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Introduction
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What’s up. Meditay here. Let’s talk about the anatomy of the Central Nervous System.
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In this segment, we will be talking about the external anatomy of the spinal cord.
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basically, go through everything you need to know in regards to what the spinal cord is
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and what you’ll find grossly on the spinal cord. Alright, so the Central Nervous System consists of
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two parts. The encephalon, and the spinal cord. So in this is video, we’re first going
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to go through the Topography of the Spinal cord, basically where it is,
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where it starts and ends. Then we’ll focus on the external surface of the spinal cord,
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basically going through all the grooves and fissures you see there. We’re also going to go
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through the segments of the spinal cord and look at its relationship with the vertebral column.
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Then we’ll go through the enlargements we see on the spinal cord. After that we’ll look at
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the anatomy of a spinal nerve, and understand its 4 branches, and then quickly understand the
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types of reflex arches we can have through the spinal cord. The internal structures and all the
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nuclei and tracts will be covered in the next video so that this video doesn’t get too long.
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Alright, so here we see a posterior view of the vertebral column, if we remove one vertebra and
Topography of Spinal Cord
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zoom in, you’ll see the spinal cord right here, going through the vertebral canal.
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So let’s go ahead and take the spinal cord out. Now the spinal cord is covered by a meningeal
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layer called Dura Mater. And if we remove the dura mater, you’ll find the Arachnoid mater,
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and if w remove the arachnoid mater, we’ll see a very thin and delicate connective tissue covering
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called the Pia mater, and if we remove that, we’ll finally get to the actual spinal cord.
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These three are what we call Meninges and they cover the whole central nervous system. We’ll go
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through these in a separate video. But now, let’s do the topography of the spinal cord.
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So the spinal cord starts off at the Foramen Magnum, all the way to the L1/L2 vertebra region.
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The length of of spinal cord varies a lot, but in general it’s about 40-45cm long. If we now
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remove the bones, you’ll see that the spinal cord ends by a structure called the Medullary Cone,
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or Conus Medullaris, from here, a very thin thread goes out called Flium Terminale, which literally
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translates as the terminal thread because it’s consider as the continuation of the spinal cord.
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And if we see here, the Filum Terminale continues downwards together with a lot of different nerves
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supplying the lower part of your body with nerves. And this area with all the nerves, is called Cauda
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Equina. So that was it for the topography. Let’s now take a small part of the spinal cord
External Surface of Spinal Cord
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and look at the external surfaces of it. So here is the external surface of the spinal cord.
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This is the internal surface of the spinal cord which we will talk about in the next video,
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and these nerves we see here on the sides are the Spinal Nerves.
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Alright, now for some orientation. The anterior part is where you’ll find this deep fissure,
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and the posterior part is more flat, with small bumps. This fissure we see on the anterior side,
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is called the Anterior Median Fissure. Then posteriorly on the midline,
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you’ll find a Posterior median sulcus. On the sides of the spinal cord, you’ll find
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the Right and Left posterolateral sulci, and the right and left, anterolateral sulci. From where
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the anterior and posterior root of the spinal nerve are going to go through as you see here,
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we’ll get back to this later when we talk about the spinal nerve.
Spinal Segments
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But now, let’s talk about something called segments. Alright, so the vertebral column
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consist of 7 cervical vertebra, 12 thoracic vertebra, 5 lumbar, 5 sacral and this varies,
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but usually you have 4 coccygeal bones fused together. Adding a total of 33 vertebraes.
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Now the spinal cord is different in that, it’s divided into 8 cervical segments, not 7,
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but there’s still 12 thoracic segments, 5 lumbar, 5 sacral, but then only 1 coccygeal segment,
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which adds up to 31 spinal cord segments. Now why is the spinal cord divided into segments?
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Because at each segment, there’ll be one spinal nerve emerging from either side, like you see
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here. Meaning that picture we saw earlier, with one spinal nerve going out, is one segment. So we
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have 31 of those you see here. And if we add the nerves from my 3d anatomy program, you’ll see that
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it looks like this. Now if we take the vertebral column and the spinal cord, and fuse them
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together, you’ll see this. You’ll see the spinal nerves going out from the vertebral column through
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the Intervertebral openings, or the intervertebral foramina. But if the spinal cord has 31 segments,
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and the vertebral column has 33 vertebra. And as we know now from the topography, with the spinal
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cord ending at L1/L2 region. How are the spinal nerves arranged within the vertebral canal? And
Spinal Nerve and Vertebral Canal
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to answer that, we need to look at the spinal cord and the vertebra from this perspective.
