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Structures of the brain, bony structures and muscles of the head at the level of the thalamus.
So you're busy watching your favorite hit medical drama. On this week's episode, there's been a serious accident on the highway. One female has seriously injured. She's been quickly rushed to the ER where the team are frantically working to save her life. She is unresponsive. They suspect a head injury. She's rushed for to CT to find out more. Within minutes, they've got the results. Of course, the doctor takes one look at the scan and knows exactly what's going on. Me, however, are not so sure. You know there is a brain on the CT but after that, you're not sure what's what. Does any of this sound familiar to you? If so, stick with me. I think you're going to like this tutorial today because we're going to examine a cross-section of the brain taken at the level of the thalamus.
For those of you who want to become a master at reading MRI and CT scans, the classic way to begin learning this skill is by studying tried and tested cross-sections of the body. Just as if we were looking at a CT or MRI scan, our main focus today will be primarily to identify a wide range of structures found at this level of cross-section, and also learning about their interrelationships with one another.
Unfortunately, we will not be going into extensive detail about the function and role of each of these structures, however, this is something that you can find in detail by looking at our articles, atlas sections and video tutorials on our website. So let's get started by allowing me to introduce you to this image here which is a cross-section of the brain taken at the level of the thalamus.
When studying cross-sectional anatomy, we almost must remember that we are looking at these slices as if we were viewing them from the feet of the subject upwards towards the head. For that reason, the top of this cross-section is anterior, the bottom is then posterior. Since we're looking from the subject's feet, this would be the right side, meaning we have the left side right here.
As we go through all these structures in this cross-section, we will also have accompanying illustrations from our online atlas here on the left side to give us more perspective of the structures mentioned. Remember each cross-section we'll look at is a two-dimensional slice taken from a three dimensional structure, which means we only can get a profile of a certain structure at this level which might be different from the actual shape of the structure itself. So, it's always good to keep in mind what form the structure has superior and inferior to our section. With all of that in mind, we're going to be starting our study along the outside or periphery of our cross-section and making our way inwards from there.
We will begin with the cranial structures which will basically concern the bones of the cranium or skull. We'll also be talking about some extracranial structures meaning structures found around the outside of the skull. After that, we will then get into the nitty-gritty of the intracranial structures found at this level, which will mainly center a group of subcortical structures such as the thalamus in the center of our cross-section followed by identification of some structures and parts of the cerebral cortex. So let's waste no time and get started exploring our cross-section beginning as we said with the cranial and extracranial structures.
As I mentioned, I would like to start with the skeletal elements or bones which make up the skull and as we are examining a cross-section of the brain, we will naturally be focusing on the superior portion of the skull which is known as the neurocranium.
The neurocranium is composed of several irregular bones which form a protective casing around the brain known as the cranial vault. The first bone that we will look at is the frontal bone – the most anterior of the cranial bones visible in our cross-section and the bone which forms then your forehead. Within the frontal bone, we can see two small air-filled cavities which are known as the frontal sinuses. These sinuses are lined with mucosa and they produce fluid which drains to your nasal cavity.
Focusing now on the left of your screen, we are now looking at an anterior view of the frontal bone. Along the superior aspect of the eye sockets or orbits, we can see it has a flat horizontal section of the frontal bone which is known as the orbital part. Focusing in on these two areas, we are now looking at the orbital plates of the frontal bone. These are then named the orbital plates due to the fact that they lie over the orbits or eye sockets. The part of this orbital plate that lies directly over the orbits is known as the inferior surface of the orbital plate while on the other side which faces your brain, we have the superior surface of the orbital plate. If we can take a look at the cross-section image on our right, we can see just a portion of the right inferior surface of the orbital plate. Remember, it's the inferior surface we are seeing here at as we are looking from the feet upwards.
Now located between the right and left orbital plates, we can see a notch or indentation in this frontal bone which is known as the ethmoidal notch. Within this notch, we can see a bony structure known as the crista galli now highlighted for you in green. This particular bony feature extends off the superior surface of the ethmoid bone which sits immediately inferior to the frontal bone. It serves as an attachment point for the dura mater, which is the outermost meningeal covering of your brain.
We're going to move slightly posterior now, let's take a look at our next cranial bone which is the sphenoid bone. The sphenoid bone is known as the keystone of the skull due to its central position within the skull and articulation with several of the cranial bones. Looking at the image on the left, we can see the sphenoid bone is found deep within the skull and can be seen posterior to the frontal bone. The illustration also shows a part of the sphenoid bone slightly curving up the cranial vault on the side as pointed out here. This portion of the sphenoid curving upwards is what we see on the side of the cross-section highlighted in green in the right image right here. This part of the sphenoid bone is known as the greater wing, and the extent to which it travels upwards can be appreciated a little bit more in this image right here.
