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Horizontal sections of the brain at the levels of the genu of the corpus callosum and the habenula.
Hey guys! This is Nicole from Kenhub, and welcome to our tutorial on the anatomy of the horizontal sections of the brain. In this tutorial, we’re going to be hoping you understand the orientation and structural anatomy of brain slices. Initially, we’re going to try and give you a better understanding of which part of the slice lies anteriorly and which posteriorly in the brain, and most importantly, why, by identifying structures that are unique in either the front or the back of the brain. Then, we’ll cover the lateral sides of our horizontal sections by exploring them from the surface of the brain to the thalamus.
Before we delve into the anatomy of brain slices, it’s extremely important to remember the very crucial distinction between the gray matter and the white matter of the brain. So you might remember that gray matter refers to the regions of the brain occupied by the cell bodies of neurons, and these regions appear in a slightly darker pink color in the cross-sections.
The white matter refers to the whiter regions you can see where neuronal axons are often covered in lipid-rich sheets of myelin which appears white when fresh. The dense layers of myelin causes most lights to be reflected back to the viewer and the areas that are not myelinated heavily such as the gray matter absorb much more light and does appear darker.
Destruction of myelin would result in change of color in a gross specimen, but it’s important to note that although the demyelinated area might have a color similar to that of a gray matter area, it would not actually be considered a true gray matter area because as we’ve already explained, gray matter areas contain neuronal bodies while demyelinated areas still contain neuronal axons.
So, again, we’ll start our tutorial by looking at the anterior part, the horizontal brain slice, and learning to identify it based on the deep brain structures. And the first structures that we must become familiar with are the anterior horns of the lateral ventricles, and their relationship with the heads of the caudate nuclei.
We’re going to refer to these structures in addition to other neighboring structures we find anteriorly in the horizontal section as the anterior regions. And this is not an official term for this group of structures, but it’s rather used to help you remember which structures you should be able to identify when you navigate yourself in a horizontal slice.
So, let’s have a closer look at the lateral ventricle.
So, how do we know if what we’re looking at lies anteriorly or posteriorly in this section? This is a really important question especially since the lateral ventricle is a large structure that spans the anteroposterior axis on either side of the brain, so how we’re going to tell which side is which? If we look closely in our image, we’ll notice that we can see ventricle-like spaces found both at the top and at the bottom of the image. So, it’s not all that hard really if you have to look at the part of the lateral ventricle that is highlighted on the top of the image which corresponds to the anterior part of the section, and with a little dose of imagination, you can see that the two horns seem to form a V-like shape and then much closer to one another than the two horns at the bottom of the image which is the posterior part of the section. Moreover, if you check out this large gray matter region which lies directly laterally to the anterior horns. The association with this large gray matter region is unique for the anterior horns.
Okay, so let’s switch our highlight now to match those gray matter regions, and each of these gray matter regions is the head of their respective caudate nucleus. Caudate nucleus is a gray matter structure which also spends the anteroposterior axis of the brains, and it’s important to note that it’s always lateral to the lateral ventricle.
So, the caudate nucleus can be used as an anatomical landmark to determine the orientation of a brain slice because its anterior segment which we just now named the head of the caudate nucleus is much bigger than the rest of the caudate nucleus. If we take a brief look at the lateral ventricle of the bottom of our slide, we’ll see a tiny area of gray matter directly lateral to the lateral wall of the ventricle, and this is the tail of the caudate nucleus.
I’m pretty sure you can see the difference in size between the head of the caudate nucleus and this tiny gray matter region in the back which corresponds to its tail.
So, the caudate nucleus is one of the basal ganglia which is a group of subcortical nuclei strongly interconnected with the cortex most commonly known for the involvement in the regulation of movement. For simplicity’s sake, let’s think of the basal ganglia as deep brain nuclei that constantly smooth out motor patterns produced by the cortex, and as a part of the basal ganglia, the caudate nucleus is thought to play a role in the regulation of motor function.
Of note, disorders or lesions of the caudate nucleus caused either too little movement or abnormal involuntary movements as well as tremor and abnormalities of muscle tone.
Now before we move on to discussing other regions in our horizontal slice, it is important to mention some other structures which can be found in the front of the brain which are no less than important than the lateral ventricles and the heads of the caudate nuclei. More specifically, we’ll talk about the white matter structures that are directly related to the anterior horns of the lateral ventricle as well as a membrane which connects these structures while also separating the lateral ventricles from each other.
