 Good day everybody. Today I shall talk about the internal capsule of the brain, how it is structured, what is its orientation, and what are the various nerve fiber tracks inside the internal capsule. My name is Dr. Sanjay Sanyal. I'm a professor and course director of neuroscience and I'm a surgeon and medical informatician by profession. Let's take a look at an axial section for the brain. When we take an axial section for the brain, we see first a thin rim of gray matter which is referred to as the cortex, the cerebral cortex. But let's focus on this white matter here. This is referred to as the subcautical white matter which becomes confluent with the central white matter and which merges with the periventricular white matter. This whole thing is composed of nerve fibers which are malenated and that's why it's referred to as the white matter. For the ease of understanding this white matter has been divided into three broad groups, the so-called commissural fibers, the projection fibers and association fibers. Let's take a look at how these fibers are disposed. This is a very special type of investigation which is referred to as diffusion tensor imaging with white matter tractography. It's a very sophisticated investigation which gives a three-dimensional representation of the white matter inside the brain. So this is a coronal view of the brain and you can see that this is one hemisphere here, this is another hemisphere, this is the longitudinal fissure, this is a commissural fiber here. These colors have been coded by the computer. This band of fibers that are running down here on either side, this is an example of a projection fiber and these fibers that you see here these are some association fibers. The thrust of our discussion today will be on the projection fibers. Let's take a look at another example of a projection fiber. This is a distinction of a brain and we can see this radiating fan shaped structure here. This is a projection fiber which has been given a specific name called the corona radiata radiating like a crown. So this is a projection fiber. Another example of a projection fiber is this. As you can see this is a more diagrammatic representation. The fibers are radiating like a crown that's why it's called the corona radiata. We shall see subsequently that this corona radiata has got many components which are all labeled here. I would like you to take a look at this fact that this corona radiata has been indented from the lateral aspect by one nucleus here which is referred to as the lentiform nucleus and another important point is this corona radiata which is located just inside the brain inside the white gray matter of the brain. As these fibers converge cordially they become more tightly packed and in the region of the basal ganglia they come to be known as the internal capsule which is going to be the main topic for today. This is another picture a DTI division tensor imaging with white matter tractography of the projection fibers. More specifically one component of the projection fibers inside the internal capsule which we shall describe which is referred to as the corticospinal tract. So this is one side corticospinal tract descending through the ponds into the medullary pyramids and this is the other corticospinal tract. So this is a DTI division tensor imaging with white matter tractography. Let's come straight to the internal capsule itself which is the most important component of the projection fibers. During the fifth week of intrauterine life a swelling appears on the lateral wall of the steel and catholic vesicle and this is referred to as the striatal eminence. When the internal capsule fibers are descending down developing they split the striatal eminence into two components a medial part and a lateral part. So this is the descending internal capsule fibers and this medial part of the striatal eminence comes to be known as the caudate nucleus and the lateral part of the striatal eminence comes to be known as the lentiform nucleus or more specifically the putamen and the two together is referred to as the striatum. And finally the descending internal capsule fibers they descend down to the sub cortical structures lateral to the thalamus. So this is a quick overview of the development of the internal capsule. As the internal capsule fibers are descending down this picture shows how it splits the caudate nucleus from the putamen through these striated structures here and this is the reason why these two structures together are referred to as the striatum. So in simple terms the word striatum refers to the caudate nucleus and putamen together and this striated appearance is the place where the fibers of the internal capsule are descending down. This is another picture to show you how the coronary data fibers they converge in the region of the basal ganglia to form the internal capsule and this structure that we see here this is the putamen or the lentiform nucleus and this structure is the caudate nucleus. Now what we shall do we shall take an axial section through the brain to understand exactly the structure and the orientation of the internal capsule and for that we will take an imaginary line an axial or transverse section through the brain at this level which I shall show you in the next slide. So this red line represents roughly the level of the axial section and this picture also shows you a diagrammatic representation of the brain the same picture which I have shown earlier and the axial section has gone approximately at this level and this is yet another picture to show you the level of section. Once we take an axial section and we look at the brain from the top this is what is referred to as the axial view of the internal capsule and what do we see? So we see this cut section of the brain an axial section has been taken as demonstrated in the previous slide. So this is the right cerebral hemisphere this is the left cerebral hemisphere this also shows the same thing this is actually an MRI image and this is a diagrammatic representation of a section of the brain. Let us focus straight on the most important part of our discussion that is the internal capsule. We