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Overview of the thalamus and surrounding structures.
When we think of learning the gross anatomy of the brain, the first thing that comes to mind are the humps and bumps of the cerebral cortex, right? We know these as the sulci and gyri, of course, but what about the deeper structures of the brain that lie within and beneath the cerebral cortex? Those not visible without some kind of dissection.
Say, for instance, we wanted to look at the thalamus – the central hub of the brain. Here we can see the thalamus which is that oval green structure in the center. Now, we have to dissect part of the cortex to get a better view of the thalamus. What might that look like if we removed some of the cerebral cortex and what structures would we expect to find around it? Are you curious to find out? Then let's see what lies beneath as we explore an illustrative dissection of the thalamus.
Before we jump into today's tutorial, let's take a quick look at what you can expect to learn in this tutorial. So, today we're going to be preparing you for your gross neuroanatomy lab by examining this illustration of the thalamus and its neighboring structures. We’ll begin by first looking at the thalamus and some structures of what's known as the diencephalon and brainstem. We’ll then be moving back up to examine some subcortical landmarks of the cerebrum which immediately border the thalamus and, as always, we'll wrap up with some quick and interesting clinical notes.
So I should note that we won't be specifically focusing on the various nuclei and the internal structure of the thalamus today, but we do have a separate video here on Kenhub all about those deep internal structures of the thalamus. So let's waste no time and begin exploring the gross anatomy of the thalamus.
First of all, let's figure out exactly what we're looking at. In this image, we see the cerebral hemispheres from a posterosuperior view. So we've resected the cerebral cortex down to the level of the temporal horn of the lateral ventricles where the hippocampi lie and forward as far as the head of the caudate nucleus. In the center, you can see the thalamus highlighted in green remembering that we're looking at it from a posterior perspective.
So now let's move on to our main event, the thalamus, which is also sometimes referred to as the dorsal thalamus, and it is a paired or bilateral structure forming the largest part of the diencephalon. To refresh your memory, the diencephalon is formed from the thalamus, the epithalamus, the hypothalamus, and the prethalamus.
The thalamus is elongated along the anteroposterior axis and is often described as roughly egg-shaped, but it's a little bit smaller than your breakfast boiled egg measuring approximately four centimeters in length. The medial surfaces of the thalami are usually connected by an interthalamic adhesion. It is not known whether any fibers cross over to the other side, so that's why we don't call it a commissure.
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 the regulation of motor activity and consciousness. In order to play all these roles harmoniously, the thalamus is located between the cerebral cortex and the brainstem and you could say that the thalamus is the central gateway to the cerebral cortex since the majority of sensory information that comes into the brain passes through the thalamus.
We’re not going to be looking at all the specific nuclei that are found in the thalamus today. Instead, we'll be looking at its general internal structure. For this, we're looking at the thalamus from the left lateral view, and from a structural point of view, the thalamus is mostly comprised of gray matter and contains up to fifty different nuclei with a few small areas of white matter.
The gray matter nuclei are primarily divided into an anterior group, a medial group, and a lateral group by a Y-shaped white matter sheet called the medial medullary lamina that is also known as the internal medullary lamina. We also find white matter on the posterior aspect of the thalamus in the layer called the stratum zonale, and on the lateral aspect, as the external medullary lamina.
Another external structure is the pulvinar of the thalamus. Can you spot it? It might be easier if we switch to this posterior view with the cerebral cortices removed. This will help you see that the pulvinar is a little extension from the posterior aspect of the thalamus extending past the third ventricle and overhanging the superior colliculi which we will look at shortly. The purpose of the pulvinar is still debated, but it is believed to be in a wide array of functions including vision, language, sensation, and pain.
Inferior to the pulvinar, we find a tiny little bump, the medial geniculate nucleus, also known as the medial geniculate body. It is the relay station of the auditory pathway receiving input from the inferior colliculi. Just lateral and slightly superior to it, you can see a similar-looking elevation called – you guessed it – the lateral geniculate nucleus. The lateral geniculate nucleus is the primary visual relay center receiving axons from the retina through the optic nerves and tracts and from the superior colliculi which control eye movement. Although the geniculate nuclei are part of the thalamus, it has become customary to group them together into a structure known as the metathalamus.
We’re now moving on to a structure of the diencephalon which is related to the epithalamus known as the stria medullaris of the thalamus. It is a thin tract of white matter which extends from the septal nuclei, hypothalamus, and anterior nuclei of the thalamus which terminates at the habenular trigone, which we’ll have a look a bit more closely at in just a moment.
