Full transcript
0:02In this video, it's going to be a short one, we're going to talk about the neuroglia
0:09cells, the support cells. We will also talk about the myelin sheath, some
0:14disorders of the myelin sheath, and then how nerve fibers can regenerate.
0:22There are about one trillion neurons in the nervous system (give or take).
0:27Neuroglia cells will outnumber them by ten to one, so you have much more of the
0:34support cells than the actual neurons. So the neuroglia has a variety of
0:40different functions: protection, binding those neurons together, giving
0:46them a framework, guiding them where they should go, especially in the fetus
0:53when you are producing the neurons. Remember that after about age 7 you do
1:02not do any more mitosis of the neurons and so the only cells that do mitosis are
1:09going to be the neuroglia cells. There are four kinds of neuroglia cells in the
1:15central nervous system. The oligodendrocytes are the ones that make
1:19the myelin - we'll talk about that myelin a little bit later. That increases the
1:25speed of the electrical signals. The ependymal cells are lining the cavities,
1:31these open spaces in the brain. It produces the cerebrospinal fluid and
1:37moves that cerebrospinal fluid. These ependymal cells will have cilia to
1:43help move that along. Microglia are actually white blood cells
1:47and so they wander through doing phagocytosis and of course they
1:53destroy the bacteria by merging that phagosome with a lysosome. The last type
2:03of neuroglia in the central nervous system are the astrocytes. So they look like
2:08little stars - "astro" refers to a star shape. So they cover
2:16most of your brain surface and help produce this nice little framework for
2:23your neurons. They also have these little "feet" -
2:27they cover blood vessels and so they form a blood-brain barrier. You do not
2:33want to certain things, such as certain electrolytes from your blood, to
2:39leak into your brain. That will cause an imbalance in your central nervous system.
2:44They absorb excess neurotransmitters - too much of that neurotransmitter can
2:49actually be deadly. If you destroy neurons in your brain, that empty space
2:57is going to be filled up by astrocytes. They do mitosis - so we can call that
3:02astrocytosis - and they actually produce some scar
3:06tissue or sclerosis. If we look at the central nervous system neuroglia, you
3:12can see this figure shows them - here's your astrocytes looks like a star and it's
3:18covering this blood vessels - the little feet are covering the blood vessel.
3:21Ependymal cells are producing the cerebral spinal fluid. You can see the
3:26cilia on one side of it. This is your oligodendrocytes - you can see the little
3:32fingers coming around and making the myelin on the neuron's axon and then this
3:38is the microglia. This is that type of white blood cell moving around doing
3:43phagocytosis. In the peripheral nervous system, we have two types of glial cells.
3:50The Schwann cells are the ones that make the myelin sheath so the myelin is
3:58very similar to the myelin that the oligodendrocytes produce but it is made
4:04in a different way. They are also able to help regenerate and repair damaged axons.
4:13This is something the oligodendrocytes cannot do. Satellite cells are very
4:19similar to the astrocytes in your central nervous system. So they
4:25are covering the cell bodies (of neurons) that are found in ganglion and they have a
4:31kind of an insulation around these cell bodies and regulate the chemical
4:37environments. Let's look at the myelin sheath. So this is the insulation around
4:43an axon. Remember the axon can be the longest part up to 3 feet long and so
4:50you want to speed up the electrical signal in this long section. So the
4:57oligodendrocytes are the ones that make the myelin in the central
5:02nervous system and the Schwann cells are the ones in the peripheral nervous system.
5:05That myelin is actually very fatty. It is actually the plasma membrane
5:12from those glial cells and so you remember what the plasma membrane is
5:18made out of - about 20% protein in that membrane and then 80% lipid. These are
5:25the phospholipids that you learned in chapter 3. So that myelination - when we
5:33use that term, this is when you make that myelin sheath - so it is made of,
5:41is actually part of these living cells. The oligodendrocytes in the central
5:47nervous system; the Schwann cells in the peripheral. It'll start (myelination) even when you are
5:53fetus - about week 14 of fetal development and it goes all the way through
6:01your childhood until about late adolescence. So remember what that
6:06myelination is doing - it's speeding up the electrical signal. So dietary fat,
6:13obviously since most of this plasma membrane is phospholipid, is going to
6:18be extremely important for the central nervous system to develop correctly. So
6:23why do you need that speed? Things like balance - you are constantly sending
6:29information up to your brain on how to keep your balance and that requires
6:34speed. Let's look at how we make myelin in the peripheral
6:39nervous system. The entire Schwann cell is spiraling around a
6:46single axon - so there can be as many as a hundred layers. There's no cytoplasm; so
6:53you are squeezing out the cytoplasm between these membranes, these layers.
7:00The neurilemma is the last loop around and so this is your Schwann cell that has
7:10the nucleus, most of the cytoplasm, all of the organelles and so kind of the
7:18last loop is the neurilemma. On the outside of that neurilemma, you will have a
7:24connective tissue membrane - that's your endoneurium. And that is actually kind of
7:30protective layer on top of your Schwann cell. So we look at this (figure) - this is
7:37cut through so you can see your axon. And then here is your Schwann cell. The
7:44last loop is the neurilemma. You can see this a little better (next figure) as it's producing
7:51the myelin. So here's the Schwann cell around the axon; he loops and loops and
7:57loops - the inside layers are very, very tight.