The next bony structure we will see in the cross-section is the temporal bone highlighted here in green. Focusing first on the image on the left, we can see the temporal bone is found just posterior to the greater wing of the sphenoid bone which we saw in the previous slide. In our cross-section, we can just barely see the temporal bone as the section has been cut along the superior border of this bone. And just on top of the temporal bone in the cross-section on the left, we find the temporalis muscle. This muscle is pretty easy to remember as it is pretty much called exactly the same as the bone it rests over on top of.
Continuing on with the bones of the skull, let's have a look at this bone now highlighted in green which is the parietal bone. The parietal bone like the temporal bone is a paired bone meaning there is one on the right and left side of the skull. It is found superior and posterior to the temporal bone and contributes to a large portion of the posterior neurocranium which we can see especially while in our cross section.
Last but certainly not least, we have our most posterior cranial bone which is the occipital bone. Despite being a relatively broad bone in its inferior portion, the occipital bone tapers somewhat to form the superior angle or apex of the occipital bone. This actually explains why the occipital bone only appears as a small bony portion of the posterior skull at this level of the cross-section.
Our final structure in this part of the tutorial is this one highlighted in green and located just under the skin that we see here. We find a very thin layer of connective tissue overlying the bones of the cranium, this is called the epicranial aponeurosis. The name itself tells you exactly where and what it is. It's a fibrous tissue above the cranium. To be more specific, we can describe it as the flat fiber sheath situated between the frontal and occipital bellies of the occipital frontalis muscle.
Now that we have covered the cranial and extracranial structures of this cross-section, let's continue now with the intracranial structures visible for examination. And actually, it's only fitting that we start with the central character of our show here which is, of course – wait for it – the thalamus.
This particular cross-section is sliced horizontally through both the right and left thalami, both of which are now highlighted in green. The thalamus is a relatively large gray matter structure located in a part of the brain known as the diencephalon deep to the cerebral cortices. Those of you who have already studied the thalamus, this central position of the thalamus is critical to its role as the information traffic cop at this busy crossroads of the brain.
Between the two thalami, we find a small hollow structure which is known as the third ventricle. The third ventricle is part of what is known as the ventricular system which produces cerebrospinal fluid that suspends the brain within the skull.
Another structure that is located immediately adjacent to the thalamus is the fornix. The fornix, highlighted in green here, is a white matter tract that begins from each cerebral side of the brain at this structure here which is known as the hippocampus. From here, the fornix proceeds anteromedially – that means, forwards and towards the midline – curving around the thalami and goes on to terminate anteroinferior to the thalamus at the mammillary bodies which are not visible in our atlas image.
Let's change to a lateral view of the fornix where we can see the anterior and posterior ends of this C-shaped structure in our cross-section here.
Next, I would like to introduce you to a number of structures which are located around the thalami which are known as the basal ganglia. These are essentially condensed groups of nuclei primarily involved in the planning and control of voluntary movements of the body. Beginning medially, the first member of the basal ganglia which we can see in our cross-section is the caudate nucleus - in this case, specifically, the head of the caudate nucleus which you can see here anterior to the thalami.
Immediately adjacent to the caudate nucleus are two important structures which are collectively known as the lentiform nucleus. The first of these structures and the smaller of the two is the globus pallidus which we can now see highlighted in both of our images. This particular structure plays an important role in monitoring and controlling voluntary motor behavior. Just lateral to the globus pallidus and posterior to the head of the caudate nucleus, we see the larger putamen highlighted in green here. Like the globus pallidus, the putamen serves similar functions but also plays a role in some types of learning. When you are trying to remember the relationship of the putamen and the globus pallidus, remember that putamen starts with a P and is the most peripheral of the two structures here.
Separating the thalamus and the caudate nucleus from the globus pallidus, we have this structure which is known as the internal capsule. The internal capsule contains white matter fibers carrying then information to and from the cerebral cortex, brainstem, and the spinal cord. You can think of the internal capsule as a highway in which motor signals travel from the cerebrum to the peripheral nerves and sensory information traveling in the opposite direction.
Lateral to the putamen, we have another white matter structure which is known as the external capsule. This structure is similar to the internal capsule, however, is more involved in carrying signals between different regions of the cerebral cortex itself.
Beyond the external capsule, I would like to point out this structure here which is known as the claustrum. This somewhat irregular structure is one of the more mysterious and less well-known parts of the brain and it is often debated as to whether it forms part of the basal ganglia or cerebral cortex. The functions of this relatively small structure remain equally uncertain but it's always good to be aware of and identify this somewhat enigmatic structure.