From a gross perspective, the stria medullaris is situated at the border of the medial and superior surfaces of the thalamus below the taenia thalami which is the structure now highlighted in green. The stria medullaris arises at the anterior pole of the thalamus and travels posteriorly to carry afferent fibers to the habenular trigone.
The habenular trigone that we just saw is not a part of the thalamus. In fact, it is a part of the epithalamus. The trigone is a small shallow depression which contains the habenular nuclei, which we mentioned are the destination of the stria medullaris. Here you’ll also find the habenular commissure where the nuclei from the opposite sides of the brain communicate.
Just posterior to the habenular trigone, we find the pineal gland, which is also part of the epithalamus and is also known as the epiphysis cerebri, and it sits quite snug in a small depression between the superior colliculi. It is an endocrine gland responsible for the release of various hormones including melatonin which regulates the sleep cycle.
Just inferior to the thalamus, we can see these four bumps which are the colliculi, and these belong to the midbrain. We’ve actually already seen these structures, so we know that the two superior elevations on either side of the pineal gland are called the superior colliculi and they are a relay station for the reflex movements of the eyes and pupillary reflexes. Just below them, we finds the inferior colliculi, which are synaptic relay stations of the auditory pathway. The four elevations collectively are called the quadrigeminal plate and are all posterior extensions of the midbrain.
More inferiorly still, we see a small portion of the superior surface of the cerebellum. The cerebellum contains cerebellar nuclei which allow the cerebellum to play an important role in motor control. If you want to learn more, the cerebellum is yet another topic covered in a separate video here on Kenhub.
So you've probably already realized that we've been moving away from the thalamus, so let's continue now and look at some subcortical structures of the cerebrum which border the thalamus.
Let’s start by drawing our attention to an obvious landmark, the longitudinal cerebral fissure, which marks the division on the left and right cerebral hemispheres. Within it, you'll find the corpus callosum which we'll discuss in a moment.
Another groove we can see from this perspective is the calcarine sulcus, which is highlighted now. The calcarine sulcus is actually the inferior border of the cuneus – a smaller lobe of the occipital lobe. The calcarine sulcus forms an infolding of the cerebral cortex which results in an enlargement in the medial wall of the posterior horn of the lateral ventricle and it's called the calcar avis. The name actually comes from the resemblance of the structure to a rooster's spur.
Connecting the two cerebral hemispheres, we see the corpus callosum, and the corpus callosum is the largest bundle of white matter commissural fibers of the cerebral cortex. These fibers connect the left and right cerebral hemispheres. The corpus callosum is comprised of four parts – the rostrum, the genu, the truncus or the body, and the splenium. In the left image, we see a part of the truncus of the corpus callosum.
Superolateral to the thalamus, we see a paired structure called the caudate nucleus. What you can see highlighted on both images is the head which gradually tapers to become the thin, curved tail of the caudate nucleus. The head of the caudate nucleus forms part of the floor of the anterior horn of the lateral ventricle.
Immediately lateral to the caudate nucleus and the thalamus, we can see the internal capsule which is the structure now highlighted in green. It contains both ascending and descending fibers that connect the cerebral cortex with the brainstem, the spinal cord, and the thalamus, and it is divided into five parts – the anterior limb, the posterior limb, the genu, a retrolentiform part, and the sublentiform part.
The part we're looking at here is actually the posterior limb. It is bordered by the thalamus and the caudate nucleus medially and the lentiform nucleus laterally.
Another structure that we see from this view is the hippocampus. The hippocampus is a paired structure located in the medial temporal lobe and is part of the limbic system. Its name comes from the Greek word for seahorse due to the curved shape that the hippocampus and the fornix form together. Ah yeah, okay, it's a little bit of a stretch. The function of the hippocampus includes the storage of long-term memory and spatial navigation.
From the deepest part of the hippocampus, a bundle of fibers arises known as the alveus. As the fibers of the alveus travel posteriorly, they aggregate medially to form the fimbria of the hippocampus that can be seen here in our illustration. So, basically, the fimbria are a bundle of fibers that carry both afferent and efferent information to the hippocampus. These fibers pass medially and upward 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 – the fornix – and these fibers run in both directions between the mammillary bodies and the hippocampus and a part of the limbic system. The fornix has a C-shape and is comprised of a body, two posteriorly-situated crura which we discussed in the previous slide, and the columns anteriorly. The hippocampus and the fornix are covered in detail in another video, so check that out if you want to find out more.
The final structures we can't ignore when looking at the gross anatomy of the thalamus and its surrounding structures are the ventricles, and we'll start with the paired lateral ventricles. Even though I've highlighted the areas adjacent to the thalamus, the ventricles are such three-dimensional structures that it can be difficult to picture what exactly we're looking at. So let's have a look at it from a lateral view.
