Neuroanatomy for Medical Examinations

Thanks everybody and welcome to our talk here on essential neuroanatomy. Now if you are taking step one there is going to be a lot of information here that I'm not going to cover that can come up on your exam. Now My videos are all generally geared towards step 2 and step 3, which requires quite a bit less anatomy. You know, you're going to be primarily focused on the basics that are relevant to the various diseases that are within the scope of step 2 and step 3. However, for step 1, because it is a basic science test, you will be responsible for quite a bit more anatomy just in general, and that goes beyond neuroanatomy as well. So... If you're taking step one, this is a good video for you to watch because I'm going to explain to you some more things that you'll need to know. I'm not going to cover them, but I will explain what is going to come up on your exam. So then you can go watch other YouTube videos or use your books to to give you more information that's going to be on your exam. OK. If you haven't had the chance yet, please consider subscribing to my Patreon. You can get there by clicking the link in the description of the video or on the i button on the upper right hand corner. I very much appreciate all the contributions I can get to help offset the cost of these videos, and I thank all those of you in advance who have already donated. And definitely feel free to subscribe. Okay, so we're going to go over sort of the external brain anatomy, get some regions, under our belt, and then we'll go over some internal brain anatomy. We'll talk briefly about the cranial nerves and some information that's particularly important for step one there. We'll talk about the arterial circulation of the brain. That's going to be very important. And then we'll talk about the spinal pathways, sensory, and motor. This is the external brain anatomy. So what you need to know are your lobes. That's... very fundamental. So obviously in the front, we have the frontal lobe, kind of in the middle towards the back, we have the parietal lobe, and then we have the occipital lobe in the very back, and the temporal lobe is lateral along the sides. Now what separates the frontal lobe from the temporal lobe is right here, and I apologize, I don't have any other color than black, but this here that... divides the frontal and temporal lobe is called the lateral sulcus. Okay, sulcus. All right, now what separates the frontal lobe from the parietal lobe is called the central sulcus. And now that's going to be pretty important because on either side of the central sulcus, we have gyri. Now, in front of the central sulcus, We have the precentral gyrus. And this part of the brain is responsible for initiating motor activity, so for movement. And then right behind the central sulcus, we have the postcentral gyrus. And that is sensory. So as we're going to see when we go through the... the pathways through the spinal cord, they're going to either start or terminate there. All right, now the occipital lobe, I just will point out, this is very important for vision. The temporal lobe is very important for hearing. And the frontal lobe, particularly towards the very front, is responsible for behavior and impulse control. Okay. So that's just a general idea of the external brain anatomy as it pertains to the cerebrum. There's also the cerebellum that has a lot to do with coordination, as you know. If you're taking step one, you'll want to know more details about the cerebellum. We're not going to go into that here. And then we have the brainstem, which of course is made up of the midbrain, the pons, and the medulla. All right, now this is a little bit more detailed here. So I already talked about precentral and postcentral. So those are primary motor area and primary sensory area. And then, of course, right here, we have the central sulcus. There are other areas that are obviously going to be very important. I will draw your attention here to Broca's area. It's in the frontal lobe, very close to the temporal lobe. And then we have Wernicke's area, which kind of spans through the temporal and parietal lobe. Broca's area is important for initiating speech, and Wernicke's area is important for understanding speech. So they both play a role in language, they just do different things. And then as mentioned in the occipital lobe, we have the secondary and primary visual areas. Occipital lobe, you need to think number one thing is vision. And then as I mentioned, auditory information gets sent to the temporal lobe. Now, if you look at the prefrontal cortex, what you're going to see is this sort of mapping. where different parts of the prefrontal, and for that matter, the postfrontal cortex, process information or control different parts of the body in sort of a mapped way. And so what you'll want to remember is that the upper extremities are lateral, and the lower extremities are medial. So if you look at the most medial portion of the cortex, what you see is its foot. and toes and hips and legs. And then as you go laterally, you kind of move upward in the body. So here you have the shoulder and the arm, and then up here you have the hand. And then on the most lateral side, you have areas pertaining to the face. Now, as we're going to see, the middle cerebral artery is responsible for perfusing the lateral brain. and the more medial parts of the brain is supplied by the anterior cerebral artery. And this is why when we see middle cerebral artery strokes, they tend to be upper extremity symptoms. And when we see anterior cerebral artery strokes, they tend to be lower extremity symptoms. And this is just comparing the somatosensory cortex, which is postcentral, and the somatomotor cortex, which is precentral. Okay, now this is the basal ganglia. And I'm not going to go into the physiology of the basal ganglia, the neurotransmitters and all that stuff. It is very important for step one. So it is one of the things you want to put on your list to review. But I will point out the disorders