 Okay, so good day everybody. Welcome to our second episode of the circumventricular organs, those areas which are outside the blood-brain barrier. Before I start describing the circumventricular organs proper, since they are located around the ventricles, let me give you a quick overview of the third ventricle and the fourth ventricle and the structures which are surrounding it. So this portion that we see in front of our visual field, this is the third ventricle. Because the third ventricle is a midline structure, this hemisection has gone right through the cavity of the third ventricle. Therefore we are seeing only the lateral surface of the third ventricle that is the right surface. This is the right cerebral hemisphere. So this is the floor of the third ventricle. And we can see the mammary body here and there's some other structures here which are part of the hypothalamus. This is the anterior wall of the third ventricle. This is constituted by a sheet of gray matter which is known as the lamina terminalis. This is a very important sheet of gray matter. It stretches from the rostrum of the corpus callosum and gets attached to the anterior surface of the optic chiasma. This is the section of the optic chiasma. This is the roof of the third ventricle and we can see the chloride plexus of the roof of the third ventricle here. The chloride plexus is situated in the roof and above that we have this structure here which is not part of the roof of the third ventricle, but it is an important component and that is known as the pharynx. And further up above we can see this whole thing is the corpus callosum which is not part of the roof but it is situated above. Now let's come to the posterior wall of the third ventricle. Going from below up we can see that there's a small structure here. This is known as the posterior commissure. Just above that we see the pineal gland here. And above that there is a small structure called habinular complex which is hardly visible here. And finally we notice that this is the opening of the aqueduct of sylvius. This aqueduct of sylvius it runs through the midbrain here and it connects the third ventricle to the fourth ventricle. So these are the walls of the third ventricle. Coming to the lateral wall itself we can see that the upper two thirds of the lateral wall is formed by the thalamus and this is the interthalamic adhesion. While the lower one third of the lateral wall is formed by the hypothalamus. We will be referring to the sum of the nuclei and the hypothalamus in our subsequent part. And connecting the third ventricle this is the fourth ventricle here. In the anterior wall I forgot to mention there is a structure here which is called the anterior commissure. Now let's come to the fourth ventricle. The fourth ventricle again this is the floor of the fourth ventricle. It is composed in the upper half by the pons because this is the pons. And the lower half is composed of the medulla this is the medulla. Note the midbrain does not constitute any part of the floor ventricle because the aqueduct of sylvius is the one which is running through the midbrain. This is the tent shaped roof of the fourth ventricle. This upper one upper half is a sheet of white matter it is called the superior medullary vealum. And the lower is the other half of the roof of the fourth ventricle. This is called the inferior medullary vealum. The inferior medullary vealum does not have any neural tissue. It contains only epindaema and pyrometer and we can see the coroid plexus of the fourth ventricle. So these are the salient points about the fourth ventricle and the third ventricle. With this background now let us come to one more point before I proceed any further. We can see one opening here where my probe is going. This is known as the interventricular foramen of Monroe. This is the foramen which connects each lateral ventricle where my probe is to the third ventricle independently. So this is the right interventricular foramen of Monroe connecting the right lateral ventricle to the third ventricle. So with this background let us start describing the various circumventricular organs. What we shall do, we will go in a clockwise direction around the third ventricle and we will finish in the fourth ventricle. We are not going to describe any of the structures which we mentioned in the optic recess. Now there is one more point I need to mention about the third ventricle. There are four projections of the third ventricle into various structures. This projection that we see here, this is known as the chiasmatic recess or the optic recess. A little bit of the third ventricle projects into the optic chiasma. There is another small projection which is not visible clearly here that is called the infundibular recess where it projects into the infundibular stem of the pituitary stalk. The pituitary stalk is supposed to be located here. It has been removed from this specimen. There is another small projection here which is known as the pineal recess. It projects into the pineal stalk and finally there is a small projection above that which is called the suprapenial recess. Now we are