Well I'd like to make just one additional point before leaving this section to move on into the base of ganglia. and what I'd like to talk about are these parallel streams that run through different divisions of the stratum. So again what we have in view here are the different parts of the striatum. Out here laterally we have the putamen which is on the lateral aspect of the internal capsule. On the medial aspect of the internal capsule in bulging in to the lateral wall of the lateral ventricle we have the caudate nucleus and then where the caudate nucleus and the putamen join together, inferior to the anterior limb of the internal capsule. That's what we call the eventual striatum. Or in this region, primarily, we call this the nucleus accumbens. Now I would like to emphasize that the basal ganglia is all about modulating movement. And what distinguishes one stream from another, is what is the object of the move Movement. Now I'm going to ask you to give me a little bit of license with this idea of movement and apply it somewhat creatively. Well, let's begin with a more concrete application of the concept of movement. The putamen is all about moving the body. So the putamen is the principal recipient of the inputs form the frontal motor cortex and the parietal somatic sensory cortex. So the putamen is about modulating the movements of the body. The caudate nucleus received input from the frontal cortex, specifically the parts of the prefrontal cortex that sit just in front Of the motor cortex. the caudate nucleus is also receiving input from the anterior part of the premotor cortex, which would include our frontal eye field. So, the caudate nucleus is concerned with movement of the eyes, and with the eyes also goes our mind. Our cognitive faculty. So, I like to think of the caudate nucleus as mainly modulating movement of mind. That is, going from one thought to another. Often as we visually inspect our world. So, what we look at is typically what we're thinking about. Not always but, for the most part, eye movements and thought. Go together, so the cautade is moving the mind. The nucleus accumbens is receiving input from the ventromedial parts of the fore brain that are concerned with processing emotion or affect. So the nucleus accombands/ g is going to be involved with moving affect. So, what do I mean by moving affect? Well, I mean the nuances of our emotional life as we go from one moment in our day to another. we have ups and downs in our emotions. And that's normal. We ought to have a normal emotional response to the events in our lives that move our emotions. That movement of emotion is facilitated by the circuitry that run through the ventral part of the basal ganglia, including those circuits that are converging in the nucleus accumbens. So, please remember, the basal ganglia modulate movement. And they do so, operating upon different modalities of information that our converging on different parts of the striatum. Okay, let's move on to the next section now. If we go back a section, now we are cutting through, really, the fat part of the internal capsule. So, what we have again is This now broad internal capsule that helps to orient us to our view, but we still have elements of the striatum that we saw in previous section the coda nucleus is bulging out forming the lateral wall of the lateral ventricle and that's a nice healthy looking coda nucleus that's what we should see. And then now we have the putamen which is lateral to the white matter of the internal capsule. And inferior to the location of the cardiac nucleus. But there is now the globus pallidus taking shape. Between the fibers of the internal capsule, the putamen, and this other white matter pathway that we see now extending across the midline. So perhaps some of you know what this white matter pathway is down here. I'll give you just a moment to shout it out. Hopefully you will recognize this as the anterior commissure. So the anterior commissure is a helpful landmark because it helps us recognize where we would expect to find the globus pallidus. The globus pallidus sits in a bit of a triangular region Between the putamen, the internal capsule, and the interior commissure. In the next section, we are getting near the posterior aspect of the basal ganglia, so perhaps we have a section that is roughly in this orientation here. So we are losing the putamen and the globus pallidus. it's still present as is our caudate nucleus. So again, let's reorient, here is our internal capsule. That streaming in to the region of the cerebral peduncle. And on the medial side of that capsule We expect to find the caudate nucleus which is this little bit of gray matter here. It's getting smaller, because now we are developing that tail that's going to follow the course of the lateral ventricle into the temporal lobe. On the lateral side of this internal capsule, we have the putamen. It's smaller than it was in the previous image, but it's still present. And we also have just a small bit of our globus pallidus left. It is just where we would expect it to be. It's between the internal capsule and the putamen. Another major component that's made its appearance in this section. Is, of course the thalamus, which is this large egg-like structure that we see on the medial side of the internal capsule. Now you may be wondering about this limbic loop. Where are the structures that the nucleus succubus projects to, and so on? So let me just point those out while we're making our way through these sections. So I did, in fact, show you the nucleus accumbens which is this ventral and anterior region where the caudate nucleus and the putamen seem to fuse together around the anterior margin of the internal capsule. The nucleus accumbens sends inputs into the ventral pallidum. And I recognize this triangular region as the globus pallidus. The ventral pallidum begins to take shape just underneath this anterior commissure and extends posteriorly just a bit. So this is the region that we would call the ventral pallidum