see this Y shaped or V shaped structure on this side and this V shaped structure on this side these are the two internal capsule mind you these fibers are projection fibers because they are descending down or ascending up in a vertical axis what we are seeing here is basically the cut section the axial section just to bring us up to speed the relationships of the internal capsule are here we see the structure here this is the caudate nucleus which I mentioned earlier this whole composite structure that we see here this is the lentiform nucleus it is called the lentiform because it is roughly shaped like a lens a convex lens and this structure that we see here this is the thalamus so this is the right side internal capsule as we can see that the lentiform nucleus has indented the internal capsule from the lateral aspect and therefore the internal capsule which should have been a straight vertical sheet of white matter has become roughly V shaped and that is what gives us the parts of the internal capsule namely this portion of the internal capsule is referred to as the anterior limb of the internal capsule which is located between the caudate nucleus mediately and the lentiform nucleus laterally this portion of the internal capsule is referred to as the posterior limb of the internal capsule which is situated between the thalamus mediately and the lentiform nucleus laterally and this curved portion of the internal capsule which joins the two limbs is referred to as the genu of the internal capsule the genu in latin means a knee or a bend so this is the internal capsule as seen in an axial section just a quick word before why is this called the word capsule in the initial stages of learning when the anatomists dissected an axial section of the brain they saw this sheet of white matter here and they saw a thin sheet of white matter on the lateral aspect of the lentiform nucleus so they said that the lentiform nucleus is enclosed in a sheet of white matter on the medial side and a sheet of white matter on the lateral side and they called it a capsule so therefore they named this the internal capsule and they named this sheet of white thin sheet of white matter on the lateral aspect as the external capsule but that is just a matter of historical interest now in the next slide what i'm going to demonstrate is what are the various fiber tracks which are running in the anterior limb the genu and the posterior limb of the internal capsule and for that i've made a diagrammatic representation of the internal capsule the right side so to bring us up to speed this is the anterior limb of the internal capsule this whole thing is the posterior limb of the internal capsule and the portion which is joining the two is the genu of the internal capsule and as we have already mentioned on the medial side of the anterior limb we have the chordate nucleus shown diagrammatically here on the lateral side we have the lentiform nucleus which actually is indenting the internal capsule and giving it this V shape open laterally and on the posterior medial aspect of the posterior limb we have this thalamus here let's take a quick look at the posterior limb of the internal capsule because that is clinically more important the posterior limb of the internal capsule we can divide it then the fibers into two broad groups as shown by this artificial color coding this lateral bundle of fibers here this is the corticospina tract which i had shown earlier and i'm going to show it again in another dti image this band of fibers that we see here this is the thalamocortical fibers which carry all the general sensations from the body to the somatosensory cortex of the parietal lobe so let's take a look at the breakup of the fibers in the corticospina tract first because clinically this is very important first this is the anterior portion the fibers in the anterior part of the lateral portion of the posterior limb of the internal capsule are the corticospinal fibers to the upper limb here the fibers are the corticospinal tract fibers to the trunk and it also carries the corticorubril fibers that is from the cortex to the red nucleus of the midbrain going further posteriorly these are the fibers the corticospinal fibers tract fibers which go to the lower motor neurons of the lower limb and finally at the posterior most tip we have certain special type of fibers which come from the temporal lobe to the pontine nuclei they are part of the frontal pontine tract in this case they refer to as the temporal pontine fibers we shall not bother about them in detail anymore so this whole segment is the corticospinal tract corticospinal tract fibers to the upper limb to the trunk to the lower limb remember that these are all fibers which are descending down and we are seeing the cut section in an axial view let's take a look at these fibers here this arrow represents the fact that the fibers descending the impulses are traveling up they are the thalamocortical fibers they go from all the sensory thalamic nuclei to the somatosensory cortex in the parietal lobe and they carry the general sensation from the body so this is the thalamocortical fibers carry now let's take a look at this portion here in circle in yellow this is the genu of the internal capsule now what do the genu carry the genu carries the corticoneuclear or the corticobulbar tracts and what are the corticoneuclear the corticobulbar tracts they are the counterpart of the corticospinal tracts the only difference being they start from the face area of the primary motor cortex and they end in the motor nuclei of the cranial nerves three four five six seven nine ten eleven and twelve so these are the corticobulbar tracts and they supply the motor nuclei of the cranial nerves supplying the head and face so this is the genu one final word about this genu in the posterior limb the whole genu in the posterior limb are supplied by some very special arteries which are branches of the middle cerebral artery and these arteries are known as the lenticulostriate arteries or the arteries of cerebral hemorrhage of sharko and these are the ones which are responsible for the vast majority of strokes involving the genu