that affect the various parts of the basal ganglia. So first off, as you probably are aware, the substantia nigra is affected in Parkinson's. Okay, those are dopaminergic neurons. The subthalamic nucleus, when damaged, can cause hemibolismus. And the caudate and putamen are implicated in Huntington's. Now, notice that the caudate and the, to a lesser degree, the putamen on this view here, this is a coronal view. are relatively close to the ventricles. Now we're going to see how that's important. Now this is an axial view here, and you see pretty much all the same things. This is just another way that you can see it. Notice here, I'm trying to find it here. Do they have the, oh, I hope, okay, so I didn't put it. So it's not on here, but this is the internal capsule. This is a very important area of the brain where motor signals are conducted. And so you see it in between the putamen and the thalamus. And then here we have the caudate. And notice that the caudate is right up against the lateral ventricles. Okay, now this is a patient with Huntington's. Now, what do you notice here? So notice these, and again, I apologize. I wish I had read. Notice these big ventricles. This is called hydrocephalus. ex vacuo. And so it's not hydrocephalus, but it looks like hydrocephalus because you have a degeneration of the caudate and putamen. So the caudate and putamen are supposed to be right here and taking up this space, but because it degenerates, it causes the ventricles to get enlarged, making it look like you have a hydrocephalus. Notice also here you have pretty significant cortical atrophy. Okay. All right, now moving on to the arterial circulation of the brain. This is super, super, super important. You want to know where each of these arteries, you know, kind of where their areas are with respect to the brain. Because when it comes to stroke syndromes, you're going to need to know, okay, if you have a patient coming in with these symptoms, which arteries are we going to be looking for a blockage? So you'll want to know. where each of these arteries go. So let's start. So we have the anterior circulation here, and the posterior circulation down here. And notice that they are connected by these anterior and posterior communicating arteries. These are the posterior communicating arteries. And it's the posterior communicating arteries that will join the anterior and posterior circulation. And then we have a similarly named anterior communicating artery. They will join the middle cerebral arteries to one another. They'll also give off anterior cerebral arteries. So this kind of circle that's made here with the posterior communicating arteries, it's called the circle of Willis. Let's look at the anterior. carotid circulation. So what you've got here is you start with the internal carotid artery. Okay, we know where that is, that runs up the neck, comes off the common carotid artery. And as we get into the brain, the internal carotid will then give off terminal branches, and that's the MCA. And the MCA is going to go, as we've mentioned, to the lateral part of the brain, the lateral part of the cerebrum. It comes off the internal carotid artery, it continues into the lateral sulcus, and it gives off various branches primarily to the anterior temporal lobe, the lateral frontal lobe, and even parts of the parietal lobe. So it's the lateral brain, the lateral cerebral cortex. Now, this artery here technically is ACA. So you have a branch, the MCA branch, and then the ACA branch. And the ACA will kind of continue up this way as it goes on to supply the medial part of the cerebrum. And then what's connecting here is the anterior communicating artery. Now, technically, the anterior communicating artery is just this section right here. This here and this here are anterior cerebral artery. Okay, now I want to point out, and I put it here, this is not anatomically correct, but I want to draw your attention. With the internal carotid artery, the first major branch is the ophthalmic artery, and that supplies the retina. So that's very important for you to know. And then we have the posterior communicating artery, which will communicate with the posterior circulation. So the ophthalmic artery goes to the eyes. The ACA goes to the middle or medial cerebrum. The ACA will go to the lateral cerebrum. And the posterior communicating artery communicates with the posterior circulation. Now, what about the posterior circulation? So you can think of this as starting with the vertebral and anterior spinal arteries. So coming up, the vertebral artery, we have PICA, posterior inferior cerebellar artery. And this... supplies the exact parts of the brain that we would think of, the posterior inferior cerebellum, as well as parts of the brainstem. Then they come together and they form, well, they give off the anterior inferior cerebellar artery, which supplies the anterior cerebellum, as well as parts of the brainstem. And then as they come together, they form the basilar artery, and you only have one basilar artery. This is not paired. And there are various branches that are called pontine arteries, and they obviously supply the pons. Now you have one more paired set of arteries that come off the basilar, and that is the superior cerebellar artery, again supplying parts of the cerebellum. And then finally, we have the posterior cerebro. artery. And that, again, is going to be cerebrum. So this will actually supply the occipital lobe. Okay, and then finally, we have the posterior communicating artery, which will communicate with the anterior circulation. So just to recap here, we've got the posterior inferior cerebellar artery, which supplies the posterior lower cerebellum, the anterior inferior cerebellar artery, which supplies the anterior lower cerebellum, the basilar artery, which supplies the empontine arteries, which supply the medulla and the pons. The superior cerebellar artery, which supplies the superior cerebellum. And the posterior cerebral artery, which supplies the posterior cerebrum, namely the occipital