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Overview of the thalamus and surrounding structures.
Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we're going to be talking about the thalamus. On this tutorial, we're going to be looking at the thalamus as well some other surrounding structures. And just to give you a quick overview of what we are going to see in this tutorial, I would like to show you this image that you see now on the screen.
In this image, we see the cerebral hemispheres from a superior view and we have made a horizontal section at the level of the temporal horn of the lateral ventricles – as you can see here – where the hippocampi lie. We also created here a coronal section at the level of the caudate nucleus and another coronal section right about here at the level of the occipital lobe. With these sections, we are able to see some of the important internal structures of the cerebral hemispheres. So, in this tutorial, we will not specifically focus on the various nuclei and the internal structure of the thalamus but rather with the position of the thalamus into the hemispheres and we will also describe some of the structures that are closely related to it.
So, let's start with our centerpiece here highlighted now in green, this is the thalamus. The thalamus is a large, paired – as you can clearly see here – symmetrical structure of grey matter and is the main part of the diencephalon. As you can see here on this image, the two symmetrical thalami are located deep in the cerebral hemispheres and they surround the third ventricle that is now seen here highlighted in green. They are elongated along the anteroposterior axis and are ovoid in shape.
From a structural point of view, the thalamus is comprised of grey matter and is made up of several nuclei some of which we will discuss in the next slides. From a functional point of view, the thalamus is of utmost importance for the integration of sensory information in the central nervous system and for also the regulation of motor activity and consciousness. In order to play all these roles harmoniously, the thalamus is located between your cerebral cortex and your brainstem. This location allows it to act as the central hub relaying and integrating a myriad of motor and sensory impulses between the higher centers and the periphery. So, we can say and you have to remember that the thalamus is the central gateway to the cerebral cortex, since the majority of the sensory information that comes from the periphery to the cortex and motor information that leaves the brain passes through the thalamus.
The next structure we're going to be seeing here highlighted is known as the pulvinar of the thalamus which is comprised of a number of nuclei. It is located at the caudal third of the thalamus. The pulvinar – which you can see here from the posterior view with the cerebral cortices removed - the pulvinar does not receive extrathalamic input and must, therefore, be viewed as an integration nucleus. It is integrated into the optic and acoustic control centers and it makes reciprocal fiber connections with the cortex of the parietal lobe and the dorsal temporal lobe. It also receives afferent fibers from the lateral geniculate nucleus and possibly from the medial geniculate nucleus as well.
It is important to keep in mind that although the pulvinar receives many intrathalamic inputs, its functional significance is not fully understood. From the posterior, ventral and lateral aspects of the pulvinar, two important nuclei are present and protrude from each side. Now, these are known as the medial and lateral geniculate bodies or nuclei. Now, on this image, you can see the lateral geniculate nucleus also known as the lateral geniculate body. Now, this is a nucleus of the ventral thalamus. This small ovoid nucleus contains six layers of neurons with the first two layers comprising of magnocellular cells and the remaining four layers comprising of parvocellular cells.
The lateral geniculate body is the primary visual relay receiving axons from the retina through the optic nerves and tracts and from the superior colliculi. Now, its voluminous efferent system is the optic radiation in the direction to the primary visual cortex as you can see here in this illustration where we see the optic radiations leaving the lateral geniculate as you can see here in the direction of the visual cortex. The function of the lateral geniculate nucleus is to act as a relay center for the visual pathway into the thalamus.
Next, we're going to be then seeing highlighted the medial geniculate nucleus, and this one is the diencephalic relay station of the auditory pathway representing the thalamic relay between the inferior colliculus and the auditory cortex. Now, the medial geniculate nucleus is located medial to the lateral geniculate nucleus – if you remember, this is the lateral geniculate nuclei – and receives afferent fibers from the ipsilateral inferior colliculus via the corresponding brachium and sends most of its axons to the primary auditory cortex. Therefore, it is very important to remember that the medial geniculate bodies are associated with the auditory pathway while the lateral geniculate bodies are then – remember from the previous slides? – yes, they are associated or a part of the visual pathway.
The final structure of the thalamus that we will be looking at is this one seen here highlighted in green, the stria – stria medullaris. The striae medullaris of the thalamus which is a bundle of fibers located at the medial side of the thalamus below the thalamic taenia. It receives afferent fibers from the hypothalamus, the septal thalamic nuclei and the anterior thalamic nucleus. It also synapses in the habenular nuclei of the epithalamus and, as a result of these connections, it is considered part of the limbic system.
