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Introduction
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What up. Meditay here. Let’s talk about the anatomy of the Central Nervous System.
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In this segment, we will go through the base in understanding how the anatomy of the CNS is built.
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And to do that, we’ll first go through the Parts of the CNS, then we’re going to go through the
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microscopic structures of the central nervous system, basically understand what Neurons and
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Neuroglia are and how they’re distributed in the CNS. After that, we’ll be talking about
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the distribution of white and grey matter and talk about nerve tracts. And then end by talking
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about the general nervous system development. Alright, so the central nervous system consists
Parts of the Central Nervous System
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of two main parts. There’s the Encephalon or the brain. And then the Spinal Cord.
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But the brain is also divided into functionally different parts,
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so if we look here, we have the spinal cord. And then, above the spinal cord,
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we’ll find a structure called the brainstem And the brainstem consists of the Medulla,
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or Medulla Oblongata, The Pons, and the Mesencephalon. Behind the brainstem,
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we’ll find the Cerebellum, which is an essential part of the brain for muscle memory. Above that,
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there’s the Diencephalon, which’s the area you’ll find the hypothalamus. And then we have the
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Telencephalon, which is what we call the highest order in our brain where our personality is.
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And so the way all of this works is that Nerves pass signals towards the higher senses of the
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brain, then there are nerves that interpret the signals, which then generate an impulse, basically
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activating neurons that send signals towards a muscle or an organ to activate a response.
Neurons
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And so I say neurons because that’s the primary type of cells in our Nervous System. If we take
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a segment of the spinal cord and look at it underneath a microscope, you’ll see
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that they’re composed of nerve tissue. And if we take a small segment of the nerve tissue,
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you’ll find a lot of these cells we call a Neuron. Let’s talk about the neuron a little bit. Here
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you see a simple animated neuron. They consist of Dendrites. Dendrites are what receive signals and
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send them towards the Cell Body, which contains a nucleus, of course, since it’s a cell. The
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signals are then sent through an axon, which are long fibers that can extend at a large distance.
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There are nerves that begin in your lower back and extend all the way to the tip of your toes,
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thanks to the length of the axon. And at the end there, that’s the axon terminal.
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Now, axons can either be wrapped around by many Shwan cells or Oligodendrocytes. If they’re
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in the Brain or Spinal cord, they’re called Oligodendrocytes. If they’re outside the CNS,
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they’re called Schwann cells. These cells form a myelin sheath around each segment it covers.
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So axons can either be wrapped around in a myelin sheath, or they can be free axons that
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aren’t covered by these myelin sheath. The Myelin sheath help transmit the signal much, much faster.
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And so this is how a general nerve impulse look like. A signal is sent from one cell through the
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axon into a dendrite of the next cell, which then travels towards the cell body. And then through
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the axon and into the dendrite of the next neuron. So if we go back to this picture. You’ll now see
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the Dendrites here, the cell body, and an axon going out from the cell body.
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You’ll study this more in histology, but the way you can differentiate a dendrite from an axon
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underneath a microscope is by looking at the granules within the neuron. The axon doesn’t
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have these granules, and you can see a clear margin between the axon and the body here.
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and so do all nerve cells look like this? the answer is no, unfortunately
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Nerves cells are actually characterized by their shape. We have a Multipolar Neuron
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with one axon and many dendrites that give the cell a star-like shape. These
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are widespread in the central nervous system. Then we have Pseudo-Unipolar Neuron. It’s unipolar
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because the axon and dendrite emerge from the same place from the cell body. And it’s Pseudo;
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Pseudo means false or fake. It’s false Because the Signal still has to go through the cell body
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to reach the axon. That’s why it’s fake. It’s not a straight line that has to go
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through the body and then to the axon. And we also have bipolar nerve cells
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where one axon and one dendrite emerge from either side of the cell body.
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So nerve cells differ in structure depending on where you find them, but neurons also differ in
Functions of Neurons
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function. And there are 3 general functions a neuron can have. A nerve can be an afferent or
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sensory nerve. It can be an Interneuron, or it can be an efferent or motor nerve.
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Let me give you a simplified example of how this works. Let’s say you wake up in the morning and
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see a coffee, and you don’t just see it. You can also smell the coffee or hear the coffee machine
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working. All of those neurons being stimulated will lead the signal towards the central nervous
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system as sensory neurons or afferent neurons. Then, these signals are interpreted in your brain
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through interneurons. And suppose you’ve decided that you want the coffee.
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In that case, the brain is going to engage motor neurons, or efferent neurons, to activate
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muscles in order to pick up the coffee cup. So Remember, Afferent neurons Arrive into the CNS,
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Efferent Neurons Exit the CNS. Ok, so we now understand what a neuron is.
Neuroglia
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But you also need to visualize the fact that they’re not alone in the CNS. There are countless
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cells we call Neurogllia that give mechanical support and give nutrients and protection
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to the nerve cells, as you see here. And so we got many different types of NeuroGlia in the CNS
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We got Astrocytes, which are the largest neuroglia.