So what I've highlighted for you now is the left lateral ventricle on both images. The part of the lateral ventricle that we're seeing here is the inferior horn, and if we move a bit more anteriorly or away from us in the right image, the opening that we see here is actually the anterior horn of the left lateral ventricle. The two anterior horns of the lateral ventricles that we just saw are separated by this thin bilayered membrane known as the septum pellucidum.
The next part of the ventricular system of the brain closely related to the thalamus that we'll look at is the third ventricle. The third ventricle is located in the diencephalic part of the brain and it is a narrow chamber filled with cerebrospinal fluid that lies between the medial aspect of each thalamus and hypothalamus, and as you can see in the left image, it is interrupted by the interthalamic adhesion. Inferiorly, it is connected to the fourth ventricle.
Also associated with the ventricular system, we find the choroid plexus of the lateral ventricle, and you can see it just lateral to the body of the fornix. A choroid plexus is found in each of the four ventricles of the brain and together they're responsible for the production of cerebrospinal fluid. We’ve only briefly touched on the ventricles here, but we have a separate tutorial dedicated to them, of course, if you want to find out more.
Okay, so now that we've covered all the anatomy, let's have a look at some clinical notes.
So we're going to look at an interesting condition related to stroke affecting the thalamus known as thalamic pain syndrome or Dejerine-Roussy syndrome, and I know what you're thinking, how can there be pain in the thalamus? And you're totally right. It actually begins as numbness or tingling sensation in the opposite side of the body to the side of the thalamus where the stroke has occurred, often the hand of the arm. Over time, it can develop into intense pain in response to little or no stimulation and another symptom is that the pain usually can't be relieved by regular analgesics.
The diagnosis is usually obtained through a neurological examination and brain imaging such as CT, MRI, or angiogram of the brain. Treatment often includes a combination of opiates, antidepressants, and physiotherapy. More recently anticonvulsants, Kampo which is a Japanese medicine, and electrical stimulation have been trialed as treatments as well.
Okay guys so that's our tutorial all wrapped up. Before we say our final goodbyes, let's quickly run through what we've learned today.
So we started with the general appearance and position of the thalamus and we saw that it is responsible for the relay and integration of both sensory and motor pathways between the cerebral cortex and the midbrain. As it's quite a simple structure externally, we swiftly moved onto its basic internal structure, so we saw that it contained numerous gray matter nuclei which were divided into anterior, medial, and lateral groups by a Y-shaped white matter sheet called the medial medullary lamina and we saw another white matter sheet called the stratum zonale on the posterior aspect of the thalamus and the external medullary lamina on the lateral aspect.
On the posterior aspect, we saw the pulvinar, and right below it, the tiny medial geniculate nucleus which is a relay station for the auditory pathway. Just lateral to that, we found the lateral geniculate nucleus which is responsible for relaying visual information. We moved on to talk about the stria medullaris which is part of the prethalamus and it's a bundle of fibers connecting the anterior thalamus to the habenular trigone.
And that brought us to the structure of the epithalamus. We started with the habenular trigone which contains the habenular nuclei and the habenular commissure. Posteriorly to the habenular trigone was the pineal gland – an endocrine gland responsible for the circadian rhythm, and just inferiorly to the thalamus, we found two pairs of structures called the colliculi.
The superior colliculi are the relay station for the reflex movements of the eyes and the pupillary reflexes while the inferior colliculi are synaptic relay stations of the auditory pathway.
We quickly ran through the cerebellum, the bilateral cerebral cortices, and the important fissures that we could see in this image – the longitudinal cerebral fissure and the calcarine sulcus. We saw an infolding of the lateral ventricle – the calcar avis – caused by the calcarine sulcus.
Connecting the two cerebral hemispheres was the corpus callosum which is a collection of white matter commissural fibers connected to the cerebral hemispheres. Superolateral to the thalamus was the head of the caudate nucleus and a bit lateral to that, we saw the internal capsule. It was actually its posterior limb that was bordering the head of the caudate nucleus and the thalamus.
And, finally, we looked at the hippocampus and its associated structures – the fimbria of the hippocampus and the fornix. And, lastly, we looked at the ventricles associated with the thalamus starting with the lateral ventricle and its associated choroid plexus anteriorly separated from each other by the septum pellucidum. And sandwiched between the thalami, we saw the third ventricle, and we finished up with some clinical notes on the thalamic pain syndrome.
Okay, so that's it guys. Hope you enjoyed this tutorial. See you next time and happy studying!