Okay, so, this has really been an extended introduction to the components of the Basal ganglia, and a little bit about their function. Now let's turn to a sharper focus on the major parts of the Basal ganglia. And we're going to focus on really just two of these processing strings, two of these loops. I'd like for us to consider the organization of body movement loop, And the organization of the limbic lobes. So the body movement loop is a circuit that runs through the basal ganglia that's primarily concerned with the movements of our limbs. The limbic loop is a circuit that runs through the basal ganglia that's concerned with our affect and our motivation. And that will be important when we consider the neurobiology of addiction. All right, so let's, let's look at what's common about these two parallel processing streams that run in the basal ganglia. they both have cortical input. In the body movement loop, the cortical input is primarily From the motor cortex, and the parietal cortex where we find our somatic sensory signals. So the input is coming from a fairly large region of the frontal parietal area of the cerebral mantle. In the limbic loop, the cortical input is coming from Telenscephalic structures that we associate with the ventral and medial parts of the fore brain. This includes the amygdala, the hippocampus, the orbito and medial parts of the frontal lobe, just in front of the motor cortex. This includes the cingulate gyrus and regions of the temporal cortex. This cortical input comes into striatum, but to a region that we call the ventral striatum. Which, as we'll see in just a moment, sits inferior to our putamen. And there is a division of this ventral striatum that I'm going to want to highlight for you. That's called the nucleus accumbens, and I'll show you that region when we look at some sections through the basal ganglia. It's worth mentioning now because the nucleus accumbens, is ground zero, for research on addiction, in the human brain. Well The striatum then sends inputs to the pallidum. And for the body movement loop, the pallidum is the globus pallidus. In particular the internal segment of the globus pallidus. For the limbic loop, the pallidal region is called simply the ventral pallidum. It's a division of the pallidum that's inferior and just a little bit posterior to that part that we recognize as the globus pallidus. Now, the pallidum provides the output for the basal ganglia, so in the body movement loop, the pallidum, here, is the internal segment of the globus pallidus, which projects to our motor thalamus. The motor thalamus, in turn, sends signals back into the motor cortex. In the limbic loop the vental pallidum projects to a different thalamic structure. It's called the mediodorsal nucleus of the thalamus. And this is the thalamic nucleus that sends signals back to many of these[UNKNOWN] structures. That provided the original input to the ventral part of the striatum. In particular, the cingulate gyrus and other parts of the orbital and medial prefrontal cortex. So again the point here is to lay out the specific processing stations that are involved. And these different processing strains but also to highlight what's common here. Cortex to striatum, striatum to palldium, pallidum to thalamus and then thalamus back to cortez. Okay I think that we're ready to actually look into the brain and see what some of the structures actually are. So, first with an illustration. What we have here is a coronal section through the fore-brain, and this allows us to see the various components of the striatum there, that are visible at this level. we see the putamen, and the caudate nucleus, divisions of the striatum that are separated by the internal capsule. just on the medial side of the putamen is where we find the globus pallidus and as we'll see the globus pallidus has two parts to it. An external part and an internal part. The output from the pallidum is directed towards the thalamus and we see our motor thalamus here in this level this is a structure that we recognize as the ventral anterior/ventral lateral complex of the thalamus. And those terms provide some indication as to what part of the thalamus is in question. And then, of course that part of the thalamus is what gives rise to projections on up To the motor cortex. Now this circuitry is modulated and some very important that we're going to spend some time talking about. The principal modulation that we will consider is coming from the mid brain and it comes from a division of a region that we call the substansia nigra. Now you, you may have heard about the substantia nigra. you may have seen the substantia nigra. The substantia nigra is one of those wonderful terms in brain anatomy that tells you what it is. It's the black substance. If you look at a brain at autopsy, and make a section through the midbrain, you will see a dark substance. That forms a bit of a line that is nearly sitting on top of the cerebral peduncles. So that dark substance is what we call the subtantia nigra pars compacta. The dark substance is there as a result of the biosynthesis of the neurotransmitter that's released by these cells. That transmitter is dopamine. And over a life time of making dopamine these cells accumlate a dark pigment which gives the dark substance its name. Dopamine is released at the axon terminals of these cells and the projections from this parts compact of the substansia nigra is targeted to the striatum. To the putamen, and to the cottay nucleus. So dopamine from the mid brain becomes an important neuromodulator of basal ganglia function. We'll spend a bit of time talking about that. There's another structure that provides an important Modulatory influence over the basal ganglia, and that