lobe. All right, and this is just kind of a recap here. Now, on your exam, particularly if you're taking step one, you will probably be given a visual, kind of like this. Now, in real life, arteries are not this red. and veins are not this blue. But this is a possibility of a picture that you might see. And so what you want to know is your major landmarks. So notice this vessel right here that's connecting these two in the front. This is anterior communicating. And so then you know from that point on that here you're dealing with MCA. And what's being connected by these anterior communicating arteries is the... anterior cerebral artery. And so this here is the internal carotid artery. And then this here that's going backwards is the posterior communicating artery. So here is your circle of Willis. And then you can see here the basilar, which is giving off these various pontine branches and so forth. So this is important for you to be able to understand. the major arteries by looking at an anatomic specimen. So if you understand it first by looking at a cartoon, a model, then kind of transition your way into understanding it and looking at various actual specimens that will go a long way for you, particularly for step one. You will not be expected to do this for step two or three. Now, there are a variety of stroke syndromes that we can encounter based on blockage of any of these vessels, and they will naturally cause different symptoms because the different parts of the brain do different things. Now, if you're taking step one, you will probably want to know all of these. If you're taking step two or three, the big ones you want to know, MCA, ACA, ophthalmic. You'll probably want to know PCA and... maybe ASA. It's good to know all of them, but those are the most important ones for you to know because they're the most commonly affected. Now, another thing that's important for you to know is where berry aneurysms take place, also known as saccular aneurysms. So these are the main areas here in the red that are affected by berry aneurysms. The most common place that we see it is in the... First part of the anterior cerebral artery, so right here. And it's right around where we have the anterior communicating artery coming off. We have a bifurcation. And so that area is the number one most affected place to get an aneurysm. Then followed very closely by the posterior communicating artery, which is right here. And then the MCA, and that's more distal when we're talking MCA. So we're talking over here. All right, so those are the most common sites of these berry aneurysms. Now, this is something you've got to know for step one, and I'm not going to go through it. But you want to know where the cranial nerves come off and what fissures they go through. So these little... foramina in the skull. So you want to know all of them. So cranial nerve one comes through the cribriform plate. Cranial nerve two, which is the optic nerve, goes through the optic canal. Superior orbital fissure, I think of orbital like eye, and all of these have to do with eye. Oculomotor, trochlear, abducens, and ophthalmic. Foramen rotundum is V2, the maxillary. Foramen ovale is V3, the mandibular. And the way I remember this is SRO, standing room only. So S is superior orbital fissure. It'll be all of the I nerves, including the ophthalmic nerve, V1. And then R is V2, and O is V3. The internal acoustic meatus is 7 and 8, facial and vestibular cochlear. The jugular foramen is 9, 10, 11. and the hypoglossal canal is 12. Now, you'll want to, again, be able to look at a skull and know where each of these foramen are, because you may get asked to point one out. And that's strictly a step one thing. All right, now we're going to talk about the spinal pathways, and this will be the last thing we talk about. Now, this is relevant, particularly for step one, but step two and three, it does have some relevance as well. So with these spinal pathways, we have two sensory pathways and one motor pathway. We're going to start out with the sensory pathways. So we're going to talk about the DCML and the spinothalamic tract. Those are your sensory pathways. And then we'll talk about the one motor pathway. The DCML, also known as the dorsal column medial lumbiscus tract, is responsible for transmitting information about fine touch, vibration, and proprioception. So what happens is you have a nerve that has a receptor. It transduces a signal from the skin all the way to the spinal cord. So you have this neuron that enters dorsally. So all your sensory information is going to enter dorsally. So up here. And so as you enter dorsally, you come up into this tract here. So this is the dorsal column. It's right here. And now as nerves enter. they're going to enter from medial to lateral. So what you see here is your lower, if you look at these words here, your lower extremities are going to be more medial. And then as your upper extremities come up, as you move upwards, they come kind of laterally. Okay, so you get added from middle to lateral. So in the very middle, you have your sacral, and then more laterally, you have the nerves coming on at cervical. levels. Now, as we go up, we have our first synapse at the medulla. So that is our first synapse. So all of this right here is one nerve. Okay, that's the first nerve. And then we have a synapse at the medulla. And right after that synapse, we have a decacation. Okay, so decussation just means that you cross to the other side. And so these fibers, they cross from one side to the other. They're called the arcuate fibers, and they come off these nuclei at the medulla called the cuneate and gracile nucleus. You may also hear it referred to as nucleus cuneatus or nucleus gracilis. And so now we have the second nerve, and this is going to go all the way up to the thalamus. And particularly, it's the ventroposterolateral nucleus of the thalamus. And then from there, we have a third neuron that then goes to the sensory cortex. So what is important here is that you understand that we have a decussation. And