Hello wonderful people, it's Medicosis Perfectionist where medicine makes perfect sense. Welcome to a new playlist. Medicosis Neuroanatomy, making sense of one of the most challenging subjects in medicine.
If you watch this playlist, Neuroanatomy, in its entirety, I promise you, you will understand the subject better than 95% of doctors. First, let's get organized. We have central nervous system and peripheral nervous system.
The central nervous system is made of two things only, the brain and the spinal cord. The peripheral nervous system is anything else. How about the nerves that come out of the brain?
Well, they are outside the brain, so they are peripheral nervous system. How about the nerves that come out of the spinal cord? Also peripheral nervous system. So the cranial nerves and the spinal nerves are peripheral nervous system.
Only the brain and the spinal cord are central nervous system. Now smash that like button, click the subscribe button and let's get started. So please make sure to save this neuroanatomy playlist.
Here are my 10 commandments of neuroanatomy. Thou shall understand the difference between motor and sensory. Draw your line in the sand between anterior motor and posterior sensory. Motor is afferent, sensory is afferent.
Why sensory afferent? Because it goes to the brain. Why is motor?
efferent because it leaves the brain and goes somewhere else. Number two, thou shall understand the difference between CNS and PNS, central nervous system versus peripheral nervous system. Central nervous system is brain and spinal cord, PNS is everything else. Any nerve is PNS, with one exception that we'll discuss later.
There is only one cranial nerve that is part of the central nervous system rather than the peripheral one. Third, thou shall understand the difference between brain and spinal cord. And between cranial versus spinal.
Between somatic and autonomic. Autonomic is also known as visceral or splanchnic. These words are used interchangeably.
Of course, you know that the autonomic nervous system includes what? Oh, sympathetic and parasympathetic only, and that's it. Shut up.
It is sympathetic, parasympathetic, and don't forget for your gut, the enteric nervous system, or ENS. That's why you always talk about your gut feelings, so to speak. Your gut has its own brain.
Next, understand the difference between myelinated and unmyelinated. Myelinated fibers appear white in color. Unmyelinated fibers appear gray in color. Hence, white matter versus gray matter.
Hence, white ramus communicans versus gray ramus communicans. If you want them in plural, white rami communicantes and gray rami communicantes. My goodness. Commandment number seven. Thou shall understand the meanings of these terms.
Nucleus, ganglion, tract, nerve, lemniscus, synapse, decussation, commissure, extraoceptive, proprioceptive, ipsilateral, contralateral, bundle, fasciculus, peduncle, pathway, and much more. In other words, thou shall study hard, cause mediocre students get on my nerve. See what I did there?
Next, thou shall understand the distinction among prosencephalon, mesencephalon, and rhombencephalon. You also need to learn about tillencephalon versus diencephalon, and metencephalon versus myelencephalon. You need to understand the organization of nuclei in the brain stem.
How many nuclei does the facial nerve have? How about the oculomotor nerve? The tenth commandment, thou shall not covet your neighbor's taste buds. Thou shall understand the difference between the anterior two-thirds of the tongue and the posterior one-third of the tongue. When it comes to the name of the nerve, the name of the nucleus, the name of the ganglion, the difference between general sensations and taste sensations, which salivary glands, etc., etc.
And bonus, if you can understand the distinction between these. Here is general somatic efferent. This is branchial efferent, general visceral efferent.
When I say efferent, I mean motor. I mean we're going to leave the brain and go somewhere else. Versus those who end in A, which are afferent.
General sensory afferent, special sensory afferent. Special Visceral Afferent, General Visceral Afferent. If you can understand these 10 commandments today, you'll be in a very good shape to dig into the cranial nerves, which is the topic that comes next in this neuroanatomy playlist.
In this doozy playlist, you'll learn about the brain, spinal cord, cranial nerves, spinal nerves, the meninges, the cerebrospinal fluid, arterial supply, venous drainage, the internal structures of the brain, the nuclei, the tracts and tractology. Now let's go over these commandments one by one. by one. First, motor versus sensory. Draw your line in the sand.
In embryology, this was called the sulcus limitans, which limits the anterior from the posterior. Anterior during embryology was called the basal lamina. The posterior was the alar lamina. The anterior became motor, the posterior became sensory for the most part. Of course, there are exceptions.