8:02There is no cytoplasm in between the cell membrane. Now remember these are the
8:08cell membranes of the Schwann cell. The last loop has all the organelles, the
8:15nucleus of that Schwann cell - that is called the neurilemma. So this very
8:21packed plasma membrane is called the myelin sheath. Now when we look in the
8:26peripheral nervous system you also have an extra connective tissue membrane on
8:32the outside of this - kind of protecting the axon, protecting the Schwann cell -
8:37called the endoneurium. Let's look in the central nervous system - the
8:45oligodendrocytes can myelinate several nerve fibers, several axons.
8:52So the whole cell does not wrap around the axon like the Schwann
8:59cell - what he does he sticks out a "hand" and he kind of uses his fingers and wrap
9:07around part of an axon. So there's no neurilemma
9:12and there's no endoneurium. If we look at it (figure) here is one oligodendrocyte -
9:19you can see a big nucleus in there. He sticks out this one "hand", wraps
9:25around that axon. Sticks out another "hand", wraps around that axon. Sticks out
9:31another, wraps around. So he can't have that connective tissue membrane around
9:36it - no endoneurium - and there's no neurilemma because the cell itself is not
9:46wrapping around, it's just the little "hands".
9:50So you are going to have multiple Schwann cells, multiple oligodendrocytes to cover
9:57up one axon. That myelin is going to be segmented - that means you have pieces
10:04that have the myelin and pieces that are exposed. So the gaps between that
10:10show the axon, the exposed axon, are called nodes of
10:16Ranvier and the internodes are the myelin covered segments. The trigger zone
10:25is going to be at the very beginning of the axon, the initial segment, and so this
10:31is where you start a nerve signal and so we called that particular nerve signal
10:38an action potential. You always hear about brain tumors. So most brain tumors,
10:49especially if we're talking about in adults, are not coming from neurons. These
10:56tumors could originate from the meninges. It could have spread, metastasized, from
11:05other tumors like from melanoma. It can be from the glial cells because the
11:13glial cells can do mitosis throughout their life. So if it's glial cells, they
11:19become gliomas - they're very, very malignant and,
11:23unfortunately, we have to treat them mostly with radiation or surgery because
11:29our blood-brain barrier is so good, with chemotherapy it blocks a lot of the
11:37drugs from getting into the brain and treating those tumors. Let's look at
11:45some of the diseases of the myelin sheath. So one of them is multiple
11:50sclerosis. This is in your central nervous system. So the oligodendrocytes
11:56and the myelin sheaths are beginning to breakdown and the myelin is now
12:01replaced with scar tissue. And so your nerve conduction is going to be
12:07disrupted because of this scar tissue. You have some symptoms such as
12:14double vision and tremors, speech defects, numbness. Typically this occurs between
12:22the years of 20 and 40 but it can occur earlier or later and most MS is due to
12:31an autoimmune disorder. So you were infected with a particular virus. Now
12:40your immune system is making a T-cell or an antibody that is confused - that now is
12:48destroying your myelin sheath. So you can see here (figure) on this normal nerve; (next figure) this is
12:54the exposed (axon) breaking down that myelin sheath on here with MS. The nerve signal
13:02is going to be disrupted if the myelin sheath is being damaged. Here's two scans
13:09of the brain - normal healthy brain (first scan), you can see the plaques
13:15(scar tissue) here in this brain. Another disease of the myelin sheath is
13:22Tay-Sachs. And so this is a genetic disorder where you are accumulating a waste
13:30product and it's going to disrupt the conduction of those nerve signals.
13:37It is always fatal, typically before age 4. You'll be learning about the genetics of
13:45this in A&P 2. So what's happened is that the organelle that should be
13:53breaking down this waste product is not functional - and that would be your
14:01lysosome that is not functional in this particular disease. The speed of the
14:08nerve fibers, that (electrical) signal, depends on two factors. The diameter of the fibers,
14:14so how big around is your axon - the larger the axon, then you
14:23have more surface area and you have faster signals. And then the major factor
14:29that affects the speed of that electrical signal is myelin - you have
14:35myelin, it goes much, much faster. So you have slow signals (you don't care about
14:43speed) if you're sending it to your intestines, so speed is
14:50not an issue. Where you need very fast signals is to your skeletal muscles. This is
14:55going to help with coordination and balance. With nerve regeneration - you
15:05don't have really regeneration in the central nervous system only in the
15:09peripheral and you can only help with repairing the axon if the cell body (the
15:19soma) is intact and you still have some neurilemma from the Schwann cell. So
15:26what happens is that if the axon has been broken, then
15:31everything from the injury on down to the tip of the axon degenerates and
15:36that's got to be cleaned up. The stump of that axon then starts "sprouting". Your
15:43Schwann cell is actually going to start moving in and they make a little tube, a
15:49regeneration tube, that that axon will grow through. Now we've got some problems
15:57because sometimes it's not fast or perfect and sometimes it's not possible.
16:04So it can take up to two years to regenerate a nerve. Sometimes it connects
16:12up with the wrong muscle - so that little tube (the regeneration tube) went to the
16:17wrong place. If we look at your central nervous system, nerve fibers don't
16:24regenerate at all. So we look at this (figure) - here is my axon in the peripheral (nervous system). If
16:31it's been damaged, then everything - all of this - degenerates. The end of it is going
16:38to start to sprout. You need to have this little regeneration tube, here, from your
16:44Schwann cell to guide the end of axon and get it to the correct muscle. All
16:55right so this is the end of video 2 - the next video is very important. We're going
17:00to be looking at the electrical signal.