so if there's some sort of pathology or stroke at the level of the brain or further up than the thalamus, then what you're going to have is a contralateral presentation. So if it affects the right part of the cerebrum, it's going to give you symptoms on the left part of the body and vice versa. Whereas if you have a lesion at the spinal cord, because it hasn't decussated yet, it's going to be ipsilateral. So this is just another way to look at it here. Notice that we have our first neuron that goes all the way up to the medulla. And then we cross over. Those are your arcuate fibers. And then we continue going up to the cerebrum. We stop at the thalamus. We have another synapse there. And then we go to that ipsilateral somatosensory cortex from the thalamus. And know this is VPL. Okay, now moving on to the spinothalamic tract. This carries information about pain, temperature, crude touch, and pressure. And there are actually two distinct spinothalamic tracts. And so you'll see them right here. So we have a lateral that's right here. and an anterior, which is right here. And so they're both kind of in the same area, sort of in the anterior part of the spinal cord. But there are two tracts, and they actually carry fibers with different information. All right, now what happens here is you have a sensory nerve that comes in, and it comes in dorsally. So it comes in this way. And what will happen is that first neuron will travel up a couple levels. And so it travels up right about right here. And that area is called the Sauer's tract, where it'll travel up. Sometimes it can even travel down. But for the most part, it travels up. And so it travels up a ways. And then it'll start to... And that's all the same nerve, by the way. There's no synapse there. And it'll go up a couple levels. Then it'll start making its way inward. And there's a... synapse right there, so about right here, and this is at the nucleus proprius. So then you have a second nerve that comes in like that, and practically immediately it's going to cross. Now that is distinct for the spinothalamic tract versus DCML. The spinothalamic tract decussates in the spinal cord, okay? Very important to know that, very important to know that. So what you have is this decussation, and this decussation happens right in front of the central canal right here, and that becomes clinically relevant. So when you're dealing with a syringomyelia, where you have dilation of the central canal, typically at the cervical levels, you will get pressure on that anterior white commissure that carries those decussating fibers. And so that is why if you get a syringomyelia, which is at the cervical level, you're going to get that loss of pain and temperature in a cape-like distribution. It'll be upper extremities and kind of the back area. And that's because it places pressure on that anterior white commissure. So then at that point, so you're coming all the way over here or over here, and you travel up that tract and you go all the way up to the thalamus. And then again here, we have another We have another synapse, and then it's going to continue traveling to the postcentral or sensory gyrus. All right, so that's spinothalamic. The big thing to know is that you decussate at the spinal cord. And in a future talk, I'm going to talk about stroke syndromes, and you'll see how all this will come together. So this, again, here is this anterior white commissure, and it's right in the front. of the central canal. All right, this is just a comparison here. And I will point out that with spinothalamic injuries, if it's a spinal injury, you're typically going to have contralateral to the injury because we have that early decussation. However, at the exact level of the injury, so not below, but at the exact level, because you have fibers there that have not crossed yet, you'll have a little bit of ipsilateral. So you'll have ipsilateral roughly at the level of the lesion, and then everything below that is going to be contralateral. Now the corticospinal tract is our motor tract, and there's only one as far as major tracts. There's some minor tracts, but As far as what you're responsible for, for step two and three, it's just this corticospinal tract. Now, they are right here in the red, and there is lateral and anterior. Lateral being primarily the limbs, anterior being primarily axial, and that helps. You can remember that because they start with the same letter. So here, we're starting at the precentral gyrus, the motor gyrus, motor cortex. So our first neuron comes from the precentral cortex, and we come down to the medulla, and we cross at the medulla. Okay, so that decussation is called the pyramidal decussation, and it's at the lower medulla. However, there is no synapse. This is all the same nerve. You just happen to cross at the medulla. So it's still the same nerve. We're not synapsing. Now we travel down those corticospinal tracts, again right here. And we go all the way down to the level that we want to come off at. And at that point, you synapse, and that is a lower motor neuron. There's also interneurons, but mostly I want you to be aware that you synapse with the lower motor neuron. And that neuron is going to come out at the anterior horn. And so you'll come off anteriorly like that. And you'll go all the way on then at that point to the muscle that you want to affect. Alright, so that is the corticospinal tract. And so if you have injury to the brain, then you're going to have contralateral signs. This is why if you get an MCA stroke, for instance, say right here, it's going to affect the contralateral side. Whereas if you have an injury to the spinal cord after the decussation, it's going to be to the ipsilateral side. So this is another great way of looking at it here. You can see in a little bit more detail here, you have your pyramidal decussation at the level of the medulla, and then you travel down the corticospinal tract, ultimately coming to the lower motor neuron, you synapse there, and then that will go to the skeletal muscles.