There always are. But that's the most important big picture. Let's practice. I want to move my biceps. Well, to move your biceps that's motor look where it starts in front of the line which passes by the central sulcus okay let's say that i want to feel the feather of my arm oh to feel something is sensory look where it is cool i want to move my eye to the right to the left up and down this is frontal eye field look where it is in front of the line but i want to see you oh to see me vision that sensation primary visual cortex Look where it is, behind the line.
Posterior baby, let me talk and articulate my thoughts. To talk is to move, therefore look at me, I am here. And this is Broca's area.
But I wanna hear you and understand you. Well, look at that, that is Wernicke's area. Surprise, surprise, the hearing area and the understanding area are very closely related, which makes perfect sense. There are hundreds of thousands of students who go through school, they just memorize this stuff like the sun shines they are, and they never take a second to understand why are they located this way.
It's all because of your embryological development. Anterior is motor, posterior is sensory. Please take a moment to pause and review. Move my extremities, feel my extremities.
Move my muscles of articulation to talk, hear, and understand you. Move my eyeball versus see you. And the same exact concept applies to the spinal cord. Here's the lovely spinal cord, cross-section, anterior is here, posterior is there.
You draw your line in the sand. Anterior is motor, posterior is sensory. Let's say that I felt a feather touching the skin of my back, feeling the feather.
Oh, that is sensory. So look where I'm going. I'm going. posteriorly, afferent, because I'm sensory, so I'm going to the back of the spinal cord.
Okay, but let's say that I want to contract my biceps. If I want to contract my biceps, I better start here at the anterior horn cell, which is anterior to the imaginary line, and I go forwards like this, efferent, until I reach the beautiful biceps to contract it. What's the name of the nerve that supplies the biceps muscle? Please comment below. Embryology review.
this is your sulcus limitans, this is the basal lamina, and this is the alar lamina. Another important fact is that motor is efferent, which means it starts in the central nervous system and goes somewhere else. Let's say it starts here in the brain and decides to leave the brain this way.
However, sensory is afferent, it's the opposite. It starts somewhere else and ends up in the brain like this. Motor is efferent, but sensory is afferent.
Next, CNS versus PNS. Brain and spinal cord are part of the central nervous system. Everything else is peripheral nervous system.
So the cranial nerves, the spinal nerves, any other nerve that you can imagine is peripheral nervous system. So the radial nerve, ulnar nerve, sciatic nerve, you name it, are peripheral nervous system. How about oculomotor nerve, vagus nerve, facial nerve, also peripheral nervous system. How about the nerves of the brachial plexus or lumbosacral plexus?
These nerves are peripheral nervous system. Next, brain versus spinal cord. Oh that one is easy. The brain is upstairs, spinal cord is downstairs.
The brain is made of four brain, midbrain, hindbrain. The hindbrain consists of the pons, the medulla, and the cerebellum. But what's the brainstem?
Brainstem is three things on top of one another. Midbrain, pons, and medulla. Then the medulla is continuous with the spinal cord.
So let's say that the brain is cerebrum, cerebellum, and brainstem. The brainstem is midbrain, pons, and middle. Then the spinal cord has many segments, about 31 segments. 8 segments in the cervical area, then 12 in the thoracic area, 5 lumbar, 5 sacral, and 1 coccygeal.
We're talking about the spinal cord and not about your vertebral column. If I was talking about your vertebral column, you'll have 7 vertebrae in the cervical area, 12 thoracic, 5 lumbar, 5 sacral, and 4 fused coccygeal. Then we talk about the cranial nerves and spinal nerves. They are part of the peripheral nervous system.
The cranial nerves are 12 pairs, 12 on the right, 12 on the left. Spinal cord are 31... One pairs, 31 on the right, 31 on the left, and they follow the same segments. So you have 8 pairs of cervical spinal nerves, 12 pairs of thoracic, 5 pairs of lumbar, 5 pairs of sacral, and 1 pair of coccygeal spinal nerve. You'll learn more about the spinal cord in a separate video titled Anatomy of the Spinal Cord.
What's the structural unit of the nervous system? It's the neuron made of a soma, or cell body, and an axon. Some axons are myelinated, like this one, and they will appear white in color. Others are unmyelinated, and they appear gray in color. That was the structural unit.