and the posterior limb of the internal capsule now let's take a look at the anterior limb of the internal capsule the anterior limb of the internal capsule the fibers are divided into for the sake of understanding into two groups the ones labeled green are the fibers which are coming from the frontal lobe to the pontine nuclei they are referred to as the frontopontine fibers or the frontopontine tracts now this is not just one fiber set of fibers they have a whole bundle of fibers and they are part of the corticopontine tract just for the ease of understanding I mentioned a few quick functions of these frontopontine fibers one of them is what is known as emotional smile the mimetic smile they carry fibers from the frontal lobe to the pontine nuclei to the seventh nerve motor nucleus smile emotional smile then we have the fibers for the horizontal conjugate gaze which goes from the frontal eye field to the paramedic pontine nucleus reticular formation nucleus in the pons for horizontal gaze then we have fibers for accommodation which do not go to the pons but they go to the nucleus in the midbrain and finally we have the fibers for mixturation voluntary control and coordinated mixturation which go from the mixturation center in the frontal lobe to the pontine mixturation center in the pons and therefore the m stands for mixturation so emotional smile horizontal gaze accommodation mixturation these are some of the just a few of the functions which are performed by these frontopontine tract fibers of course there are many more and there are many other functions this is just a few this central portion of the anterior limb the one marked in red here the fibers are going up they are also thalamocortical fibers like these but there's a considerable difference between these thalamocortical fibers and these thalamocortical fibers while these thalamocortical fibers carry general sensation these thalamocortical fibers they do not go to the parietal lobe instead they go to the frontal lobe and one example of these thalamocortical fibers are that which is part of the papae circuit which goes from the anterior nucleus of the thalamus to the cingulate chyrus it is a part of the papae circuit so this is just one example of course there are many such thalamocortical fibers which are traveling in the anterior limb of the internal capsule so this is a diagrammatic representation of the internal capsule so in the anterior limb the genome and the posterior limb with the various types of fibers traveling which have been given color coding and the direction of travel of the impulses are also mentioned by arrows and finally the blood supply to the anterior limb of the internal capsule is the anterior central branches of the anterior cerebral artery so this is in a nutshell the various fiber tracts which are located in the internal capsule and this is again a diffusion tensor imaging and white mitrotractography showing the disposition in a three-dimensional view of the corticost spinal tract as we can see it is coming from the frontal lobe it's coming from motor cortex and it is descending down and here it is the corona radiator here it becomes the internal capsule and it descends further down so this is the and incidentally in this picture also we can see some fibers of another type of group of fibers which are referred to as the commissure fibers in this case it is the corpus callosum that is just by the way so this is the importance of the projection fibers which become the internal capsule and the very important component of these projection fibers namely the corticost spinal tract why are we so bothered about the internal capsule and the corticost spinal tract again this is just the same picture which I had shown earlier and I have put a quick tabular representation of some lacuna stroke syndromes involving the posterior capsule involving the internal capsule we have something called pure motor stroke or pure motor hemiparesis where the location of lesion can either be in the corona radiator which I had shown earlier it can be in the internal capsule or it can be the basilar part of pons or it can be the medial medulla all of them produce opposite side face arm and leg paralysis pure motor then we have another syndrome mind you this is just a few of them there are some 20 25 different lacuna stroke syndromes described another one is called the dysarthria clumsy hand syndrome where the location of lesion is in the genome and the anterior part of the posterior limb so therefore the patient presents with one sided C at 7 9 10 12 upper motor neuron cranial palsy and weakness of the arm because we remember we had said that the corticost spinal tract fibers to the upper limb are located here so the dysarthria clumsy hand syndrome the location of stroke is here then we have another entity called the ataxic hemiparesis where the location of lesion is in the posterior part of the posterior limb of the internal capsule and here the patient presents with again opposite side weakness where the lower limbs are more weak than the upper limbs and that's why it is referred to as ataxic hemiparesis there these are just a few of the stroke syndromes involving the posterior limb of the internal capsule so this more or less concludes our discussion of the projection fibers the coronary the internal capsule and the clinical correlations of the posterior limb of the internal capsule specifically pertaining to the corticobulbar and the corticospinal tracts however remember that there are many other fibers also which are present in the brain just to give you a quick overview there's something called the commissure of fibers the largest commissure fiber is the corpus callosum which connects the neocortex of both the hemispheres and there's also something called association fibers which run in an anterior posterior axis and connect different regions of the brain in each hemisphere so this concludes our presentation on the projection fibers namely the internal capsule in the coronary radiator and the various tracts which are located thank you for watching and if you have any questions or comments please put them in the section below have a nice day