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Internal structures of the brain seen at the level of the thalamus.
Have you ever peeked at an MRI or CT scan and wondered how you're supposed to know what you're looking at? Well, it's not as difficult as it might first appear. Clinicians use certain landmarks to guide them as they analyze the images. In this tutorial, we're going to be looking at the major structures scientists and clinicians use to orient themselves in coronal sections of the brain, especially when taken through the thalamus.
So, today, we're going to be exploring the structures visible in a coronal section of the brain. What is a coronal section of the brain you ask? Well, a coronal or frontal section is a vertical slice of the brain which divides the brain into anterior and posterior parts and, of course, we'll be making our cut at the level of the thalamus.
First, we'll discuss grey matter structures including the thalamus, the caudate nucleus, the lenticular nucleus, and the substantia nigra then we'll look at white matter structures running through the brain including the corpus callosum, the fornix, and the white matter tracts traveling between the brain and the spinal cord. Next, we'll look at some of the named cortical regions visible in this coronal section, and finally, we'll identify a few additional structures – the lateral ventricles, the optic tracts, and the mammillary bodies. But, firstly, let's get oriented.
Remember, we're looking at the brain as if we're standing in front of a person looking at their face. This is superior and this is inferior. This, of course, is the midline of the brain and these are the left and right sides, which are opposite to the viewers since the subject is looking towards us. Right and left is always relative to the subject.
Parts of the brain appear lighter than others. This reflects what we call grey matter and white matter. The brain is made up of neurons and other supporting cells known as glial cells. Grey matter is a collection of neuronal cell bodies while white matter is a collection of myelinated axons bringing messages away from the cell bodies. And just like in our illustration here, white and grey matter can be distinguished in MRI images. Let’s first take a look at some grey matter structures.
As we just said grey matter is made up of cell bodies. This is where neurons make synapses and the brain integrates their signals. So, let's begin at the center of our illustration at the thalamus, which is the most major landmark visible in this particular section. The thalamus, in green here, is the largest nucleus in the brain. It is often nicknamed the traffic control center of the brain. It receives and distributes information between the peripheries and higher centers of the brain such as the cerebral cortices. Basically, it tells many of our nerves where they should go or when they can and can't go. It’s therefore involved in sensory and motor functions in addition to sleep, consciousness, attention, and memory.
The thalamus is bilateral in structure. That means that you have two thalami, one on either side of your body. The two thalami face each other medially across the third ventricle and are variably connected at this point known as the interthalamic adhesion. In humans, it is not believed that fibers cross between the right and left thalami in this structure, so that's why we don't refer to it as a commissure or connection.
The area of neural tissue that lies just below or ventral to the thalamus that we saw here earlier is called the subthalamus. The subthalamus contains a special nucleus which is appropriately named the subthalamic nucleus. It communicates with the globus pallidus, a structure that makes up part of the basal ganglia which we'll see shortly. Let’s move a bit more inferior to look at a part of the brainstem visible in this section.
We can see the midbrain here. The midbrain contains a collection of dopamine-releasing cells called the substantia nigra. This structure looks very dark in a real brain and its name literally means “black stuff.” We mentioned that the subthalamic nucleus communicates with the basal ganglia. Let’s look at these ganglia now.
The basal ganglia are a collection of nuclei that are essential for the control of movement. The basal ganglia are subdivided into the striatum and the lenticular nuclei. The striatum is made up of the caudate nucleus and putamen and the lenticular nucleus is made up of the putamen and globus pallidus. Before we identify in a coronal section, let's look at the caudate nucleus from the side so we can see the whole nucleus and appreciate its shape.
The caudate nucleus, in green, is a C-shaped structure. Because it is curved, we can see the nucleus in two places in the coronal section at the level of the thalamus. Up here, just lateral to the lateral ventricles and below the corpus callosum which we'll look at later, we can see the body of the caudate nucleus in green. The body of the caudate nucleus is much larger than the tail of the caudate nucleus here in the temporal lobe. It is smaller due to the fact that this is the part of the nucleus which tapers off towards the amygdaloid body.
In this image, we can see the lenticular nucleus highlighted green in cross-section. Let’s have a look at its two components a little bit closer.
The putamen seen here is the largest and most lateral part of the lenticular nucleus. The putamen is a relatively large nucleus and tends to fuse to the head of the caudate nucleus that we just saw. This is the globus pallidus which receives its name from the multitude of myelinated fibers passing through this region. Globus pallidus means “pale globe.” It is a cone-shaped nucleus just medial to the putamen. The globus pallidus is made up of two nuclei separated by lamina and we can see both parts here.
The internal segment is closer to the internal capsule. It makes up the apex of the cone. The internal segment of the globus pallidus provides the major output from the basal ganglia to the thalamus. The larger external segment here is closer to the putamen and it makes up the base of the cone. The external segment of the globus pallidus sends projection fibers to the subthalamic nucleus that we discussed earlier.
Let’s move out from the center of the brain now and just lateral to the lenticular nucleus you can see a sliver of grey matter called the claustrum. It’s barely visible in routine dissections and diagnostic images of the brain, but we've highlighted it in green just here.
So, we've looked at several grey matter structures that clinicians used to read diagnostic images, let's now have a look at some white matter structures.
Remember that white matter is made up of axons surrounded by myelin. This is where neural signals are being transmitted around the brain. This is the corpus callosum. It is the largest collection of white matter fibers in the human brain and contains hundreds of millions of axons. These axons share information between left and right cerebral hemispheres.