So, we are now looking at the structure which lies in front of the anterior horn of the lateral ventricle, and this is the genu of the corpus callosum – a major commissural structure which connects the right hemisphere to the left hemisphere. The corpus callosum can be found in the midline of the brain and runs in the anteroposterior axis. It’s divided into three regions. It’s more anterior part is known as the genu, its most posterior part is known as the splenium, and the part in between is known as the body.
As we’ve already discussed, the genu can be found anteriorly, but what we didn’t mention is that it also lies anteriorly to the horns of the lateral ventricles. So if you noticed a large white matter structure in that area, you can be absolutely sure that it is the genu of the corpus callosum. And we’ll talk about the splenium of the corpus callosum in our discussion of the posterior regions of the structures a little bit later in this tutorial.
So, let’s now move on to another white matter structure on the anterior part of our image, and this is the column of the fornix. The column of the fornix is a paired structure which can be found very near the midline on the posterior medial portion of the anterior horn of the lateral ventricle, and the reason we cannot see it on both sides of the brain is because as we move to lower horizontal slices of the brain, the fornix bends and continues its way towards the mammillary bodies.
For those of you who are wondering what the function of the fornix is, let’s summarize it to the fornix being white matter and containing output from the hippocampus to other regions of the brain, particularly the frontal cortex and the thalamus. The fornix also carries commissural fibers that connect the contralateral hippocampi as well as input from the aforementioned regions.
Finally, let’s shift our attention to this structure between the corpus callosum and the column of the fornix. This midline structure is the septum pellucidum – a thin veil that separates the two anterior horns of the lateral ventricles in the midline. So if you can see the two horns of the lateral ventricle really close to each other but only separated by a thin veil, you should know that you’re looking at the anterior part of the brain, and that the two horns that you’re seeing are the anterior horns and that the veil is the septum pellucidum.
You should also be able to note the relationship between the caudate nuclei and the lateral ventricles and be able to identify the gray matter near the lateral ventricles as the head of the caudate nucleus.
So now that we’re finished our discussion on the anterior regions, let’s move a little bit further posteriorly, and we’re going to be talking about the structures near the midline of the brain which we have not already covered in the anterior region section. We’ll need to know these structures for two reasons. First of all, after discussing them, the ventricular system will become much clearer and more easy to understand, and secondly, we need to be familiar with these structures both for our discussion on the posterior regions of structures as well as for our discussion on the basal ganglia.
Okay, let’s start then with the third ventricle, and this is a cavity in the middle of the brain which is connected to either lateral ventricle by the interventricular foramen of Monro. It also communicates with the fourth ventricle via the cerebral aqueduct, and the third ventricle is bounded by the thalamus on either side and floored by the hypothalamus.
So in this image, you can see some structures highlighted in green on either side of the midline, and these are known as the thalami. Each thalamus is a gray matter region consisting of many smaller nuclei. The thalamus serves as the relay station of sensory signals ascending to the cortex and motor signals descending from the cortex to the periphery. And it’s also involved in several other functions including the regulation of consciousness, sleep, and memory.
The medial side of each thalamus adheres to the contralateral medial thalamus via the thin band highlighted just here, which is known as the interthalamic adhesion. And in terms of its function, we’re not really sure whether this adhesion contains any fibers that truly cross the midline to connect one thalamus to the other as essentially the function is uncertain.
Moving further posteriorly, the structure we can see highlighted is the pineal gland. So, some interesting facts here. The pineal gland actually became popular through the philosophical work of Rene Descartes where it was proposed to be the seat of the soul, and since then, it’s remained popular as a mystical extrasensory perception organ in certain non-scientific circles. So no such things can be proven scientifically, of course, other than the fact that the pineal gland produces the hormone known as melatonin, and this hormone helps to modulate sleep patterns. What you should definitely remember about the pineal gland is that it is located in the midline and on the posterior aspect of the brain. As such, spotting the pineal gland in a cross-sectional image will definitely help you orient yourself as being in the posterior portion of the brain.
Let’s now shift our attention to this little structure just here which seems to connect the pineal gland to the thalamus, and this structure is the habenula. The habenula consists of groups of nuclei which seemed to be involved in reward processing and may also be involved in pain processing, reproductive behavior, and other limbic functions.
Alright, we’ve now finished our discussion on the structures one can find on the anterior of the brain near the midline. So let’s move on now to discuss the structures that can be found posteriorly. Again, we’ll be unofficially be naming this group of anatomical structures as the posterior regions, and there are three plus one major structure we should immediately recognize on the posterior portion of a horizontal brain slice.