How about the functional unit? That's the reflex arc. We start with a stimulus, which will be felt by a receptor.
Then I have an afferent fiber. Then I have a center, such as the spinal cord. Then I have an efferent fiber. an effector organ like my biceps which causes a response.
So if I touch the candle, that's the stimulus, it's going to be felt by a receptor for heat or thermoreceptor, and then it will travel through an afferent fiber towards my spinal cord, which is the center. Then I have an efferent fiber which will go to the effector organ such as my biceps to flex my arm away from the candle. Next, somatic versus autonomic. Somatic is voluntary, which you can control, but autonomic is involuntary, you cannot control. The word autonomic is synonymous with visceral or splanchnic.
The somatic could be motor or sensory. The autonomic also could be motor or sensory. What is a mixed nerve? It's a nerve that has more than one thing. It has motor fibers and sensory fibers, or motor and sensory and autonomic.
What are the types of autonomic nervous system? We have sympathetic nervous system. parasympathetic nervous system and enteric nervous system.
The enteric nervous system is subdivided into myenteric plexus and submucosal plexus. We have talked about all of these in great detail in my physiology playlist. Next, myelinated versus unmyelinated. Some neurons have myelin and they appear white and we call this the white matter. Other neurons are not myelinated and they appear gray.
We call this the gray matter. In the spinal cord, The gray matter is on the inside, but the white matter is on the outside. So if I look at this, oh, this is white matter, which means the nerve fibers here are myelinated. But let's look here. Oh, it's gray.
So the nerve fibers here are unmyelinated. The opposite is true for the brain. Look, the gray matter is not central anymore. It is peripheral. Ooh.
What does that mean? It means that I have... unmyelinated fibers here or it could simply be that I have the somas here the cell body there because the cell bodies are not covered by myelin only some axons are so it could be that the fibers are rearranged like this so I have a soma and a axon soma axon soma axon so to recap in the spinal cord the gray matter is on the inside but in the brain the gray matter is on the outside how is this important well it is very important important indeed. Because we agreed that this white matter means myelinated, so let's just draw some myelin. So look at this.
This is myelin fibers, myelin, myelin, and myelin. So these fibers are myelinated. That's how they appear white. Okay.
What if I have a disease known as multiple sclerosis? It demyelinates the fibers of the central nervous system only. So who's going to suffer? Only the brain and spinal cord and nothing else. Okay.
what's going to happen here? What's going to happen is that you will destroy your myelin. Oh, so where will be the disease?
On the outside or on the inside? The disease is where the myelin is. It will be on the inside part of the brain. And that's why on MRI, you will see oligoclonal bands, signs of demyelination. And when you find them on MRI in a patient with MS, these oligoclonal bands are on the inside and not on the outside.
Because the oligoclonal bands... are where the myelin used to be. Cosmeticin makes so much sense once you understand what the french toast you're talking about. So white matter is myelinated. Okay, who myelinated these fibers?
If they are in the central nervous system, the oligodendrocytes myelinated those fibers. But if these fibers are in the peripheral nervous system, let's say the ulnar nerve or the radial nerve, they were myelinated by the Schwann cells. Next, a matter of definitions.
if you wish to converse with me, said Voltaire, define your terms. So let's define our terms. Remember that this is a neuron.
It's made of soma, or cell body, and axon. A collection of somas in the center nervous system is a nucleus. A collection of somas in the center nervous system The peripheral nervous system is a ganglion. But a collection of axon fibers in the CNS is a tract, and a collection of axons in the PNS is a nerve. That's why you hear of the sciatic nerve, ulnar nerve, radial nerve, musculocutaneous nerve.
All of these are nerves because they are a collection of axons. What if I told you that 99% of doctors cannot give you these simple definitions? How about the definition of a fasciculus or a bundle? That's a group of nerve fibers, usually in the CNS, but they have different origins, terminations, and functions. They just happen to be in close proximity to one another.
What's a peduncle? Oh, I'm glad you asked. Here is my lovely forebrain, and here is the midbrain, followed by the pons, followed by the medulla. If the midbrain wants to communicate with my cerebrum, it does so by a cerebral peduncle. But if the same midbrain here wants to communicate with the cerebellum behind it, it does so by means of superior cerebellar peduncle.