This is the fornix. The fornix is a curved fiber path with a shape similar to the caudate nucleus and we're looking at its body just here. Axons in the fornix bring information from the hippocampus to the mammillary bodies, which we can see right here. The mammillary bodies send their projection fibers to the thalamus.
Next up is the optic tract. It carries visual signals from cranial nerve two – the optic nerve – to the visual cortex in the occipital lobes.
Our last group of structures are large sheets of white matter known as capsules which travel between the cortex and the brainstem. There are three named capsules that separate the deep nuclei of the brain. The largest white matter capsule – the internal capsule – is the most medial. We can see it here separating the thalamus from the lenticular nucleus.
The internal capsule is the matter topically organized, meaning axons from adjacent regions of the body maintain the same pattern in the capsule. This way, we can organize the capsule into limbs with designated functions. At this level, we're looking at the posterior limb of the internal capsule. Its axons carry motor signals from the face, arm, trunk, and leg.
The external capsule is slightly smaller and is not separated into limbs. It lies here between the putamen and the claustrum. This capsule transmits association fibers which project between various areas of the cerebral cortex so that you can process information from multiple sensors and come to a conclusion. These are the fibers that help you decide – well, it looks like a duck, sounds like a duck, it smells like a duck, so it must be a duck!
Finally, the extreme capsule is the most lateral and we can see it here between the claustrum and the insular cortex. It also transmits association fibers.
That brings us to the end of our exploration of the white matter structures, so let's now take a look at some of the other structures in our coronal section beginning first with the cerebral cortex.
The cortex is the neural tissue that surrounds the outside of the brain. The regions of the cortex are named by their structure and function. This region is called the insular cortex. It is lateral to the lenticular nucleus and white matter tracts that we saw before. If we look at the brain from the side, the insular cortex is deep within the lateral fissure and we must retract the other regions of the cortex to see it.
Up here, we can see a small piece of cortex superior to the corpus callosum. This is the cingulate gyrus, a part of the limbic system. The limbic system is responsible for a multitude of functions but is most well-known for its involvement in regulated emotion and memory formation. Moving down, we can see the temporal lobe. There’s a special part of the temporal cortex right here that's responsible for the creation of memories. It is called the hippocampus because its curved shape resembles a seahorse.
Let’s move on to talk about the spaces within the brain that contain cerebrospinal fluid. These spaces are called ventricles and you have four of them – two lateral ventricles, the third ventricle, and the fourth ventricle. The lateral ventricle is the largest. It has several parts – two bodies, a posterior horn, an atrium, two temporal horns, and two anterior horns. In a coronal section at the level of the thalamus, we can see the lateral ventricle in two places. This part, inferior to the corpus callosum, is the central part of the body of the lateral ventricle. The right and left bodies of the lateral ventricle are separated by the septum pellucidum and the fornix.
This space here within the temporal lobe is the temporal horn of the lateral ventricle. It is the most inferior part of the lateral ventricle. The hippocampus that we saw earlier here makes up the floor of the temporal horn. This space between two thalami is the third ventricle. The interthalamic adhesion that we saw connecting the right and left thalami runs through the third ventricle here.
Next up is the choroid plexus which is a massive vascular tissue found in various parts of the ventricles that is responsible for the production of cerebrospinal fluid.
And that concludes our exploration of the coronal section of the brain. Let’s wrap up our tutorial now with a quick clinical correlation to what we learned today.
Earlier, we looked at a special part of the brainstem called the midbrain and its collection of dark cells, the substantia nigra. These cells project to the basal ganglia supplying it with dopamine making it crucial to the control of movement. In Parkinson's disease, the pigmented cells of the substantia nigra begin to die off which results in symptoms such as tremors and postural instability. Many patients with Parkinson's disease can be treated with a drug called levodopa that helps to replenish dopamine lost from dying cells. In other cases, a surgery called deep brain stimulation in which electrodes placed in the brain modulate the basal ganglia can help to improve symptoms.
Okay, so before we finish this tutorial, let me take you through a quick summary of what we talked about today.
In this tutorial, we looked at grey and white matter structures visible in a coronal section of the brain at the level of the thalamus. First, we identified the right and left thalami - the traffic control centers of the brain and the interthalamic adhesion that connects them.
Then, we identified the grey matter structures surrounding them – the subthalamic nucleus that communicates with the globus pallidus featuring an internal segment and an external segment; the basal ganglia, a collection of nuclei that along with the globus pallidus includes the caudate nucleus and the putamen; the putamen and the globus pallidus together are called the lenticular nucleus; the caudate nucleus, a C-shaped nucleus that can be seen in two places at its tail and its body; just lateral to the basal ganglia, we saw the claustrum; and below it, we saw the substantia nigra.
Next, we identified some white matter structures – the corpus callosum which connects the left and right cerebral hemispheres, the fornix which carries axons from the seahorse-shaped hippocampus to the mammillary bodies at the base of the brain, and the optic tract which carries visual information to the occipital lobes.
Several sheets of white matter form capsules in the brain. These are the internal capsule carrying motor and sensory signals, the external capsule carrying association fibers, and the extreme capsule just deep to the insular cortex. The insular cortex is a small part of the cerebral cortex deep to the lateral fissure. We saw another named piece of cortex called the cingulate gyrus above it.
Finally, we looked at the ventricles of the brain and the choroid plexus that makes the cerebrospinal fluid within them. In this coronal section, we saw the third ventricle between the two thalami, the body of the lateral ventricle adjacent to the corpus callosum, and the temporal horn of the lateral ventricle in the temporal lobe.
And that's it for our tutorial today. Thanks for watching and happy studying!