And these are one, the posterior horns of the lateral ventricles; two, the tail of the caudate nucleus; and three, the splenium of the corpus callosum. The plus one/extra structure is the cerebellum which you should definitely not miss if you see it. And for now, we’re going to start with the gray matter structures in these regions because we’ll refer to these later when we’ll be discussing the horns of the lateral ventricle and the splenium of the corpus callosum.
Okay, so, let’s talk about the tail of the caudate nucleus. The tail of the caudate nucleus is a small gray matter structure which can be found in the back of the brain, and as we would expect from what we know about the relationship between the caudate nucleus and the ventricular system, the tail can be found lateral to the posterior horns of the lateral ventricles, and this is pretty obvious in this image that you can see here.
We’re now looking at the posterior horns of the lateral ventricle, and let’s compare these now to the anterior horns of the lateral ventricles. And as you can see lateral to the anterior horns, you’ll find a large gray matter structure which you may remember as the head of the caudate nucleus. And here in the posterior region, the only gray matter structures near to the posterior horns of the lateral ventricles are one big gray matter nucleus near the midline which we already know to be the thalamus. And this small gray matter structure just here more laterally to the posterior horns medially is the tail of the caudate nucleus.
The third structure easily recognized in the posterior region of a cross-section is the splenium of the corpus callosum, and you can see it highlighted just here. As you can see, it can be found posterior and medial to the lateral ventricles and the splenium is considered to be the thickest part of the corpus callosum.
Moving further to the back of the brain, we should now be able to recognize the cerebellum. The cerebellum is pretty easy to tell apart. Its cortex does not consist of thick gyri as the cerebral cortex, but of these delicate wrinkle-like structures which are actually called folia.
Now if you can see the cerebellum in our horizontal brain slice, you should immediately know two simple and crucial facts. First of all, the part of the brain where you can see the cerebellum is the posterior part of the brain, and secondly if you’re seeing the cerebellum, you’re looking at a rather low section of the brain. So it’s far more likely that the cortex next to the cerebellum belongs to the occipital lobe rather than the parietal lobe.
You might also be wondering why it’s important to know which lobe you’re looking at, and the main reason for this is that on the occasion of a brain lesion such as a stroke or a tumor, finding a lesion in a particular brain region on MRI could potentially explain the clinical findings of a rigorous neurological examination.
Okay, now that we’re finished with the three plus one major structures, we should be pretty familiar with the posterior portion of the brain, so it’s time to cover some more delicate structures in more detail, and we’re going to begin by looking at the contents of the posterior horn of the lateral ventricle.
So let’s begin our discussion with an anatomical structure which you might encounter in other regions of lateral ventricles, on the roof the third ventricle as well as in the fourth ventricle. So, this structure is none other the choroid plexus, and you can see two of these highlighted in the posterior horns of the lateral ventricles, but if you look closely, you might also notice a similar structure where the anterior horn of the ventricle to your left and it’s the third ventricle.
So, choroid plexuses are tissues comprising of cuboidal cells surrounding capillaries, and their main function is to filter the contents of the blood in order to produce cerebrospinal fluid.
The hippocampus is another structure contained in the posterior horn of the lateral ventricle, and as you can see, it is a ridge of gray matter which can be found on the floor of the temporal horns of the lateral ventricles. In this section, we can only see the posterior part of the hippocampus near the posterior horn of the lateral ventricle.
The hippocampus plays a significant role in memory and in spatial navigation and it’s of quite a bit of clinical significance as the hippocampus is one of the first regions afflicted by Alzheimer’s disease and both memory loss and disorientation can be regarded as a result of hippocampal lesions.
Neural fibers arise as a band of white matter from all the length of the hippocampus on its anteroposterior axis and the band which can be seen along the medial edge of the hippocampus is what you now see highlighted. And this structure is known as the fimbria of the hippocampus.
So remember this structure over here? That’s right. This is the splenium of the corpus callosum, and you should note that as the fimbria moves closer to the splenium of the corpus callosum, it gradually flattens and separates from the hippocampus until it turns into the crus of their respective fornix. And in this picture, you can see this transition from the fimbria to the fornix.
The last structure we’ll be looking at in the posterior horn of the lateral ventricle is the crus of the contralateral fornix, and you can compare this now to this structure of the fimbria we just looked at on the other side of the brain. So, as the crura move further superiorly, they reach a point where they meet and this point, if you remember, is the commissure of the fornix.
After that, the fornix continues on anteriorly and under the corpus callosum until the two begin to separate from each other, and we start seeing the septum pellucidum. Columns of the fornix are located posteriorly to the septum while the corpus callosum is located anteriorly to the septum.