Then the pons to the cerebellum is middle cerebellar peduncle. The medulla with the cerebellum is inferior cerebellar peduncle. Let's do it again. This is a cerebral peduncle. Superior, middle, and inferior cerebellar peduncles.
So a peduncle is a thick bundle that supports part of your brain. and usually communicates between two structures. What's a pathway? A pathway is a series of neurons transmitting impulses inside or outside the CNS, and you will learn about pathways in the next video, which talks about the olfactory nerve and the olfactory pathway.
You'll also learn about the visual pathway in an upcoming video, the auditory pathway, etc, etc, etc. What's a commissure? Oh, look at this.
Here are your... two cerebral hemispheres. Who communicates between them?
A commissure. You can have a gray commissure made of gray matter or white commissure made of white matter. A commissure connects part of your CNS on one side to the same part on the opposite side. You look at this, that's a tract. When two tracts cross like this, we call this a decussation or crossing.
What's a limniscus? A limniscus is a band of ascending fibers always going upstairs until they reach the thalamus. A limniscus has to be a group of fibers in the brainstem and they have to end up in the thalamus.
How many limnisci do you need to know? Four. Medial and lateral, spinal and trigeminal. Medial limniscus, lateral limniscus, spinal limniscus, and trigeminal limniscus. Next, ipsilateral versus contralateral.
Well, if I'm referring to the right cerebral cortex, and let's say the right half of my face, they are on the same side, so they are ipsilateral. But the right cortex with the left half of my face are contralateral, on the opposite side. Last set of definitions today is extraoceptive versus proprioceptive. If I'm feeling touch, or pain, or temperature, these sensations are called extraoceptive sensations. But If I'm feeling the position of my muscles or ligaments or joints in space, this is proprioception.
Please do not continue watching this video if you do not fully grasp the definition of all of the following terms. Nucleus, ganglion, tract, nerve, bundle or fasciculus, peduncle, pathway, commissure, decussation, limnuscus, extraceptive vs proprioceptive. and ipsilateral versus contralateral. Next, do you remember the trilaminar embryo, endoderm, mesoderm, and ectoderm? Yes, I do.
Where did the nervous system come from? Only from the ectoderm. The ectoderm is subdivided into neuroectoderm and surface ectoderm.
The surface ectoderm is for the epidermis of your skin, hair, nail, etc. Forget it. But the neuroectoderm is the one that actually gives us the nervous system. And it will give us two structures, neural tube and neural crest. The neural tube will give us the CNS, but the neural crest will give us the PNS.
Neural tube will give us the cells that myelinate the CNS, known as the oligodendrocytes, but the neural crest will give us the cells that myelinate the peripheral nervous system, namely Schwann cells. Don't forget that all of the ganglia, by definition, are a collection of somas in the peripheral nervous system. That's why the ganglia are made by the neural crest. So the CNS and the cells that myelinate the CNS come from the neural tube.
But the PNS and the cells that myelinate the PNS come from the neural crest, both of which come from the neuroectoderm, which is ectodermal in origin. Here is a tough question. Question for you. Can you name one ganglion that is not in the peripheral nervous system and instead it's in the central nervous system? Please comment below.
Now let's talk about prosencephalon, mesencephalon, and rhombencephalon. During embryological developments, do you remember the neural tube? Yeah, the neural tube will give us spinal cord and brain. The spinal cord is called the posterior neuropore.
And up here is the anterior neuropore. Anterior end, posterior end, which later become the upper end and the lower end. This is the tip of your forebrain, and this is the lowest part of your spinal cord.
These are the three vesicles that will make up your brain. Prosencephalon, mesencephalon, and rhombencephalon. The word prosencephalon reminds me of a protrusion.
Look at this protrusion. That's your forebrain. The word meso means what? It means in the middle. If you remember, in organic chemistry, we had something called a mesocompound, which looked akin to this.
It was a compound that had a central, I mean internal plane of symmetry, so that I can divide it with a knife like this, and I will have two mirror images. That was called a mesocompound. Meso, because there is a mirror in the middle. Meso means middle, hence mesencephalon midbrain.
The midbrain is mid, it's in the middle. between the forebrain and the hindbrain. After this, I have the rhombencephalon or the hindbrain. Prosencephalon will give me telencephalon and diencephalon.
