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