So now that we’ve explored the structures of the anterior, posterior, and midline of a horizontal brain slice, it’s about time now that we take a look at the lateral sides of the brain.
So as you can see here, there are many gray matter structures and white matter structures to discuss. Let’s have a first look before we go into further detail, and as you can see, there’s a layer of cortical gray matter and deep brain gray matter.
The cortical layer of gray matter which we’re now seeing highlighted is known as the insular cortex or the insula of Reil and this piece of cortex resides in the Sylvian fissure which can be seen right here under the folds of the temporal cortex, the frontal cortex, and the parietal cortex.
Okay, let’s move on now to the deeper gray matter. What you’re seeing highlighted here is known as the dorsal striatum, and as you can see, it consists of several different structures, one of which we already know. And if you’re thinking that this is the head of the caudate nucleus, then you are correct. Now, the striatum is a word derived from the word “striae” and if we look closer, we’ll be able to see that each of these deep gray matter nuclei seemed to reach to each other via thin bridges of gray matter. These bridges are the striae from which the striatum takes its name, and we’ll talk about the individual nuclei in the striatum a little bit later in this tutorial.
Finally, let’s have a quick look at what lies in between the cortex in each of the gray matter nuclei we briefly saw previously. Of course, the answer would be white matter, but it’s important to know how to call some very specific regions of white matter in a horizontal brain slice.
For instance, there are some very specific white matter regions near the striatum which we call neither tracts nor fasciculi like we call other white matter regions in the spinal cord. Instead, the most accurate term would be to call them capsules, after using an appropriate adjective to describe how deep they are within the brain parenchyma.
The word “capsule” seems most appropriate because these white matter regions when encountered in a gross dissection appeared to encapsulate the basal ganglia. So, I hope this little introduction has made you more familiar with the basics one should know when exploring the lateral sides of a horizontal brain slice. And let’s move on now to exploring the capsules in this striatum in a bit more detail.
We’re now looking at a region of white matter just medially to the insular cortex, and this region is known as the extreme capsule, because it is the outermost of all the capsules. The extreme capsule contains neural fibers which connect the insular cortex to a deeper structure known as the claustrum. And studies have shown that the extreme capsule contains fibers which extend from the regions associated with comprehension of speech to regions associated with the motor elements of speech. So, it’s speculated to be carrying fibers associated with the functions of speech.
What we’re now seeing highlighted is the claustrum – a word meaning enclosed or hidden space – which is a thin layer of gray matter in between much thicker sheets of white matter – the extreme and external capsules. And recent research suggests the claustrum to be involved in the regulation of consciousness, however, this is a deep region that can be studied with difficulty, not much yet is clarified about it.
The external capsule is a layer of white matter between the claustrum and the striatum. Be careful not to confuse it with the extreme capsule we saw earlier as the external capsule contains cortical association fibers which are fibers which connect different parts in the same hemisphere.
So, we finally reached the striatum and, particularly, the lentiform nucleus. Lentiform means “lens-like” so with a lot of imagination, we might be able to believe that the anatomist that discovered it might have been thinking about the shape of a crystal lens when they were naming it. And as you can see, the lentiform nucleus is only one part of the dorsal striatum, the other part being the caudate nucleus.
The lentiform nucleus comprises of two different nuclei – the putamen laterally and the globus pallidus medially. And if we look closely, we can actually see the three areas of gray matter separated from each other only by a thin white matter laminae. Don’t be confused by that as the two innermost regions of gray matter can be considered as portions of a bigger nucleus.
Okay, let’s talk about the nuclei that comprise the lentiform nucleus in a little bit more detail. So, we’ve highlighted the outermost portion of the lentiform nucleus which is known as the putamen, and the putamen is also one of the nuclei of the basal ganglia. As such, it plays a significant role in motor control particularly in motor preparation and regulating the amplitude of movement collaborating with many other regions of the basal ganglia for these functions.
The innermost segments of the lentiform nucleus is known as the globus pallidus which literally means “pale sphere”, and while it may be difficult to imagine why this structure came to be called a sphere, it’s easy to remember the pale part of the name since if you look at it on a gross horizontal section, it’s actually a lot paler in color compared to the putamen or the caudate nucleus. The globus pallidus is also part of the basal ganglia and as such you can also imagine it plays a role in the motor regulation and control.
So, let’s move a little bit medially now. The structure highlighted in this image is the internal capsule, and this is a very important structure of the central nervous system as it contains many ascending and descending pathways. And as you can see, the internal capsule encapsulates the medial side of the lentiform nucleus. The internal capsule has two limbs. The anterior limb contains descending fibers originating from the frontal cortex and heading downwards to the pons. These are known as frontopontine fibers. It also contains thalamocortical fibers – that is, fibers that originate in the thalamus and ascend towards the cortex.