Mesencephalon will give me the midbrain and rhombencephalon will give me metin and myelin. Metin is the pons and the cerebellum. Myelin is the medulla. How do I remember that myelin is the medulla and not the other way around? Easy.
The word myelo means core, referring to the spinal cord. What's the continuation of the spinal cord as I go up through the foramen magnum? It is the middle. And you will learn about telencephalon, diencephalon, midbrain and all of this in upcoming videos. But for now I want you to know this.
Prozencephalon is the forebrain. It will give you telencephalon up here on the outside and diencephalon which is deep on the inside. The diencephalon is anything that has the word thalamus in it, including thalamus and its metathalamus, hypothalamus, epithalamus and subthalamus. The telencephalon on the outside will give you the cerebral hemispheres.
on more of the outside and basal ganglia towards the inside. Outer cover followed by this middle cover followed by this inner cover. Your brain and spinal cord are surrounded by cerebrospinal fluid. Who made that cerebrospinal fluid?
Ventricles. Ependymal cells lining the choroid plexus in the ventricles. Lateral ventricles make CSF and then that CSF goes to that third ventricle. This CSF leaves the third ventricle and goes to the fourth ventricle. and then leaves through many foramina to surround your brain and surround your spinal cord.
What's the name of the ventricle of the telencephalon? Lateral ventricle. What's the name of the ventricle between the diencephalon or between the thalami? Third ventricle.
Near the mesencephalon you have the cerebral aqueduct of sylvius, and for the rhombencephalon what do we have? We have the fourth ventricle. In the next videos we'll talk about the cranial nerves.
For now just remember that cranial nerve 1 and 2 come out of the midbrain. 3 and 4 come out of the midbrain, 5, 6, 7 and 8 exit through the pons, 9, 10, 11, 12 come out of the medulla, and this is the total of 12 pairs of cranial nerves. When you think of emotions and behavior, think of your limbic system, such as the amygdala.
When you think of your hypothalamus, think regulation. Regulation of temperature, regulation of glucose, regulation of appetite, thermostat, glucostat, epistat, respectively. When you think of your midbrain, remember the eye reflexes, like the cornea reflex, light reflex, etc. They have their center in the midbrain.
As for the middle, it has 4 famous centers that you cannot forget. Heart and lungs, get it in, get it out. Cardiovascular respiratory, swallowing and vomiting. Heart and lungs, get it in, get it out.
Mr. Spinal Cord has the micturition reflex, defecation reflex, and the center for erection. Remember again that we have somatic and autonomic. The autonomic could be sympathetic nervous system, parasympathetic, or enteric.
The enteric is in your gastrointestinal tract, subdivided into myenteric plexus for motility and submucosal plexus for secretions. The M goes with the M and the S with the S. Recall that your sympathetic is thoracolumbar but the parasympathetic is craniosacral. What do you mean by cranio? I mean cranian nerves 3, 7, 9, and 10. oculomotor, facial, glossopharyngeal, and vagus nerves.
What do you mean by sacral? I mean S2, S3, S4. S2, S3, S4 keeps my pee-pee off the floor. If you If you understood this mnemonic, please comment below, explain it.
Recall, if I am somatic motor, I'll leave the anterior horn cell. But if I am autonomic motor, I'll leave the lateral horn cell, and then I will leave this, go to the efferent, and then reach the spinal nerve, and then I'm here. But I gotta go through a ganglion because I'm autonomic.
I'll have preganglionic fibers and postganglionic fibers. Pre goes to the ganglion, post, whereas post leaves the ganglion. Usually the preganglionic fibers are myelinated, so they appear white, and we call this the white ramus communicans.
The postganglionic fibers leave the ganglion, they are unmyelinated, they appear gray, hence the gray ramus communicans. The ganglion is a relay station for regulation. So if I want to draw this, this will be the soma that I will start with. Okay, dendrites, beautiful.
And then here is the axon, amazing. And then as I reach the ganglion, that's the end of the axon. In the ganglion, there is a synapse, so another neuron starts, and the cell body, and we have the axon going this way. If you want to learn more about the difference between preganglionic and postganglionic, and how ganglia work, etc., check out my physiology playlist, especially the series on the autonomic nervous system. For now, remember that preganglionic is myelinated, postganglionic, unmyelinated, preganglionic appears white, postganglionic appears gray.