You know, you're going to be primarily focused on the basics that are relevant to the various diseases that are within the scope of step 2 and step 3. However, for step 1, because it is a basic science test, you will be responsible for quite a bit more anatomy just in general, and that goes beyond neuroanatomy as well. So... If you're taking step one, this is a good video for you to watch because I'm going to explain to you some more things that you'll need to know. I'm not going to cover them, but I will explain what is going to come up on your exam. So then you can go watch other YouTube videos or use your books to to give you more information that's going to be on your exam.

OK. If you haven't had the chance yet, please consider subscribing to my Patreon. You can get there by clicking the link in the description of the video or on the i button on the upper right hand corner.

I very much appreciate all the contributions I can get to help offset the cost of these videos, and I thank all those of you in advance who have already donated. And definitely feel free to subscribe. Okay, so we're going to go over sort of the external brain anatomy, get some regions, under our belt, and then we'll go over some internal brain anatomy. We'll talk briefly about the cranial nerves and some information that's particularly important for step one there.

We'll talk about the arterial circulation of the brain. That's going to be very important. And then we'll talk about the spinal pathways, sensory, and motor.

This is the external brain anatomy. So what you need to know are your lobes. That's...

very fundamental. So obviously in the front, we have the frontal lobe, kind of in the middle towards the back, we have the parietal lobe, and then we have the occipital lobe in the very back, and the temporal lobe is lateral along the sides. Now what separates the frontal lobe from the temporal lobe is right here, and I apologize, I don't have any other color than black, but this here that...

divides the frontal and temporal lobe is called the lateral sulcus. Okay, sulcus. All right, now what separates the frontal lobe from the parietal lobe is called the central sulcus.

And now that's going to be pretty important because on either side of the central sulcus, we have gyri. Now, in front of the central sulcus, We have the precentral gyrus. And this part of the brain is responsible for initiating motor activity, so for movement.

And then right behind the central sulcus, we have the postcentral gyrus. And that is sensory. So as we're going to see when we go through the... the pathways through the spinal cord, they're going to either start or terminate there. All right, now the occipital lobe, I just will point out, this is very important for vision.

The temporal lobe is very important for hearing. And the frontal lobe, particularly towards the very front, is responsible for behavior and impulse control. Okay.

So that's just a general idea of the external brain anatomy as it pertains to the cerebrum. There's also the cerebellum that has a lot to do with coordination, as you know. If you're taking step one, you'll want to know more details about the cerebellum.

We're not going to go into that here. And then we have the brainstem, which of course is made up of the midbrain, the pons, and the medulla. All right, now this is a little bit more detailed here. So I already talked about precentral and postcentral.

So those are primary motor area and primary sensory area. And then, of course, right here, we have the central sulcus. There are other areas that are obviously going to be very important.

I will draw your attention here to Broca's area. It's in the frontal lobe, very close to the temporal lobe. And then we have Wernicke's area, which kind of spans through the temporal and parietal lobe.

Broca's area is important for initiating speech, and Wernicke's area is important for understanding speech. So they both play a role in language, they just do different things. And then as mentioned in the occipital lobe, we have the secondary and primary visual areas.

Occipital lobe, you need to think number one thing is vision. And then as I mentioned, auditory information gets sent to the temporal lobe. Now, if you look at the prefrontal cortex, what you're going to see is this sort of mapping. where different parts of the prefrontal, and for that matter, the postfrontal cortex, process information or control different parts of the body in sort of a mapped way.

And so what you'll want to remember is that the upper extremities are lateral, and the lower extremities are medial. So if you look at the most medial portion of the cortex, what you see is its foot. and toes and hips and legs.

And then as you go laterally, you kind of move upward in the body. So here you have the shoulder and the arm, and then up here you have the hand. And then on the most lateral side, you have areas pertaining to the face.

Now, as we're going to see, the middle cerebral artery is responsible for perfusing the lateral brain. and the more medial parts of the brain is supplied by the anterior cerebral artery. And this is why when we see middle cerebral artery strokes, they tend to be upper extremity symptoms. And when we see anterior cerebral artery strokes, they tend to be lower extremity symptoms. And this is just comparing the somatosensory cortex, which is postcentral, and the somatomotor cortex, which is precentral.

Okay, now this is the basal ganglia. And I'm not going to go into the physiology of the basal ganglia, the neurotransmitters and all that stuff. It is very important for step one.

So it is one of the things you want to put on your list to review. But I will point out the disorders that affect the various parts of the basal ganglia. So first off, as you probably are aware, the substantia nigra is affected in Parkinson's.

Okay, those are dopaminergic neurons. The subthalamic nucleus, when damaged, can cause hemibolismus. And the caudate and putamen are implicated in Huntington's. Now, notice that the caudate and the, to a lesser degree, the putamen on this view here, this is a coronal view.

are relatively close to the ventricles. Now we're going to see how that's important. Now this is an axial view here, and you see pretty much all the same things.