ready to start describing the various circumventricular organs. So let's start with one of the very important ones which is located in the floor of the third ventricle. The floor of the third ventricle in this region, this is called the median eminence of the tuber scenario. What is this median eminence? Median eminence is the raised most central part of the tuber scenario which is a convex shaped mass of grey matter in the ventral most part of the hypothalamus. So this is the location. This median eminence of the tuber scenario is lined by tansites. It does not have a blood-brain barrier and this is where it has a very important nucleus of the hypothalamus which is called the archaeate nucleus. This archaeate nucleus, it samples the blood through tansites which I described in my first part of the video and once it detects the levels of hormones in the blood through the tansites, it releases its own regulatory hormones which can either be releasing hormone or inhibitory hormone. And one important inhibitory hormone that it releases is prolactin inhibitory hormone. It releases these hormones and that's how through the tuber-o-hypophysia tract it connects with the adenohypophysis and it regulates the release of the hypofysia hormones. So therefore this archaeate nucleus in the median eminence of tuber scenario has a very important endocrine function. It controls the basal levels of hormones in the body especially FSH and LH and ICSH. So that is the first circumventricular organ, the median eminence of tuber scenario in the floor of the third ventricle mediated by the archaeate nucleus. Now let's take the second circumventricular organ proceeding clockwise. As I told you earlier this is the lamina terminalis, the thin sheet of grey matter. It has got many functions. One of the functions here is that this has got a vascular organ here and that is called the organum vascularum of lamina terminalis, OVLT or in simple terms it is also called the vascular organ of lamina terminalis. What does this do? This acts as an osmoreceptor. It detects the osmolality of the serum. The normal osmolality of the serum is 280 to 300 milliosmoles per kg body water. When there is increased osmolality that means the serum becomes more concentrated it generates thirst and therefore it stimulates the person to drink water. At the same time it also sends its impulses to a very important nucleus which is located here that is called the supra-optic nucleus and the supra-optic nucleus in turn releases anti-diuretic hormone and therefore causes water retention. Therefore this organum vascularum acts as a fluid electrolyte balance maintenance organ. Apart from that this organum vascularum also provides, because it is highly vascular it provides a vascular outlet for two other nuclei of the hypothalamus. There is a nucleus right behind the lamina terminalis here which is called the pre-optic nucleus. One of the secretions of the pre-optic nucleus is LHRH Lutinizing hormone, releasing hormone. So the luteinizing hormone, releasing hormone is released through this organum vascularum. The second nucleus which is located right here just above the chiasma, optic chiasma is called the supra-chiasmatic nucleus. This supra-chiasmatic nucleus is responsible for circadian rhythm control and it stimulates the pineal gland to release melatonin but it also releases another hormone of its own that is called somatostatin or growth hormone inhibitory hormone. This GHIH is also released through this organum vascularum of lamina terminalis. So therefore OVLT provides a vascular outlet for the release of pre-optic secretion as well as supra-chiasmatic secretion. So that's an additional function of this organum vascularum of lamina terminalis. Now let's come to the third circumventricular organ going further clockwise. If you recall, I had mentioned this structure here, we had called this the fornix. The fornix is not a part of the third ventricle, it is above the chloride plexus but if you take a close look at this, this is the anterior column of fornix. This is an important outflow tract from the hippocampus and it contains the hippocampum avillary tract. This anterior column, it winds in front of the foramen of Monroe and in fact it constitutes the anterior boundary of the foramen of Monroe and then it disappears in the lateral wall of the third ventricle and it ends finally in the mammary body here. Therefore this fornix contains the hippocampum avillary tract but let's focus on the circumventricular organ which I want to talk about. There is a circumventricular organ just under the fornix here in the region of the foramen of Monroe. That is why it is called the sub-fornical organ. What does this sub-fornical organ do? This sub-fornical organ, this contains angiotensin 2 receptors. Therefore it responds to angiotensin 2 levels in the blood and therefore it responds to decreased higher concentration of serum. Angiotensin stimulates angiotensin 2 receptors and the sub-fornical organ in turn projects, sends nucleus, sends axons which projects to the supropoptic nucleus which is located here and it sends axons to the organobascularism of