, And d, the nodes of Ranvier.

Now the Schwann cells are really dense in myelin. Myelin's made of cholesterol and also proteins, and it serves a couple of different functions. It mostly insulates and speeds up the impulse by just making sure the impulse doesn't get lost to their surrounding atmosphere. And so then 6, the nodes of Ranvier, are in between the Schwann cells.

These location also, the nodes of Ranvier, are full of ion channels. And they're actually even more densely situated than they are in the axon hillock. Both of these places are locations where the neural impulse gets like rebooted, due to the opening and closing of ion channels. It keeps the impulse alive over distance.

So another thing I wanna say about the Schwann cells is that the Schwann cells are the name for the cells that surround the axon, but only in the peripheral nervous system. So the axons in both the central and peripheral nervous systems do have cells comprised of myelin, but they have different names depending on the location. And that's part of what we're gonna go over in the next couple of slides.

So before I talk about the different kinds of neurons and glial cells, I wanna just show you a chart overview of the PNS and the CNS, and this is just to reorient you. So, let's look at glial cells. There are six major categories, four in the CNS and then two in the PNS. And we've already hit on Schwann cells. So, a glial cell, also called neuroglia, is a cell that provides support for neural cells.

And in some cases it's an axon, as with the Schwann cells and the myelin coding that we looked at. And in other cases the support is about providing nourishment to the cell body or removing harmful elements in the environment. So I mentioned before that Schwann cells make up the myelin sheath in the peripheral nervous system.

In the CNS, the cells that have this function are called oligodendrocytes, but they're built differently. So one oligodendrocyte can wrap its extension around dozens of neighboring axons. And for any given axon in the CNS you typically have several oligodendrocytes, each wrapping one foot around a portion of the axon.

So it's very much a network model with tons of glial cell bodies, and they're pretty much popping up between axons in every direction. Whereas in the PNS it's all very smooth and streamlined, because the entire body is contained here, pretty mush flush with the axon. So going back to our chart, I next wanna talk about the glial cells that support the spine.

And thats right here, it's call the ependymal. Ependymal cells line the brain ventricles as well as the spinal cord. They help produce cerebral spinal fluid and they also can absorb the excess, so basically they're regulating the amount. And in addition to that, it seems like they have neuro-regeneration properties, but there isn't a lot of research yet on the extent of this.

Next I'm gonna look at these two in parallel, because they have very parallel functions. So we have astrocytes in the CNS and satellite cells in the PNS. And they both provide nourishment, structure, and they help with message signaling, and also maintaining ion balance. And that would be the ions that are in the extracellular environment, so outside of the neural cell.

And these two particular types of glial cells provide most of their support to the soma, to the cell bodies. So let's look at each of these separately. The astrocytes are one of the most abundant types of glial cells in the brain, and they regulate blood flow in the brain. They're also able to fuel neurons with glucose, and in addition to that they transport a wide variety of neurotransmitters.

Another function they perform is they regulate electrical impulses in the brain and they provide a lot of support to endothelial cells. And those are cells that line the surface of blood vessels. And scientists used to think that astrocytes played a major role in the blood brain barrier, and that's because they wrap their feet around these endothelial cells.

And so it appeared as though they were helping with this barrier function, but as time's gone on, it looks like they really aren't playing that big of a role after all. Satellite cells cushion and nourish and provide structure for the soma of the ganglia, and so that includes the sensory, or also called afferent nerve, and also the motor neurons.

And motor neurons are efferent. They have a lot of cytoplasm and that helps provide cushioning as well as just some structural support and nourishment. And two other functions, they moderate aspects of the pain response and they house a variety of enzymes. They can provide glutamine and lactate and a couple of other substances as well directly to neurons.

Microglia are macrophages. So they remove damaged nerves, DNA fragments, could be plaque, and they also have the ability to destroy harmful intruders. Although there aren't that many, because the blood-brain barrier keeps most of them out. However, in those cases they do serve an immune function.

Once they're full and they can't macrophage anymore, they are called gitter cells. And these show up on brain scans if there are enough of them in a dense region and give scientists an indication that there's been an infection in a particular area of the brain. And then eventually they're washed out through the blood or through the cerebrospinal fluid.

Okay, practice question for you, which of the following would not be found in a sympathetic ganglia, pseudo-unipolar neurons, astrocytes, satellite cells, or Schwann cells? So this is to teasing you basically on your overview of the nervous system. The answer is B, astrocytes. Sympathetic ganglia are part of the sympathetic nervous system, which is part of the autonomic nervous system, which is part of the peripheral nervous system.

And so without even thinking that much about A, C, D, if you can remember right off the bat that astrocytes are in the central nervous system, that would be the odd man out here. But if you wanted to be a little bit more thorough about it in your thinking, we can remember that the pseudo-unipolar neurons are found in the ANS. And that both of these are support cells, glial cells in the PNS.

So astrocytes is the answer, because it's found in the central nervous system.

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