This is just another way that you can see it. Notice here, I'm trying to find it here. Do they have the, oh, I hope, okay, so I didn't put it.

So it's not on here, but this is the internal capsule. This is a very important area of the brain where motor signals are conducted. And so you see it in between the putamen and the thalamus.

And then here we have the caudate. And notice that the caudate is right up against the lateral ventricles. Okay, now this is a patient with Huntington's. Now, what do you notice here? So notice these, and again, I apologize.

I wish I had read. Notice these big ventricles. This is called hydrocephalus. ex vacuo. And so it's not hydrocephalus, but it looks like hydrocephalus because you have a degeneration of the caudate and putamen.

So the caudate and putamen are supposed to be right here and taking up this space, but because it degenerates, it causes the ventricles to get enlarged, making it look like you have a hydrocephalus. Notice also here you have pretty significant cortical atrophy. Okay.

All right, now moving on to the arterial circulation of the brain. This is super, super, super important. You want to know where each of these arteries, you know, kind of where their areas are with respect to the brain.

Because when it comes to stroke syndromes, you're going to need to know, okay, if you have a patient coming in with these symptoms, which arteries are we going to be looking for a blockage? So you'll want to know. where each of these arteries go.

So let's start. So we have the anterior circulation here, and the posterior circulation down here. And notice that they are connected by these anterior and posterior communicating arteries.

These are the posterior communicating arteries. And it's the posterior communicating arteries that will join the anterior and posterior circulation. And then we have a similarly named anterior communicating artery.

They will join the middle cerebral arteries to one another. They'll also give off anterior cerebral arteries. So this kind of circle that's made here with the posterior communicating arteries, it's called the circle of Willis.

Let's look at the anterior. carotid circulation. So what you've got here is you start with the internal carotid artery.

Okay, we know where that is, that runs up the neck, comes off the common carotid artery. And as we get into the brain, the internal carotid will then give off terminal branches, and that's the MCA. And the MCA is going to go, as we've mentioned, to the lateral part of the brain, the lateral part of the cerebrum. It comes off the internal carotid artery, it continues into the lateral sulcus, and it gives off various branches primarily to the anterior temporal lobe, the lateral frontal lobe, and even parts of the parietal lobe. So it's the lateral brain, the lateral cerebral cortex.

Now, this artery here technically is ACA. So you have a branch, the MCA branch, and then the ACA branch. And the ACA will kind of continue up this way as it goes on to supply the medial part of the cerebrum.

And then what's connecting here is the anterior communicating artery. Now, technically, the anterior communicating artery is just this section right here. This here and this here are anterior cerebral artery. Okay, now I want to point out, and I put it here, this is not anatomically correct, but I want to draw your attention.

With the internal carotid artery, the first major branch is the ophthalmic artery, and that supplies the retina. So that's very important for you to know. And then we have the posterior communicating artery, which will communicate with the posterior circulation. So the ophthalmic artery goes to the eyes.

The ACA goes to the middle or medial cerebrum. The ACA will go to the lateral cerebrum. And the posterior communicating artery communicates with the posterior circulation. Now, what about the posterior circulation? So you can think of this as starting with the vertebral and anterior spinal arteries.

So coming up, the vertebral artery, we have PICA, posterior inferior cerebellar artery. And this... supplies the exact parts of the brain that we would think of, the posterior inferior cerebellum, as well as parts of the brainstem.

Then they come together and they form, well, they give off the anterior inferior cerebellar artery, which supplies the anterior cerebellum, as well as parts of the brainstem. And then as they come together, they form the basilar artery, and you only have one basilar artery. This is not paired. And there are various branches that are called pontine arteries, and they obviously supply the pons. Now you have one more paired set of arteries that come off the basilar, and that is the superior cerebellar artery, again supplying parts of the cerebellum.

And then finally, we have the posterior cerebro. artery. And that, again, is going to be cerebrum. So this will actually supply the occipital lobe.

Okay, and then finally, we have the posterior communicating artery, which will communicate with the anterior circulation. So just to recap here, we've got the posterior inferior cerebellar artery, which supplies the posterior lower cerebellum, the anterior inferior cerebellar artery, which supplies the anterior lower cerebellum, the basilar artery, which supplies the empontine arteries, which supply the medulla and the pons. The superior cerebellar artery, which supplies the superior cerebellum. And the posterior cerebral artery, which supplies the posterior cerebrum, namely the occipital lobe. All right, and this is just kind of a recap here.

Now, on your exam, particularly if you're taking step one, you will probably be given a visual, kind of like this. Now, in real life, arteries are not this red. and veins are not this blue. But this is a possibility of a picture that you might see. And so what you want to know is your major landmarks.