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Let’s now fade this picture a little bit, and go through them part by part.
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At the beginning, the spinous process of the cranial cervical vertebra,
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cranial meaning up towards the head, correspond to the same level as the spinal cord segments,
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and the spinal nerve leave above the first cervical vertebra as you see here.
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Then as we continue slowly downwards. You’ll see that the spinal nerve starts to bend. And now,
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the spinous process of the caudal cervical vertebra, caudal meaning towards the tail,
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or away from the head, it correspond to one above the corresponding cervical spinal cord segment.
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And by this I mean Vertebra C7 is at the same height as Spinal segment C8. So you could
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take Segment +1 at this point. Because the spinal cord is getting compressed.
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Then as we continue down, the spinal nerves bend even more and at this point, the spinous
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process of the cranial thoracic vertebra, correspond to the thoracic spinous segment +2.
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So you add two numbers to the vertebra you’re looking at, so vertebra T3, is at the level of
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Spinal segment T5. And as you slowly continue down, the differences start to be greater.
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The Caudal thoracic vertebrae start to correspond to the Spinal segment +3… and then as you continue
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further down.. the T10-T12 vertebra becomes at the level of L1-L4 spinal cord segment,
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and then at the Vertebra T12, L1 we’re starting to reach the end of the spinal cord, but not yet, so
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we call this area the Epiconus, because remember the medullary cone is at the end, epi means above,
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so above the end of the spinal cord. That’s what epiconus mean. This correspond to L5-S2 spinal
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cord segment, and then lastly at the L1/L2 region, you have the rest of the spinal cord,
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from S3 to S5 plus the one coccygeal segment. This scheme is just to help you visualize
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how the spinal cord is arranged within the vertebral column. Now.
Enlargements of the Spinal Cord
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As you look at the spinal cord anteriorly, you’ll notice two distinct enlargements.
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One called the cervical enlargement, or intumenencia cervicalis. Which goes from the
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segment C3 to T2, and a lumbosacral enlargement, or intumenencia lumbosacralis, going from T12
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down to the medullary cone. Now why are these significant? Because at these regions, you have
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a bundle of nerves called plexuses, supplying the upper limb and the lower limb with nerves.
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And these nerves have to be large and in a large quantity, in order to innercate all the muscles of
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the lower limbs and the upper limbs with nerves. So the cervical enlargement forms the brachial
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plexus for the nerves that goes to both arms, and the lumbosacral enlargement is for the sacral and
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lumbar plexuses, innervating structures in the pelvis and the legs. So these are very
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important. Now since we’re talking so much about the spinal nerves, let’s really understand the
Spinal Nerve Anatomy
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anatomy of the spinal nerves. Because once you understand that, the actual internal and external
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surfaces of the spinal cord becomes more logical. So if we take a segment of the spinal cord again,
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you’ll see the internal surface here. We will talk about this in detail in the next video,
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but the internal surface consist of grey matter, and white matter. And how does this coorelates
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with the spinal nerve? Because all the small neurons that go within the spinal nerves,
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will synapse with nuclei in the grey matter. And I’ll show you how. So first you need to understand
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where the spinal nerves enter the spinal cord. The spinal nerve enters the spinal cord through
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the Right and left Posteriolateral sulci, and the right and left anterolateral sulci. Now lets
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animate it a little bit and add some structures to make it look a little more realistic.
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Here we can see the meninges. So the red that’s closest tot eh spinal cord,
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is the pia mater, the blue lining is the Arachnoid mater. And between the pia mater and the arachnoid
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mater is the subarachnoid space, filled with cerebrospinal fluid that provide nutrients to
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the spinal cord tissue. And then the outermost dense structure is the dura mater. So these are
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the meninges. And remember, the deep fissure is anterior, and the more flat surface is posterior.
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Now here you see the spinal nerve. The spina nerve is divided into two roots
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before it enters the spinal cord. It’s divided into the posterior root
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or the sensory root, and it’s divided into the anterior root,
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or the motor root. And I’ll mention this again, because I really want you to not forget this.
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Posterior root enter through the right and left posterolateral suci, and anterior root
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enter through the anterolateral sulci. But you’ll notice that on the posterior root, there’s a bulb,
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a small enlargement called a Spinal ganglion, or sometimes referred to as dorsal root ganglion.
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You’re gonna have many ganglions in the body, and the reason why ganglions are bubbly, is because
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they contain many nerve cell bodies as you see here. So dendritic fibers go from the periphery
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towards the spinal ganglion, and then the axons of these neurons go into the spinal cord.