The posterior limb of the internal capsule contains the corticospinal tract – the main pathway connecting the cortex to the spinal cord as well as ascending sensory tracts and the special sensory tracts.
Finally, the region between the limbs which is known as the genu from the Greek word “gony” which means knee, and contains mainly corticobulbar fibers. These are the fibers that connect the cortex to the brainstem. The posterior limb of the external capsule extends even behind the lentiform nucleus and this portion of the external capsule is known as its retrolenticular part. And this part contains special sensory fibers mainly from the optic system coming from the lateral geniculate nucleus of the thalamus.
And with this region, we have completed our exploration of the lateral side of the brain. And in our last part, we’re going to shift our focus to some cortical structures we should be able to recognize on a horizontal brain slice.
So, we’re now looking at the cingulate gyrus – an integral part of the limbic system involved in emotion formation and processing, learning, and memory. You might be wondering how to distinguish this gyrus from other gyri in a horizontal slice, and it’s actually pretty easy to do. You just need to remember that the cingulate gyrus is always superficially adjacent to the corpus callosum and we can see that both in the front of the brain and in the back of the brain.
If we keep looking to the back of the brain, we can also find the calcarine sulcus. This sulcus is found behind cingulate gyrus, and to better understand the orientation of the calcarine sulcus, we’ll also be looking at a lateral image. The calcarine sulcus is an important anatomical landmark of the posterior side of the brain in addition to being the region where most of the primary visual cortex is concentrated.
And that’s it for the horizontal slices, let’s move on to talk about some clinical notes.
So, stroke refers to a decreased blood flow in a certain region of the brain resulting in cell death. It’s a condition known since early antiquity and was ranked as the second cause of death worldwide after cardiovascular events. As a condition, it can present with a variety of symptoms clinically such as paralysis and can affect anywhere from a single limb to the entire body, speech defects, and higher cognitive function defects amongst other rarest symptoms.
The presentation of these symptoms suggest different regions affected by the reduced blood flow and people with a clinical presentation resembling that of a stroke undergo brain imaging which can either be a CT or an MRI scan. So both these scans yield images of the brain in slices which can either be horizontal, coronal, or sagittal, though usually horizontal slices are preferred in stroke imaging, and the basic regime behind this is that in stroke imaging, we wish to compare the contralateral sides of the brain to one another to see if the other side has any pathological finding. And pathological findings in stroke may affect the cortex or the deeper gray matter structures.
It’s not rare for strokes to leave their mark on the striatum or the internal capsule nor is it rare to find the ventricles of the affected side of the brain diminished in size and the midline shifted due to cerebral edema.
Knowledge of the anatomy of a horizontal brain slice is important to help us recognize the location of such lesions which would allow us to explain the cause of various clinical findings. For example, a lesion in the posterior limb of the internal capsule would explain contralateral paralysis whereas a lesion on the calcarine sulcus would explain loss of vision in the contralateral visual field.
Alright, so we’ve come to the end of our tutorial and before we finally finish, let’s summarize what we’ve learned so far.
So we’ve talked about several regions that we should recognize in a horizontal section and these included the anterior region, the regions nears the midline, the posterior regions, the contents of the posterior horn of the lateral ventricle, and the lateral sides of the section. Each of these regions contain several structures you should be familiar with after the end of this tutorial.
In the anterior regions, you should be able to recognize the anterior horns of the lateral ventricles and the head of the caudate nucleus, the genu of the corpus callosum, the columns of the fornix, and the septum pellucidum. Near the midline, you should be able to identify the thalamus and interthalamic adhesion as well as the pineal gland and the habenula. In the posterior regions, you should be able to identify the posterior horns of the lateral ventricles, the tail of the caudate nucleus, and the splenium of the corpus callosum.
Inside the posterior horn of the lateral ventricle, we expect to find the hippocampus, the fimbria, and the crus of the fornix as well as the choroid plexus. Moving on to the lateral regions, we should be able to identify the insula of Reil, the claustrum as well as specific parts of the striatum, and white matter capsules. Remember that the striatum consists of the putamen and the globus pallidus and that there are three white matter capsules namely the internal capsule, the external capsule, and the extreme capsule.
And, finally, remember that we talked about some notable gyri and sulci namely the cingulate gyrus as the calcarine sulcus.
Happy studying and thanks for watching.