lamanaterminalis. Therefore this sub-fornical organ, it also regulates the fluid electrolyte balance by stimulating the release of anti-diuretic hormone and therefore aiding the function of the osmoreceptors namely the ovlt and the supropoptic nucleus. So this is the third circumventricular organ, the sub-fornical organ. Now let's continue further clockwise. We come to this structure here which is on the posterior wall of the third ventricle. This is the pineal gland which I alluded to earlier. The pineal gland is stuck to the posterior wall of the third ventricle by means of a small stalk or the pineal stalk and as I had mentioned earlier there is a small recess of the third ventricle which enters into the pineal stalk. Now this pineal gland is highly vascular, highly vascular and it contains special types of cells which are known as penialocytes or epiphyseal cells. These penial cells, they release among many other secretions, they release melatonin. This melatonin is released into the CSF of the third ventricle and it is transported either via the TAN sites to the bloodstream the TAN sites, if you remember, are located in the floor of the third ventricle or they go, these penialocytes, secretions, they go directly through the CSF of the third ventricle and they go and act on the pediatric gland. So therefore, this is another very important circumventricular organ mediated by the epiphyseal cells here. Now let's come to the next circumventricular organ. If you recall, I had mentioned that just under the pineal gland there is a small structure here on the posterior wall which is called the posterior commissure. The posterior commissure per se is not relevant to this present discussion though it has got a very important role to play. Let's go and look at just under the posterior commissure exactly where the aqueduct of sylvia is beginning to form, this location here. This location is called the sub-commissural organ. This is another blood-brain barrier free area, the so-called circumventricular organ. Now this sub-commissural organ, it contains special types of cells which are epithelial and glial cells in nature. The important exception of the sub-commissure organ is that unlike the other circumventricular organs, here the capillaries are not fenestrated. So that breaks the question, how does it function as a blood-brain, as a circumventricular organ and what is its exact role? The exact role is not very clear, however there are two postulates. One postulate is that because it is located right at the beginning of the aqueduct of sylvia, it somehow controls the flow of CSF from the third ventricle to the fourth ventricle. And some studies have shown that failure of function of the sub-commissural organ can lead to hydrocephalus. So that is one possible theory that it aids in the circulation of CSF. And another theory is that it also helps in the clearance of certain compounds which are present in the CSF, but both of these things are still the object of further research. So this is the next circumventricular organ. So we have described median eminence of tuber scenario, organum vasculosum of lamina terminalis, sub-phortical organ, pineal gland, sub-commissural organ. Now let's move to the last circumventricular organ and that is in the floor of the fourth ventricle. So if you remember this was the fourth ventricle. Right at the inferior apex of the floor of the fourth ventricle where the fourth ventricle continues as the central canal of the spinal cord. Just adjacent to that where my probe is almost disappearing. It is just below the vagal triangle and the inferior margin of the floor of the fourth ventricle. There is a very vascular organ and that is called the area pastrema. This area pastrema contains modified astrocyte like neurons and it is the so-called vomiting center of the brain. It has got 5-HD3, 5-hydroxy tryptamine 3 serotonin receptors which are ionotropic excitatory receptors and these respond to stimulants like 5-hydroxy tryptamine in the blood, morphine, excess levels of oxytocin or any other toxins which stimulate these and they produce induced vomiting. So therefore they help to remove toxins from the body. Apart from that these 5-hydroxy tryptamine receptors they also get stimulated by the chemoreceptor trigger zone and they also receive afferents from the gastrointestinal tract. So therefore if we have eaten any noxious substance we feel nauseated. Why? It is because the afferents from the GIT stimulate the area pastrema. Incidentally this area pastrema 5-HD3 receptors are the target of blockade by certain therapeutic drugs like for example Ondan-Cetron which is used to treat chemotherapy induced nausea and vomiting. It acts on the 5-HD3 receptors and inhibits them and therefore it relieves the post-operative and chemotherapy induced nausea and vomiting. And finally it has also been postulated that this area pastrema plays a role in food intake and cardiovascular system control. So this completes our overview of the circumventricular organs around the ventricles and if you have any questions or comments please write it down in the comments below. Have a nice day.