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Now remember from previous video when we went through the different types of neurons.
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What kind of nerve do you think this is? These are pseudounipolar neurons, going into he spinal cord,
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so they take sensory information from anywhere in the body, and then enter the spinal cord so that
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you can sense what’s happening. So if you blow on your arm, that cold sensation is gonna enter
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the spinal cord through this neuron. And once it enters the grey matter of the spinal cord,
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it can either synapse with an interneuron and go further up towards your higher senses so
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that you can make sense of what’s happening, or it can connect directly to a motor neuron,
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which go out from the spinal cord, to move a muscle to react in any way. And there’s gonna
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be a lot of connections to the motor neuron. Any voluntary movement you wanna do, either comes form
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the interneurons ro the sensory neurons directly. So a spinal nerve consist of Sensory fibers,
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Motor fibers, and either sympathetic or parasympathetic nerve fibers. So in the
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spinal cord, you’ll find the sensory fibers back here, the motor fibers are in the front here,
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and the sympathetic and parasympathetic fibers, come from the lateral part fo the spinal cord. And
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they come from specific areas within the spinal cord. The segments C8 to L2, are responsible for
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sympathetic nerve response, while the segments S2 to S4 give parasympathetic nerve response.
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And here’s a quick way to remember this. S, stands from stress, to remind you
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that sympathetic neurons are responsible for fight or flight response, or stress response,
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meaning it increases your heart beat, makes you breath faster, more alert, all of those reacions
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are gonna come from fibers that leave your spinal cord between the C8 and L2 segments.
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The P in Parasympathetic stands for Peace, which is rest and digest. SO you’re chilling,
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you’re sleeping, your intestines are doing its work to absorb the food,
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and all of those things, come from the neurons that emerge between S2 and S4 spinal segments.
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Now back to the spinal nerve. So once the spinal nerve leave the spinal cord.
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It branches out into 4 parts. It becomes a ventral branch, which supply skin and muscles
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of your limbs and the anterior and lateral part of the trunk. And as they do that,
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they form plexuses. We’ll go through this when we go through the peripheral nervous system.
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But plexuses are a huge network of neurons that supply regions of your body. So we have a cervical
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plexus supplying areas associated with your neck and shoulders, the brachial plexus for your arms,
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the lumbar plexus for your legs and pelvis and the sacral plexus for your pelvis and legs aswell.
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So that is the ventral branch, forming these plexuses. The spinal nerve is also going to divide
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into a dorsal branch for the skin and muscles of the back and neck. Then there’s a white ramus
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communicans, which relay sympathetic nerves. And those nerves are involuntary, so they reach out
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to organs you’re not in control of, like your smooth muscles, glands and your visceral organs.
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The last branch is a branch that goes back inside the vertebral canal to supply the meninges,
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called the meningeal branch. So it goes back through the intervertebral foramen, to supply
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the meninges. So that was the general anatomy of the spinal nerve. The last thing I wanna talk
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about in this video, is a reflex arch. Within our body, we have two types of reflex arches.
Reflex Arch
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It’s either going to be a quick unconscious reflex through a monosynaptic reflex. And a more slower
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conscious reflex, called multisynaptic reflex. Now the monosynaptic reflex are simple reflexes
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that go through one synapse, remember synapse is when one neuron connect to another neuron.
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So there are only two neurons involved here. And a famous example si the patellar tendon reflex.
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Imagine you’re at your doctors office, and your doctor wants to assess your peripheral nerve
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reflex response. So he takes up a hammer and taps your patellar tendon quickly, causing your leg to
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kick out. What happens is that the impact of the hammer triggers a stretch receptor neuron
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within your muscle, that quickly fires an action potential towards your spinal cord. Which then
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quickly triggers a motor neuron to activate that muscle. We can’t suppress this reflex,
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because it’s physiological, it doesn’t connect to an interneuron, which goes up to your brain.
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A multi-synaptic reflex is different. And the withdrawal reflex is an example of that.
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So let’s start here, by a candle triggering temperature
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and pain receptors on your hand. That pain is sent through sensory neurons to your spinal cord,
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which triggers interneurons that go up to your brain and trigger an ouch response, as well as
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triggering a motor neuron to remove your hand as quickly as possible. It requires more neurons,
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and it’s a conscious movement. So that was all I had for the external structure of the spinal cord
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and the anatomy of the neuron and its reflexes. Let’s pause here so this video doesn’t get too
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long, and let’s do the internal tracts and nuclei of the spinal cord in the next video.
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