So far, we have seen the structures that make up part of the thalamus and that are visible from this perspective. In the next slides, we will be looking at some of the surrounding structures, starting with this one that you see now highlighted in green, this is the lateral ventricle. So, here we are going to be looking at these cavities that you see highlighted in green, they are the lateral ventricles – so there's a left and right lateral ventricle – and they are part of the ventricular system of the brain where the cerebrospinal fluid is produced and through which it travels. In total, there are four ventricles in the brain. The two C-shaped lateral ventricles as you can see here on this image highlighted in green – make up two of the four.
The lateral ventricle can be divided into the following parts: An anteriorly projecting frontal horn which you can see here. There is also a central part right about here which is located in the region of the frontal and parietal lobe, or lobes. There is also an occipital or posterior horn as you can see here which is a fingerlike projection posteriorly whose floor contains then the calcar avis. And there is also a temporal or inferior horn right here which is the most inferior part of the lateral ventricle. This one extends into the temporal lobe and houses the choroid plexus.
The two anterior horns of the lateral ventricles that we just saw are separated by this thin bilayered membrane, the septum pellucidum. So, as I mentioned, this is the bilayered membrane and it extends between the fornix inferiorly and the corpus callosum superiorly and anteriorly.
Now that we have looked at the lateral ventricles and the septum pellucidum that lies between them, the next part of the ventricular system of the brain closely related to the thalamus that we will look at is this one here the – highlighted in green – the third ventricle. Now, the third ventricle is located in the diencephalic part of the brain and it is a narrow slit that is bordered laterally by the medial nuclei of the thalamus, the hypothalamus and is interrupted by the interthalamic adhesion. It is connected to the fourth ventricle posteroinferiorly through a small canal called the cerebral aqueduct and here we can see the third ventricle from the medial aspect, and as you can see in its inferior most part are the infundibular and supraoptic recesses of the hypothalamus - so you can see it right about here.
For this tutorial, I will not go into detail about the ventricles as they will be covered in a separate tutorial here at Kenhub.
Next, let's have a look at two of the most important sulci that can be seen from this perspective, the first of these is very prominent and clearly definable and that is the medial longitudinal fissure, also known as the longitudinal cerebral fissure. This deep sulcus separates two hemispheres of the brain. In the depth of the longitudinal fissure lies the corpus callosum as you can see a little bit here and this fissure also houses the falx cerebri.
The second important sulcus seen from this perspective is the sulcus located below the cuneus near the primary visual area known as the calcarine sulcus. If I show you here the medial view of the brain with the calcarine sulcus highlighted in green, you can see it starts near the occipital pole right about here running forward just below the splenium of the corpus callosum which is here – this is the corpus callosum, you can see how it runs towards this structure. Anteriorly, the calcarine sulcus meets the parietooccipital sulcus at the acute angle which is found right about here. This sulcus marks the end of the visual pathway and it is the primary visual cortex.
The calcarine sulcus that we just saw forms an enlargement into the occipital or posterior horn of the lateral ventricle called the calcar avis. Calcar avis comes from the Latin meaning rooster spur because it resembles the spur of a cockerel. So, this is an enlargement as you can see into the occipital or posterior horn of the lateral ventricle. Now, an interesting point I would like to make about these structures that a century ago, it was known as the hippocampus minor and it was actually a central issue that caused major debate over evolution between the most prominent scientists of the 19th century.
The next structure we're going to be highlighting here is one that I mentioned before, the corpus callosum. This is the largest bundle of white matter commissural fibers of the cerebral cortex. Now, these fibers connect the left and right cerebral hemispheres at the base of the longitudinal cerebral fissure. The corpus callosum is comprised of four parts: the rostrum, genu, truncus or a body, and splenium. In this image, we see a part of the body of the corpus callosum.
Another important structure that we see into the lateral ventricles and close to the corpus callosum is this paired structure here known as the caudate nucleus. This is one of the two nuclei, the other being the lentiform nucleus that makes up the corpus striatum which is part of the basal ganglia. The caudate nucleus is comprised of a large head that gradually decreases and forms the body which finally becomes a thin, long curved structure which is the tail of the caudate nucleus. In this illustration here, we can only see the first portion, the head of the caudate nucleus, which as you can see forms part of the floor of the anterior horn of the lateral ventricle.
Immediately lateral to the caudate nucleus, we see this structure that is highlighted here in green, this is the internal capsule. It is one of the most important fiber systems of your brain. It lies medial to the lentiform nucleus and lateral to the thalamus and caudate nucleus. It contains both ascending and descending fibers that connect to the cerebral cortex with the brainstem, the spinal cord and thalamus.