is the sub thylamic nuclei. The subthalamic nuclei sit basically just in front of the substantianagra, on top of the cerebral peduncles, at a junction between the diencephalon and the mid brain. But the subthalamic nuclei are part of the diencephalon. The subthalamic nuclei provide input to the internal part of the globus pallidus and help to sustain activity that drives the output from the internal segment of the globus pallidus to the thalamus. So, one might quibble about whether the subthalamic nucleus are really part of the basal ganglia because they're diencephalic, I don't include them as part of the basal ganglia directly. Rather, I consider this to be an important source of modulation of basal ganglia function. Now, it's not illustrated here, but I'll also point out Well we have the substansia nigra in front of us. That there is a medial region in the vicinity of the pardus compacto that likewise contains a collection of cells that make dark substance. this is what we refer to as the ventral tegmental area or the VTA for short. The ventral tegmental area provides a dopamine projection to the ventral part of the striatum that participates in the limbic processing string. Alright, well let's look now at the forebrain and try to appreciate the complex geometry of the basal ganglia. And I'll start actually with our brain model that we have here. So I've alluded a few times as our focus has been on the brain stem that there are structures that are part of the forebrain that are sitting on top of this brain stem in this model. And what we're looking at basically is the basal ganglia. So from the frontal view we're looking at the putamen laterally. And on the medial side is the caudate nucleus parts of the striatum and when the caudate nucleus and putamen join together down below that's the region that we call the ventral striatum and specifically there is a division here that we call the nucleus succumbence. So the nucleus accumbens is simply the inferior and anterior continuity of the putamen and the caudate nucleus. Now, I want to show you the posterior view, which we would only see if we dissected away the forebrain. And let me first orient you a little bit, so this bundle of white matter that we find right here, this is called the internal capsule. And it's an important landmark that will help you recognize what parts of the basal ganglia you are looking at. And what you should remember is that the putamen is on the lateral side of the internal capsule. Whereas the caudate nucleus is on the medial side of the internal capsule. Now the two, the putamen and the caudate, they join together around the anterior side of the internal capsule with the nucleus accumbens forming the inferior continuity. But on the Posterior and dorsal aspect of the basal ganglia. The caudate nucleus extends a long tail, that runs along the medial edge of the internal capsule. And it actually extends all the way down into the temporal lobe, which is not illustrated here in this model. But we'll see a long tail of the caudate. That extends quite a distance into the temporal lobe, following the course of the lateral ventricle. Now, there's a important basal ganglia component that we cannot see in this model, and that is the globus pallidus. The reason that we can't see it is that it's buried just on the medial edge of this putamen. It sits between the fibers of the internal capsule and the putamen. Now if we look at this illustration. Hopefully you'll have some orientation now as to what were seeing. So from this lateral view. We can pre shade the Putamen. That division of the[UNKNOWN] that we find most latterly. And the putamen is continuous in its anterior extent with what we call the head of the caudate nucleus. And now from this illustration, you can see how the caudate nucleus wraps all the way around the body of the lateral ventricle, and follows the temporal horn of the lateral ventricle. Into the temporal lobe. Now again what we don't see here again in this view is the globus pallidus, because the globus pallidus is completely enveloped by the putamen. But what we do also appreciate is the position of the thalamus. So here's the thalamus. That is a large egg shaped structure. That's on the medial side of the internal capsule and it is a little bit more posterior in its extent than is the putamen so we'll note that the thalamus seems to be shifted just a little bit in the posterior direction as we look at cross sections through the brain. So lets go ahead and do that now. Alright, so we are looking at a coronal section through the forebrain taking the level that is just catching the anterior tip of the temporal lobe. And I want you to notice of course all the cortex that we find. Forming the bark around the margins of the cerebral cortex. We have white matter underneath. But now we've got this deep gray matter that is bulging out into the lateral wall of the lateral ventricle and it seems to be surrounded superiorly. Latterly and inferior with the white matter of the hemisphere. So lets look at what we have here. we are looking at a section that would have been taken very near the front edge of the internal capsule. So what we would expect to see is this continuity of the caudate, the putamen And then the ventral striatum, the nucleus accumbens down below, and that's really what we observe here. So you'll notice just a little whispy bit of white matter that is labeled number six here in the left hemisphere. That's the front edge of the internal capsule. So what must mean then that what we have in this view is all[UNKNOWN] So we have the caudate nucleus on the medial side of the internal capsule. The putamen on the lateral side, and where the two come together, we have the nucleus accumbens
