Welcome to this tutorial on the Thalamus. Now that we are into the our studies of our sensory systems and we will progress from there to our studies of the motor system we're going to encounter interesting structure that's deep near the center of the brain called the Thalamus. So, I think it's worthwhile for us to think in general terms, about the structure and the function of this part of the brain. So, thank you for joining me for this discussion. This pertains to several of our core concepts in the field of Neuroscience. Again, the complexity of the brain. confronts us every time we consider one of its component structures in relation to others. we would be talking about the derivation of the thalamus in embryological terms which reminds us that the circuits in the brain that are the foundational for function. Are all encoded. In the information that's present in our genome. Now those circuits are formed and shaped and changed in very interesting ways through activity dependent mechanisms of plasticity. But the basic architecture seems to be fundamentally, genetically, determined. And as a consequence of this genetic endowment. And the accrual of our life experiences. And how all of these factors self organize in the developing brain. We have this fantastic capacity to ask questions. Such as, what has led you to take this course with me? you've been endowed with a curiosity that arises from the structure and function of your own networks within your brains. Alright, well our objectives for this particular tutorial are that I want you to be able to briefly discuss the embryological origin of the the thalamus. And I want you to be able to identify its location in the human brain. And lastly, I want you to be able to have just a very short discussion with a friend or a family member regarding the functions of the thalamus in the mammalian brain. Okay, concerning its embryological origins, the thalamus is derived from that forward part of the developing embryo that will form what we call the forebrain. And in embryological terms, the forebrain is the Prosencephalon. Let me remind you that the nervous system develops from the walls of a tube. And those walls have an anterior end and here is the anterior end of the developing tube, shown schematically. And a more realistic view of the embryo, there's the anterior end. The posterior end is the inferior aspect of the spinal cord. But on the anterior end, we have a large swelling that begins to form. And as embryology proceeds, that swelling subdivides. There is a very large telencephalic vesicle that forms and here is where we will derive our cerebral cortex or our basal ganglia. Much of what we consider when we think about the mammalian forebrain. But the more posterior part of this structure is what forms the thalamus. This is called the diencephalon. And there are lots of other interesting developments that emerge out of the diencephalon. One of them is shown here. And that's the optic vesicle. here it is in schematic form. The optic vesicle goes out to form the neural elements of the eye, the retina. So believe it or not, the retinas in our eyes are truly brain. They are derived from the diencephalon. A division of the posterior forebrain. So, in this view here is our diencephalic region near the posterior part of the forebrain just between the telencephalon and that middle brain, the mesencephalon. And as we see the consequence of this brain development, we find that the diencephalon is found very much near the center midline of the brain. And in the more posterior and dorsal aspects of the diencephalon, we have the region that we call the thalamus. More eventual in anterior is the part that we call the hypothalamus. There are other divisions that are of, less importance to us. At least, right now, in the course. We'll speak about them as needed, as we progress. But for now, I want to focus in on this division of the diencephalon that we call the thalamus. And again, just to remind you of. Of, of where this is from, embryologically speaking. The diencephalon is derived from the prosencephalon. It's the posterior part of that large anterior swelling that sits in the transitional region between the telencephalon and the middle brain or the mesencephalon. And then the diencephalon goes on to subdivide into this dorsal thalamus and the more eventual anterior hypothalamus, both of which form the lateral walls of the third ventricle. Okay, so that's where we are from an embryological perspective. What about from a neuro-anatomical point of view thinking about the whole human brain. Well, let me now show you in a phantom view of an adult human brain, what we saw in an earlier illustration of a developing brain. That is the thalamus ends up very near the center of the forebrain. So here we're looking at a phantom view. From the lateral aspect, this orange curved structure is part of the basal ganglia, and what we see is that the thalamus is tucked in on the medial side of the basal ganglia. It extends just a bit posterior to it. So if we were to look at some cross sections, what we would discover is that the thalamus sits as something of an egg shaped structure, near the midline, but also very near the center of the fore brain. And just to the lateral aspects of the thalamus is a big bundle of white matter known as the internal capsule. And on the other side of that internal capsule are components of the basal ganglia, specifically two divisions out here called the globus pallidus and the putamen. Just above the thalamus we find the lateral ventricle. This large space that sits just above the thalamus, but also just above the thalamus and to its lateral margin is an additional component of the basal ganglia called the caudate nucleus. Well, let's look inside the thalamus if we can and I'll point you to this illustration which indicates that the thalamus is really not one thing, it is dozens of things. It is dozens of discreet collections of cells. That each maintain their own set of input and output relations to other parts of the nervous system. So the thalamus comprises some 20 or 30, depending upon your tolerance for dividing up different parts of this structure. These 20 or 30 individual nuclei, represent