So notice this vessel right here that's connecting these two in the front. This is anterior communicating. And so then you know from that point on that here you're dealing with MCA.

And what's being connected by these anterior communicating arteries is the... anterior cerebral artery. And so this here is the internal carotid artery. And then this here that's going backwards is the posterior communicating artery.

So here is your circle of Willis. And then you can see here the basilar, which is giving off these various pontine branches and so forth. So this is important for you to be able to understand. the major arteries by looking at an anatomic specimen. So if you understand it first by looking at a cartoon, a model, then kind of transition your way into understanding it and looking at various actual specimens that will go a long way for you, particularly for step one.

You will not be expected to do this for step two or three. Now, there are a variety of stroke syndromes that we can encounter based on blockage of any of these vessels, and they will naturally cause different symptoms because the different parts of the brain do different things. Now, if you're taking step one, you will probably want to know all of these. If you're taking step two or three, the big ones you want to know, MCA, ACA, ophthalmic. You'll probably want to know PCA and...

maybe ASA. It's good to know all of them, but those are the most important ones for you to know because they're the most commonly affected. Now, another thing that's important for you to know is where berry aneurysms take place, also known as saccular aneurysms. So these are the main areas here in the red that are affected by berry aneurysms. The most common place that we see it is in the...

First part of the anterior cerebral artery, so right here. And it's right around where we have the anterior communicating artery coming off. We have a bifurcation. And so that area is the number one most affected place to get an aneurysm. Then followed very closely by the posterior communicating artery, which is right here.

And then the MCA, and that's more distal when we're talking MCA. So we're talking over here. All right, so those are the most common sites of these berry aneurysms.

Now, this is something you've got to know for step one, and I'm not going to go through it. But you want to know where the cranial nerves come off and what fissures they go through. So these little...

foramina in the skull. So you want to know all of them. So cranial nerve one comes through the cribriform plate.

Cranial nerve two, which is the optic nerve, goes through the optic canal. Superior orbital fissure, I think of orbital like eye, and all of these have to do with eye. Oculomotor, trochlear, abducens, and ophthalmic. Foramen rotundum is V2, the maxillary.

Foramen ovale is V3, the mandibular. And the way I remember this is SRO, standing room only. So S is superior orbital fissure. It'll be all of the I nerves, including the ophthalmic nerve, V1.

And then R is V2, and O is V3. The internal acoustic meatus is 7 and 8, facial and vestibular cochlear. The jugular foramen is 9, 10, 11. and the hypoglossal canal is 12. Now, you'll want to, again, be able to look at a skull and know where each of these foramen are, because you may get asked to point one out.

And that's strictly a step one thing. All right, now we're going to talk about the spinal pathways, and this will be the last thing we talk about. Now, this is relevant, particularly for step one, but step two and three, it does have some relevance as well. So with these spinal pathways, we have two sensory pathways and one motor pathway. We're going to start out with the sensory pathways.

So we're going to talk about the DCML and the spinothalamic tract. Those are your sensory pathways. And then we'll talk about the one motor pathway. The DCML, also known as the dorsal column medial lumbiscus tract, is responsible for transmitting information about fine touch, vibration, and proprioception. So what happens is you have a nerve that has a receptor.

It transduces a signal from the skin all the way to the spinal cord. So you have this neuron that enters dorsally. So all your sensory information is going to enter dorsally.

So up here. And so as you enter dorsally, you come up into this tract here. So this is the dorsal column.

It's right here. And now as nerves enter. they're going to enter from medial to lateral. So what you see here is your lower, if you look at these words here, your lower extremities are going to be more medial. And then as your upper extremities come up, as you move upwards, they come kind of laterally.

Okay, so you get added from middle to lateral. So in the very middle, you have your sacral, and then more laterally, you have the nerves coming on at cervical. levels.

Now, as we go up, we have our first synapse at the medulla. So that is our first synapse. So all of this right here is one nerve. Okay, that's the first nerve.

And then we have a synapse at the medulla. And right after that synapse, we have a decacation. Okay, so decussation just means that you cross to the other side.

And so these fibers, they cross from one side to the other. They're called the arcuate fibers, and they come off these nuclei at the medulla called the cuneate and gracile nucleus. You may also hear it referred to as nucleus cuneatus or nucleus gracilis. And so now we have the second nerve, and this is going to go all the way up to the thalamus.

And particularly, it's the ventroposterolateral nucleus of the thalamus. And then from there, we have a third neuron that then goes to the sensory cortex. So what is important here is that you understand that we have a decussation. And so if there's some sort of pathology or stroke at the level of the brain or further up than the thalamus, then what you're going to have is a contralateral presentation. So if it affects the right part of the cerebrum, it's going to give you symptoms on the left part of the body and vice versa.