and the ventral pallidum provides the output from the limbic loop. To the the thalamus and specifically that output is directed to the medial dorsal nucleus of the thalamus, which is present right about there. So there's our medial dorsal nucleus. The medial dorsal nucleus provides output to the cingulate gyrus and other divisions of the prefrontal cortex. Okay, so now that you've seen the basic parts of the basal ganglia let's begin to consider the connections among the various parts. So I've already talked a little bit about this first point. What are the major inputs to the basal ganglia? Hopefully you will recall that the inputs to the basal ganglia. Are directed towards the striatum. So, much of that input is derived from the telencephalon, from the cortex. So, we see numerous projections from the cerebral cortex into the striatum, which is the caudate nucleus and the putamen. So, all of this cortical input's converging at the level of the striatum. The striatum is also the target of our dopamine inputs from the substantia nigra pars compacta. Okay? So, the dopamine is being released in the striatum. That's going to be very critical for understanding how the circuitry of the basal ganglia function. So the striatum is really the key input station for all the loops that run through the basal ganglia. For the limbic look in the basal ganglia, same principles at play. We have projections from cortex only for the nucleus accumbens, the cortex that's providing input. Would be in the temporal lobe, as well as in the orbital and medial parts of the prefrontal cortex. So these inputs from structures like the amygdala, parts of the hippocampus, and then that orbital medial prefrontal cortex are converging on the striatum and in this case it's the nucleus accumbens. Our dopamine cells, for the nucleus accumbens, again are derived from the medial part of the dark substance, that's what we call the ventral tegmental area. And just as for the dorsal processing streams, concerned with moving the body, so in this limbic loop, dopamine is released in the striatum. In the nucleus accumbens. This will be very important for understanding our nerve biological frame work for addiction. I think we are now ready to talk about the circuitry with within the basal ganglia. Now that we understand the conversion of inputs into the straitum. So we conventionally talk about two pathways that get information through the circulatory of the basal ganglia and into the thalamus. There's a direct pathway because it's the most direct connection from the striatum to the pallidum and then out to the thalamus. And here's the anatomy of the direct pathway through the basal ganglia. We've already talked about the inputs from the cortex that come into the striatum, so collectively we can collapse the striatum and call it the caudate putamen. So this is just the striatum. the striatum sends projections to the palatum/g. In the case of our body movement loop, that would be the internal segment of the globus pallidus. The internal segment of the globus pallidus then sends signals to the motor thalamaus. The ventral anterior and ventral lateral complex of the thalamus. Which in turns provides input to the motor cortex. Now don't worry about the pluses and the minuses and the transients and the tonics at this point. We'll come back to that. For now I want you to focus on the anatomy. So again we have cortex projecting to strawatum. Striatum to pallidum, pallidum to thalamus, and then thalamus back to cortex. This is the direct pathway. There's an indirect pathway. The indirect pathway, actually, I must admit, looks worse than it is. What we have in this illustration is a combination of the direct and the indirect pathway. So the direct pathway is what we find to the right hand side of this figure. The indirect pathway is the portion of this figure that is backed by that yellow field. So the indirect pathway as its name suggests must be a less direct means of getting information. From the striatum, through the palladum, and on to the thalamus, indeed it is. The indirect pathway involves an additional loop that runs from the external segment of the globus pallidus, through the subthalamic nucleus, and then back to the internal segment of the globus pallidus. So let's see how that works out. So, so here's our indirect pathway now this area that's back by this yellow color. And our indirect pathway involves projections from cortex to the striatum. Projections from the striatum now specifically to the external segment of the globus pallidus. So, this is the more lateral division of that Globus pallidus body that we saw a moment ago. The external segment of the Globus pallidus projects to the Subthalamic nucleus, and it also has direct projections to the internal segment. It seems as though this projection to the subthalamic nucleus is really important in how the indirect pathway works because the subthalamic nucleus then, projects back to the internal segment of the globus pallidus. And now we're back into the confluence of the direct and the indirect pathways, the internal segment of the globus pallidus. Projects to the motor thalamus which then sends signals into the motor cortex for the initiation of movement. So, when one considers the indirect pathway. What I want you to appreciate is the importance of this subthalamic nucleus. If the function of the basal ganglia is to modulate the way the thalamus is interacting with the cortex, then you might imagine that the subthalamic nucleus ought to be pretty important because it's one way to either turn up or turn down the activity of this internal segment of the globus pallitus. Well, so is the direct pathway for that matter, but these two inputs converging And the internal segment of the globus pallidus are very important in modulating or controlling the output of the basal ganglia that ultimately is reaching the thalamocortical circuit. So, after a short break, let's come back and let's consider the function of this circuitry in the basal ganglia.
