It becomes important when we talk about the nervous system to discuss the role of the glial cells. Glial cells are support cells for neurons. They are much smaller than neurons, but they are much, much more numerous.
And the very most numerous amongst them are the astrocytes. So the first one we're going to consider is the astrocyte. Just so you know, in this picture, the first four, 1, 2, 3, 4, all of these are support cells that exist within the brain and the spinal cord.
And down here we have two cells listed, which are the Schwann cells and the satellite cells. These exist in the peripheral nervous system and support the peripheral neurons. Okay, so the first cell, definitely by far the most important of the support cells. We're learning more and more all the time about its role is the astrocyte. The astrocyte is the most abundant of the support cells, the most abundant of the neuroglia or the glial cells.
Its job is to deliver nutrients. from the capillaries, shown here, directly to the neurons, shown here. You can see the cell body of this neuron and one of its axons or dendrites. It's involved in capturing excess salts and neurotransmitters, and it does have a role in cell signaling. It used to be thought that the only cells that existed within the brains that had any...
role in cell signaling, as in communication between cells, were the neurons. But recent research has become more and more interested in the role of the astrocyte. For example, the role of the astrocyte in schizophrenia has become of great interest to us. Okay, so just to describe exactly what's happening in this picture, neurons do not have direct contact with the blood.
They're actually very protected from the blood. It's called the blood-brain barrier. The blood-brain barrier is made of two structures, the walls of the capillaries that move through the brain and then these foot processes of the astrocytes which if you'll notice completely wrap around the blood vessels.
There's a little bit of capillary sticking out here only so that you can see that it is actually capillary but this would in fact be completely coated with the end processes of the astrocytes. So these two walls then, the wall of the capillary, the foot process of the astrocyte, does not actually allow any kind of blood to actually physically enter into the brain and interact with the neuron. However, as I suggested in the last video, neurons are highly metabolic.
They actually have a high need for oxygen and nutrients. And so it's the job of the astrocyte to grab that stuff out of the blood supply. and deliver it to the neurons.
Okay, next we have the microglia. Microglia are the opposite of astrocytes. Rather than allowing for neurons to become healthier and supporting them in a healthy way, microglia will actually clean up neurons that are damaged or dead or dying or in any way sending out some kind of a signal that they shouldn't be there anymore. So The job of the microglial cell is to monitor neural health and act as a macrophage, which means that they're able to actually consume dead cell parts and move them along.
There is no immune presence, per se, in the central nervous system, and so whenever something needs to be cleaned up, the microglia will do it. Recent research has pointed to the idea, actually, that there does seem to be some kind of... possibly immune presence in the brain, but we're not sure what the role is yet. We do know that microglia are oftentimes present in areas, for example, where there are there is any kind of cell damage in the brain.
Oftentimes you'll find markers for microglia in those regions. Next we'll look at the ependymal cells. The ependymal cells, if you look at them, they actually look a lot like epithelial cells and that's exactly the kind of role they actually take on.
Appendimal cells are oftentimes ciliated. They line the central canals of the brain and the central canal of the spinal cord, and they allow for a permeable barrier that circulates a fluid in the brain that is called CSF or cerebral spinal fluid. So you'll see that there are cilia here that help to circulate the fluids along. There is no blood inside of the brain, but the cerebral spinal fluid takes on some of the roles.
blood so that becomes a very important aspect of brain support The last cell that is found inside of the brain is called the oligodendrocyte. And the oligodendrocyte in the central nervous system, just like this guy down here, which is the Schwann cell of the peripheral nervous system, the role of both is similar in that they secrete myelin sheaths. So the processes of myelin will wrap around axons, and what they're called in the brain are fibers. So axons in the brain are called fibers. And then that allows for a faster conduction of nerve communication within these structures.
All right, again, in the peripheral nervous system, we have Schwann cells, which secrete amylin fiber, which allow for fast conduction along axons, that a signal that is coming from the cell body will be able to make its way toward the axon terminals that much faster. So... The role of myelin is incredibly important, and when it is not present or when it is damaged, for example, in multiple sclerosis, it becomes a tremendous problem.
Okay, so adjacent Schwann cells will have gaps in between here, which are called nodes of Ranvier, and this allows for myelin, whenever you have myelin present, which is in... Pretty much most neurons, I would say the vast majority of neurons, actually have myelinated axons. This allows for your nerve impulses to travel very quickly.
The last cell of the peripheral nervous system is called the satellite cell. Here you've got the cell body, and surrounding it you have what looks like five different satellite cells here. We don't actually know the entire role of the satellite cell, but it does seem to be some sort of supportive role for the cell body directly.
I think one more thing that we want to say about the myelin, because it is a really important topic, is just kind of showing a closer up view of how it looks. This is a Schwann cell that as you can see is in fact a living cell. It has a nucleus here, it has organelles. Its job is to secrete this myelin and wrap itself around.
You'll see that it wraps around many times. around an axon. These are not found in dendrites, only an axon. So you can tell you're looking at an axon because it's almost always myelinated.
So the myelin sheath will wrap around many many times. It's still a living cell. Here is a cross-section of an axon surrounded completely with a myelin sheath with a bunch of different roles around the axon.
The myelin itself is a glycol lipid. It's a fatty kind of a structure.