Whereas if you have a lesion at the spinal cord, because it hasn't decussated yet, it's going to be ipsilateral. So this is just another way to look at it here. Notice that we have our first neuron that goes all the way up to the medulla. And then we cross over. Those are your arcuate fibers.

And then we continue going up to the cerebrum. We stop at the thalamus. We have another synapse there.

And then we go to that ipsilateral somatosensory cortex from the thalamus. And know this is VPL. Okay, now moving on to the spinothalamic tract.

This carries information about pain, temperature, crude touch, and pressure. And there are actually two distinct spinothalamic tracts. And so you'll see them right here.

So we have a lateral that's right here. and an anterior, which is right here. And so they're both kind of in the same area, sort of in the anterior part of the spinal cord. But there are two tracts, and they actually carry fibers with different information.

All right, now what happens here is you have a sensory nerve that comes in, and it comes in dorsally. So it comes in this way. And what will happen is that first neuron will travel up a couple levels.

And so it travels up right about right here. And that area is called the Sauer's tract, where it'll travel up. Sometimes it can even travel down.

But for the most part, it travels up. And so it travels up a ways. And then it'll start to...

And that's all the same nerve, by the way. There's no synapse there. And it'll go up a couple levels. Then it'll start making its way inward. And there's a...

synapse right there, so about right here, and this is at the nucleus proprius. So then you have a second nerve that comes in like that, and practically immediately it's going to cross. Now that is distinct for the spinothalamic tract versus DCML.

The spinothalamic tract decussates in the spinal cord, okay? Very important to know that, very important to know that. So what you have is this decussation, and this decussation happens right in front of the central canal right here, and that becomes clinically relevant. So when you're dealing with a syringomyelia, where you have dilation of the central canal, typically at the cervical levels, you will get pressure on that anterior white commissure that carries those decussating fibers.

And so that is why if you get a syringomyelia, which is at the cervical level, you're going to get that loss of pain and temperature in a cape-like distribution. It'll be upper extremities and kind of the back area. And that's because it places pressure on that anterior white commissure. So then at that point, so you're coming all the way over here or over here, and you travel up that tract and you go all the way up to the thalamus. And then again here, we have another We have another synapse, and then it's going to continue traveling to the postcentral or sensory gyrus.

All right, so that's spinothalamic. The big thing to know is that you decussate at the spinal cord. And in a future talk, I'm going to talk about stroke syndromes, and you'll see how all this will come together.

So this, again, here is this anterior white commissure, and it's right in the front. of the central canal. All right, this is just a comparison here.

And I will point out that with spinothalamic injuries, if it's a spinal injury, you're typically going to have contralateral to the injury because we have that early decussation. However, at the exact level of the injury, so not below, but at the exact level, because you have fibers there that have not crossed yet, you'll have a little bit of ipsilateral. So you'll have ipsilateral roughly at the level of the lesion, and then everything below that is going to be contralateral.

Now the corticospinal tract is our motor tract, and there's only one as far as major tracts. There's some minor tracts, but As far as what you're responsible for, for step two and three, it's just this corticospinal tract. Now, they are right here in the red, and there is lateral and anterior.

Lateral being primarily the limbs, anterior being primarily axial, and that helps. You can remember that because they start with the same letter. So here, we're starting at the precentral gyrus, the motor gyrus, motor cortex.

So our first neuron comes from the precentral cortex, and we come down to the medulla, and we cross at the medulla. Okay, so that decussation is called the pyramidal decussation, and it's at the lower medulla. However, there is no synapse.

This is all the same nerve. You just happen to cross at the medulla. So it's still the same nerve. We're not synapsing. Now we travel down those corticospinal tracts, again right here.

And we go all the way down to the level that we want to come off at. And at that point, you synapse, and that is a lower motor neuron. There's also interneurons, but mostly I want you to be aware that you synapse with the lower motor neuron. And that neuron is going to come out at the anterior horn.

And so you'll come off anteriorly like that. And you'll go all the way on then at that point to the muscle that you want to affect. Alright, so that is the corticospinal tract.

And so if you have injury to the brain, then you're going to have contralateral signs. This is why if you get an MCA stroke, for instance, say right here, it's going to affect the contralateral side. Whereas if you have an injury to the spinal cord after the decussation, it's going to be to the ipsilateral side.

So this is another great way of looking at it here. You can see in a little bit more detail here, you have your pyramidal decussation at the level of the medulla, and then you travel down the corticospinal tract, ultimately coming to the lower motor neuron, you synapse there, and then that will go to the skeletal muscles.

Transcript for:Neuroanatomy for Medical Examinations

Transcript for:
Neuroanatomy for Medical Examinations