discreet pathways that are providing inputs to the thalamus. Then the thalamus, in each of these sub divisions, give rise to outputs that are directed mainly to the cerebral cortex. Well, how can we make sense of these 20 or 30 subdivisions of the thalamus. I would suggest pretty simple anatomical framework for doing so. When we look at the human thalamus, we see that it's roughly divided. By a white matter layer that runs right through the middle of it. It sort of has a, a Y-shape, if you can appreciate this as a shape resembling the letter Y. there is an anterior part of this thalamus in the crook of this Y that contains a group of nuclei that we call the anterior nucleus or the anterior nuclei group. This part of the thalamus maintains inner connections with the front part of the frontal lobe, a region that we call the prefrontal cortex. Specifically to a component of the prefrontal cortex in the cingulate gyrus. The medial part of the thalamus is comprised of a set of nuclei that project to other parts of the frontal lobe and the insular cortex and even the anterior part of the temporal lobe. These are all components of that pre-frontal cortical network that are more broadly distributed. There's a significant nucleus in this group called the medial dorsal nucleus of the thalamus, that helps to define what we mean by pre-frontal cortex. Now on the lateral aspect of the thalamus, we find most other cell groups that are going to be of interest to us through the course. These include a variety of nuclei that project to the somatic sensory regions of the parietal cortex. The visual regions of the occipital cortex, the auditory structures in the temporal cortex. And even the motor structures in the posterior part of the frontal lobe. So it's in this lateral group that we find many of our sensory pathways that we will be talking about over the next several tutorials. Running through the thalamus, relaying information to the cerebral cortex in an aerial-specific pattern. Now, there is one other set of nuclei, that I'd like to highlight for you, and these are a collection of small nuclei that are actually buried within that white matter lamina that forms this Y shape. These nuclei, in addition to some nuclei that are around the midline of the thalamus, tend to project very broadly to widespread regions of the cerebral cortex. We don't think of them as conveying specific signals, such as our sensory and motor nuclei that we find out here in our lateral group. Rather, we think of these midline nuclei and intralaminar nuclei as being primarily modulatory in function, rather than conveying specific information. Now let's use this anatomical framework to consider the function. Of these different kinds of nuclei that we've just described in the thalamus. the most, widely known set of nuclei in the thalamus are those that receive incoming or ascending sensory signals. And the function of these nuclei is fundamentally to relay that information to the respective divisions of the cerebral cortex. That are defined by the input patterns of those nuclei. So for example, there is feed-forward transmission of ascending sensory information through nuclei at the thalamus that are concerned with somatic sensation. They receive inputs from somatic sensory pathways that are rising from the spinal cord and from the brain stem. And they send their signals into the somatic sensory division of the cortex. Namely the postcentral gyrus. The same principals are seen in the organization in the pathways for vision, for audition and for our sense of equilibruim, or vestubular sense. Each of these senses have relays through specific nuclei of the thalamus. In fact, those divisions of the cerebral cortex that are dedicated to processing inputs about a particular sensory modality, such as the semantic sensory cortex, the visual cortex. They are considered primary because they're the first cortical area to receive input in this ascending sensory stream. And the reason they're the first is that they are the recipients of a specific set of thalamic inputs that arborize in a particular sector of the cerebral cortex. So in this Morris schematic view of the thalamic function. What I'm talking about or what we might call first-order thalamic nucleus. Or sometimes they are called specific thalamic nuclei. These are nuclei that are receiving sensory or motor signals and sending inputs up to cortical network. Through very specific and very robust connections that terminate in the middle layers of the cerebral cortex. These first order thalamic nuclei are typically those that we find in the lateral division of the thalamus. Well, in addition to these first order nuclei, we can also recognize higher order thalamic nuclei. These are thalamic nuclei that receive primary input, not from ascending sensory or motor pathways. But from descending pathways that arise from the cerebral cortex itself. So, these are thalamic nuclei that seem to participate in cortical thalamal cortical loops. That is the distribution of signals from one region of the cortex to another. Through a connection in the thalamus. And lastly I would highlight those set of thalamic nuclei that are typically on the more medial part of the thalamus but also those nuclei buried. Within that lamina of white matter, also in this category, that we might consider to be modulators of cortical function. These are what we call nonspecific thalamic nuclei that project not to the main input layer of cortex layer 4 but, more broadly, typically to the upper layers of the cerebral cortex. And these more diffuse projections seem to be very important in modulating the state of the cortical network. So rather than providing specific sensory motor signals or distributing particular higher order processing signals. These more diffuse axonal systems emanating from the mid-line and intralaminar nuclei of the thalamus perhaps have a role to play in modulating cognitive functions such as attention, or perhaps modulating our level of arousal. So, so these are, are not specific nuggets of information that are being transmitted along thalamic cortical lines but rather it's a modulatory signal that can have a widespread influence over large-scale cortical networks.