Preganglionic are B fibers, they are thin and myelinated. Postganglionic are C fibers, thin and unmyelinated. Where's the cell body for this preganglionic?
It was all the way in the lateral horn cell as we have just discussed. But where is the cell body for the postganglionic? It's inside the freaking ganglion. Next, let's talk about the organization of nuclei in the brainstem. Here's your brainstem.
This could be midbrain, pons, or middle. If a cranial nerve is leaving, and this cranial nerve is purely somatic motor, it will leave entromedially, very close to the midline. For example, the abducens nerve. It is purely motor somatic, so it leaves entromedially. But if you are mixed, if you are not pure blood motor like this, if you are mixed, if you are motor and sensory, motor and autonomic, motor, sensory, autonomic, branchial, all kinds of mixture, you will be more entrolateral.
And not just that, you could be lateral or even posterior. Anything but intramedial. This trick will save you tons of hours on your studying.
And it's not haphazard either. This is how your brain was organized during embryological development. You will learn more about the extraocular muscles. But for now, just remember that the nerves that supply the muscles that move your eye are cranial nerves 3, 4, and 6. Keep this in mind.
So they move my eyeball. But what if I want to see? Oh, sensory, not motor. Then that's cranial nerve too, the optic nerve for you.
So here is vision, which is sensation. Here is movement of the eyeball, which is motor. Another important trick. If I want to feel the feather that touched my face, I use cranial nerve number five, which is trigeminal nerve.
But if I want to move my facial muscles, i.e. motor, that's the facial nerve or the seventh cranial nerve. And last, let's talk about the anterior two-thirds versus the posterior one-third of your tongue. Now, none of this is going to make sense to you today, but keep watching my videos on the cranial nerves, and you will master this topic like it's nobody's business.
Anterior two-thirds of the... the tongue here posterior one third of the tongue is there separating between the two portions is the sulcus terminalis and you have a foramen c come in the middle anterior two thirds of the tongue who's gonna help me feel the touch that's v3 the lingual branch of the mandibular nerve which belongs to the trigeminal nerve but who's gonna help me taste the chordotempony nerve of the facial nerve which is the seventh cranial nerve let's switch to the posterior one-third of the tongue. Who's gonna help me feel the pain and the touch? Ninth nerve, which is the glossopharyngeal. Who's gonna help me taste?
The same nerve, the glossopharyngeal. Oh, so there is a difference in the anterior, but there is no difference in the posterior. That's correct.
Anterior two-thirds is V3 for general sensation and 7 for taste sensation, i.e. special sensation. General versus special. How about posterior, general and special?
are by the glossopharyngeal nerves. All right, now tell me about the gland, the salivary gland. What does the facial nerve supply? It supplies submandibular glands and sublingual glands, but the glossopharyngeal supplies the parotid gland. Okay, now please name the ganglion.
Well, I am submandibular gland, so I lay in submandibular ganglion, but I am a parotid gland, which literally means parallel. ...your ear, para-otic, so I relay an otic ganglion. Amazing. Since this is saliva, and this is saliva...
Which nucleus in the brainstem should I start in? You should start in a salivatory nucleus to salivate, to secrete saliva. The one that controls the facial nerve, anterior, is the superior salivary nucleus, but the one that contains the glossopharyngeal, which is posterior, is the inferior salivatory nucleus.
The superior salivatory nucleus is called superior because it exists in the pons, which is located superior to the medulla. the medulla is inferior to the pons anatomically speaking. So anterior before posterior.
Superior salivatory is before or above inferior salivatory. The facial is above the glossary. opharyngeal in position and the pons is anatomically superior to the medulla in position.
I know this is a difficult topic right now, but if you can memorize these distinctions, it's going to help you tremendously as we go forwards. forgot to tell you something. All of this was sensory, parasympathetic, etc.
But let's say that I wanna move my tongue up and down, right and left. This is the function of the glossopharyngeal nerve, which is the 12th cranial nerve. And there you have it, the 10 commandments of neuroanatomy. How about the bonus?
You will learn about this in the upcoming videos, where we discuss the cranial nerves in detail. By the end of this series, you will understand every single word that is written here. Do you want to learn about...
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