Another very beautiful structure that we see from this view is the hippocampus. The hippocampus is a paired structure located in the medial temporal lobe and this way fills the temporal horn of the lateral ventricle. It is part of the limbic system. It has a curved structure and the name comes from the Greek for seahorse due its resemblance to a seahorse. Now, the function of the hippocampus includes the storage of long term memory and spatial navigation.
From the most posterior part of the hippocampus, a bundle of fibers begin known as the alveus. As the fibers of the alveus travel posteriorly, they aggregate medially to form the fimbria of the hippocampus. So, remember this, as the fibers of the alveus travel posteriorly, they aggregate medially to form then these structures that we have now highlighted in green. So, basically, the fimbria are a bundle of fibers that carry both afferent and efferent information to the hippocampus. These fibers pass medially and upwards on the hippocampus and continue into the fornix as the crus of the fornix.
As we just saw, the fimbria forms another bundle of fibers that you see here highlighted in green, the fornix. These fibers run in both directions between the mammillary bodies and the hippocampus. The fornix has a C-shape and is comprised of a body, two posteriorly located crura, and the columns anteriorly. It is part of the limbic system. The main functions of the fornix include connecting the hippocampus to the mammillary bodies which you can see here on this image. So, these are the mammillary bodies and here what we talked about before, the hippocampus. It also connects the mammillary bodies to the anterior nuclei of the thalamus. Another function is to then connect the hippocampus to the septal nuclei and the nucleus accumbens.
Lateral to the fornix, we see the choroid plexus of the lateral ventricle – this structure seen here highlighted in green. A choroid plexus is found in each of the four ventricles of the brain and they are responsible for production of cerebrospinal fluid. Now, this extensively folded membrane consists of a layer of cuboidal epithelium or simple cuboidal epithelial cells surrounding a core of capillaries. Capillaries are fenestrated with specific permeability. The folding of this membrane allows it to have an expansive surface area and please note that the choroid plexuses are not present in the frontal horn of the lateral ventricle, also not present on the occipital horn of the lateral ventricle and the cerebral aqueduct.
Here, we only see the choroid plexus of the lateral ventricle but please keep in mind that there are four choroid plexuses in your brain – two in the lateral ventricles and one in the third ventricle and one in the fourth ventricle.
So, next, let's focus on this area caudal to the thalamus and the first structure that we see here is the pineal gland. The pineal gland, also known as the epiphysis cerebri, is located in the diencephalon. It projects from the posterior wall of the third ventricle as you can see here on our illustration. So, remember, previously, this is part of the third ventricle. The pineal gland is attached to both the cerebral hemispheres via the habenular commissure and the trigone superiorly and via the posterior commissure inferiorly. The pineal gland is an endocrine gland. It is responsible for releasing melatonin regulating the circadian rhythm. The regulation of wakefulness and sleep is a secondary function and is achieved through the regulation of the circadian rhythm.
Just inferior to the thalamus and on either side of the pineal gland, we can see these two bumps which are the superior colliculi or superior colliculus for singular. The superior colliculi are located on the dorsal aspect of the midbrain. Now, they are relay station for the reflex movements of the eyes and pupillary reflexes. The commissure of the superior colliculi connects the right and left superior colliculi while the brachium of the superior colliculus connects them to the corresponding lateral geniculate body.
Next, we're going to be then showing the inferior colliculus or colliculi. The two inferior colliculi also found on the dorsal aspect of the midbrain are located just below the superior colliculi which, if you remember, they're here. The inferior colliculi are the synaptic relay station of the auditory pathway. The commissure of the inferior colliculus connects the left and right inferior colliculi as well as receiving fibers from the lateral lemniscus of the contralateral side, whereas the brachium of the inferior colliculus connects the medial geniculate nucleus to the inferior colliculus. Now, please note that the inferior colliculi together with the superior colliculi form the quadrigeminal plate. We can also see here another view – or a posterior view – of the inferior colliculi.
Finally, on this tutorial, we will only see a small portion of the superior surface of this structure here highlighted in green, this is the cerebellum. The cerebellum which has an outer grey matter – cortex – and a whiter matter internally in which masses of grey matter known as the cerebellar nuclei are found. If you remember the location in your skull of the cerebellum – yes, it is located on the posterior cranial fossa. The name cerebellum comes from the Latin meaning "small brain" which is kind of similar – it's where you can see here on this illustration why it's so small. So, it is a smaller brain.
The cerebellum plays an important role in motor control. It does not initiate movement but rather it modifies the motor commands of the descending pathways to make movements more adaptive and accurate. I will not be covering the parts of the cerebellum in this tutorial but please look at our tutorials here where we cover the cerebellum in a bit more detail.