The last major structure that we can see localized to the center of the cross-section image is the splenium of the corpus callosum. The corpus callosum is another white matter structure that connects the left and right hemispheres together and allows for the exchange of information between these two hemispheres. The corpus callosum itself is split into several sections where the posterior being the splenium. The splenium of the corpus callosum is the only part of the corpus callosum that we can see at this level and it is the only section that lies inferiorly enough to appear at this particular level of the cross-section.
You have probably already noticed that most of the structures we are finding are found in pairs, that is, one in the right hemisphere and one on the left hemisphere. Now, this separation is made by the longitudinal cerebral fissure which goes right down the midline of the cerebrum, and as you see in our atlas image and cross-section. At its simplest level, each cortex of each hemisphere can be further divided into small regions known as lobes – four of which, I am now going to identify for you in our cross-section. Let's start at the anterior aspect of the cerebrum where we have the right and left frontal lobes highlighted here in green in both of our images.
The anterior portions of each frontal lobe lies within the dome of the frontal bone just posterior to the frontal sinuses. The frontal lobe is primarily involved in a wide variety of functions such as movement, emotions, reasoning, planning, parts of speech and problem solving.
Now, we're going to move laterally and as we do so, the next lobe which we will find is the temporal lobe again highlighted in both of these images in green. The temporal lobe is found inferior to the parietal lobe which is not visible at this cross-sectional level and like the frontal lobe, the temporal lobe is involved with several major functions related to auditory stimuli, memory, and speech.
If you look with me now at our image on the right hand side, you will see a well-defined groove located here between the frontal, parietal and temporal lobes, this is known as the lateral sulcus or Sylvian fissure.
Now if we were to take some forceps and separate the frontal and temporal lobes here at this groove which is known as the lateral sulcus or Sylvian fissure, we would reveal another lobe of the cerebrum which is known as the insula or insular lobe. This part of the cerebrum is involved with the processing of sensations of taste, touch, pain, and also balance.
We're now going to move towards the posterior aspect of the cerebrum – our final lobe – which we will be examining today is the occipital lobe. One of the primary functions of this lobe involves visual processing as it houses what is known as the primary visual cortex. With that in mind, I would like to bring your attention to this structure here which is the optic radiation. The optic radiation is a white matter bundle located between the thalamus and occipital lobe which takes visual information that has been received by the thalamus and then sends it to the optic radiation towards the occipital lobe.
And finally before we finish, there are two structures close to and around the occipital region which I would like to identify for you. The first of which is the occipital horn of the lateral ventricle. This is another part of the ventricular system of the brain which we briefly encountered earlier when we looked at the third ventricle. We can see in the atlas image that the posterior horns of the lateral ventricle protrude posteriorly into the occipital lobe. This is apparent in the cross-section at the thalamic level and we can see them highlighted in green in this occipital region of the cross-section.
We're going to be moving to the most posterior aspect of our cross-section here, the other structure of interest here is the superior sagittal sinus. The superior sagittal sinus is responsible for returning venous blood from the brain back into the circulation and is highlighted in both of our images. This particular sinus travels along the inner portion of the skull along the sagittal plane and curves downward on the back of the skull as you can see in our image on the left. The descending portion of the sinus is what we find in the cross-section at the thalamic level and it is found in the midline anterior to the occipital bone between the two occipital lobes.
And that's it, we have quickly worked through this cross-section. I hoped you didn't find it too challenging. The next time you come across a CT or an MRI scan taken at this level of the brain, I have no doubt you'll be a pro and identify the structures here.
Before I leave you, I would like to recap what we have learned so let us quickly go through and highlight each structure which we have discussed. First, we looked at the bony structures of the cranium starting with the frontal bone then we looked at the superior surface of the orbital plate and then the crista galli. From there, we studied the keystone of the skull which is the sphenoid bone. We then pointed out the temporal bone and then the muscle with the similar name, the temporalis muscle. Moving posteriorly, we found the parietal bone and then another bone, the occipital bone.
After we covered the bony structures, we took a look at the intracranial structures beginning with the thalamus then the third ventricle and the fornix. We then looked at the basal ganglia and surrounding structures which included the head of the caudate nucleus, the globus pallidus, putamen and the ever mysterious claustrum. We also used these structures to pinpoint several white matter structures in the center of the image including the internal capsule and external capsule which then led us onto the splenium of the corpus callosum.
Our final group of structures centered around the longitudinal cerebral fissure which we learned separates the two hemispheres. We then observed the frontal lobe, the temporal lobe, the insular lobe and finally the occipital lobe. Lastly, we pinpointed the optic radiation, posterior horn of the lateral ventricles, and the superior sagittal sinus.
And that wraps our tutorial on the cross-section of the brain at the thalamic level. Be sure to test your knowledge out now by having a go at our quiz on this cross-section. I'm certain you'll ace it by now and I will see you on the next tutorial.