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These astrocytes have long projections that wrap around the blood vessels within your CNS,
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and they form a so-called Blood-brain barrier. There are Oligodendrocytes, which are responsible
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for the myelination of nerves in the CNS. Remember, in the peripheral nervous system,
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there are Shwan cells, and in the CNS, there are Oligodendrocytes.
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So that’s that one, forming a myelin sheath. We got Microglia, which are the smallest neuroglia.
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These are basically the immune cells of the CNS. They can do everything a macrophage does,
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like phagocytosis, and migrate between the tissue. And lastly, we also have Ependymal Cells, which
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line all the cavities within our central nervous system. So that was the two um main categories of
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cells in the CNS. Neurons and Neuroglia. But the tissue in our CNS is distributed
Distribution of White and Grey Matter
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as either white or grey matter. Grey matter is tissue rich in Nerve cell Bodies and Dendrites.
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White matter is tissue rich in myelinated axons and glial cells.
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And if you look at this neuron. In reality, the whole Neuron is gray in color. They’re all gray
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underneath the microscope without any significant staining. The Axons with myelin sheath around
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are white because they’re so rich in lipid that they appear white underneath the microscope.
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So cell bodies and Dendrites are grey matter, and myelinated axons are white matter.
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And as we study the CNS, we often need to look at cross-sections to study the tracts and nuclei
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within each segment of it. Like in the spinal cord and the brain. In the spinal cord,
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you’ll find the gray matter centrally and the white matter around it. And in the brain,
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you’ll find the gray matter at the external border, we call it the cerebral cortex, and you’ll
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find grey matter in some places within the brain itself. Everywhere else is gonna be the white
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matter. So gray matter, cell bodies, and dendrites. White matter, myelinated axons. Cool.
Nervous Tracts
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Now. Here’s something you’ll see a lot when you study the CNS anatomy. Its Nerve Tracts. Nerve
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tracts are a bundle of axons that connect Gray matter to Gray matter. Or fibers that
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connect Nuclei to Nuclei. So imaging a hand that either touches something, senses temperature or
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gets pinched. All of those will activate specific nerves that lead impulses towards the spinal cord,
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leading the signals through specific places in the brain and spinal cord in order to understand
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what happened and react to it. What I want you to know is that in Grey matter, we got nuclei,
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and in white matter, you got Tracts. Now there are certain ways to classify
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these Tracts. You can either classify them as association fibers, connecting adjacent
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structures, Comisural fibers, connecting one part of the brain to the other side,
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or projection fibers, leading tracts up and down the spinal cord. I will talk more about this when
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we talk about the internal structures of the Cerebral Hemisphere because that’s when this
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classification becomes relevant to you. But the most important thing to remember,
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which you’ll hear about a lot, are Ascending tracts, leading sensory fibers. Descending tracts,
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leading motor fibers, and Indirect Tracts that interconnect certain parts of the brain. This
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is another way to classify nerve tracts. So that’s all I had about nerve tracts for now.
Nervous System Development
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Lastly, let us understand the principle of how the CNS is developed. Once you understand that,
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you’ll also understand why the CNS is built like it is and why the adjacent structures
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often have the same function. So if you look at the real primitive brain,
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we find that we have these four humps at a time of 4th week after fertilization.
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And we call those. Well, the first one is not a hump but the spinal cord.
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We then have the Hindbrain, the midbrain and then we finally have the forebrain.
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Or in Latin, the Rhombencephalon, Mesencephalon and the Procecephalon. But during development,
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your brain changes drastically. So already during the 5th week, you’ll notice these
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humps are starting to form shapes. You gonna see that the Rhombencephalon
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and the Prosencephalon are gonna divide. The Rhombencephalon divides into the Myelencephalon
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and the metencephalon, where they’re later on gonna become the Pons, Cerebellum, and
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the Medulla oblongata. The mesencephalon is just gonna stay like that. It’s called the midbrain.
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And then, the prosencephalon will divide into the telencephalon and the Diencephalon.
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And so this is what an adult brain looks like. The spinal cord is down here. And again, the medulla
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oblongata, pons and the Cerebellum are all formed by the Myelencephalon and the Metencephalon.
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The mesencephalon is a synonym for the midbrain since it doesn’t divide.
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The Diencephalon will become all the thalamus structures like the hypothalamus and the thalamus.
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And the Telencephalon is the actual brain cortex and its fibers here in blue.
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— One thing you should remember is that the closer
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we are to that spinal cord, the more basic the functions are. And so down here at the hindbrain,
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they’re responsible for simple functions. So they’ll regulate the respiratory frequence when
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you’re not thinking about it, cardiac function, vasodilation, and reflexes like vomiting,
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coughing, sneezing, and even swallowing are considered basic functions.
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And if you have any deep thoughts about something or you decide to do a simple act,
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that’s going to be your cerebral cortex giving orders to the rest of your body.
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And so this was an overview of how the CNS is distributed and its function and development.
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In the next videos, we’ll be looking detailed into the anatomy of each of these parts and understand
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how they function. So the next video will be going through the whole anatomy of the Spinal Cord.
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