Greetings. Today we're going to talk about the POSTERIOR PITUITARY, and the posterior pituitary is the one that is going to govern our blood pressure as well as the osmotic balance within the body. The posterior pituitary remains connected to the brain through a, through the region which is called the infundibulum, and that's shown here. So we have neurons, which are going to extend their axons from the hypothalamus and down through this, through this neck region and into the posterior lobe and, such as this. These neurons their axons are going to terminate on the capillary beds which are perfusing all of the, all the region of the posterior lobe. These, this is being fed by the inferior hypophyseal artery. It, it enters, breaks up into a capillary bed, and then it's going to drain by a vain to the systemic circulation. The cells which were of interest to us are, are in the hypothalamus, come from two seperate lobe sides. One's called the parventricular nucleus and the second, the suproptic nucleus. These suproptic nucleus and the parventricular nucleus are going to secrete two types of neuro peptides. And the neuropeptides are going to be oxytocin and vasopresin. So this area then is going to secrete two peptides into the systemic circulation. The first of the neuropeptides that I want to discuss is called oxytocin. Oxytocin has been a peptide that people have thought was pretty boring, because oxytocin is a peptide that is secreted, by the suckling of the, of the infant at the breast, so it's a mechanical stimulation. Of the tissue, which causes a release of oxytocin, and then in turn causes the myoepithelial cells to contract, and when they contract, then the milk is expelled. It's also being secreted at the time when we have delivery of the baby and we have stretching of the cervix and at this and of the birth canal. And as this occurs, this mechanical stretching, again causes release of the Oxytocin. Oxytocin will cause restriction or contraction of the smooth muscle of the uterus, and it helps to expel the baby. So it's been known for many years that this is what oxytocin was doing. The only problem was that it was a conundrum, and that was, is that males also secrete oxytocin. And so why would a male be secreting oxytocin when clearly Human males do not give birth, and they, to live young. And secondly, they also do not breastfeed the young. And it was about five years ago that they started to look into what is actually happening with oxytocin in the male, and what they found was is that oxytocin is released in high amounts at the time of orgasm, that is at sexual climax. And this is occurring in both the males and in the females. So oxytocin then is the bonding hormone, and it's the hormone that actually is is causing you to bond with your partner during the sexual act. And oxytocin is also a bonding hormone for the, for the mother and the father to the baby, as they stroke the baby, or cuddle the baby, or hold the baby then the oxytocin levels increase. See that oxytocin then is our love potion or our bonding hormone, and when I first told this to, to my class a couple years ago, one of the students came up to me and asked me if this was available commercially. What happens if we don't have oxytocin? If we don't make enough oxytocin, then we, we see that the mother actually rejects her baby. She is, these are the individuals where they give birth to the child, but then have no have no interest in feeding the child, taking care of the child, tending to the child in any way. So very low oxytocin levels then, there's no bonding then to the offspring, by the mother. So, eh, so this is be a pathological, situation. So that's oxytocin. So what about vasopressin? So vasopressin is secreted also to two different stimuli. Vasopresin is regulated by osmotic, stimuli, and by a volume stimulus. The osmotic stimulus is that if we have a very high rise or even a very small rise in osmolarity, and this would be the concentration of sodium which is within the bloodstream, within the plasma, then the vasopressin is released. In addition, vasopressin is released when there is a decrease of about 10% or more of blood volume. So a loss in blood pressure or in particularly, a loss in blood volume will cause release of antidiuretic hormone or vasopressin. Those are the same entity. Vasopressin, or antidiuretic hormone, works in the kidney, on the collecting tubules of the kidney, where it moves water from the filtrate. That would be the presumptive urine back across the ephithelia cells which are lining these tubules into the capillaries and into the bloodstream. So vasopressin then is increasing blood volume by moving water from presumptive urine back into the, into the blood. By doing so it also dilutes the osmolarity of the blood, so vasopressin then is just moving water across this region of the body. So what is our pathology? So the pathology for vasopressin is called diabetes insipidus. many, many years ago the Greeks would recognize individuals who were drinking a lot and peeing a lot, as having some type of pathological condition. And what they would do is they would taste the urine that was coming from these individuals, and if the urine tasted sweet it was called diabetes mellitus. This was urine that had glucose in it. these are individuals who, who were secreting glucose from the body. As well as water. In the individuals where the urine had no taste they call this diabetes insipidus. Insipidus because it has no taste. And that's the pathology that we see here, if there's a problem with the vasopressin or the anti-diuretic hormone. We can have two separate problems. One would be if there is a central problem, and that is that there's a lesion which is occurring where we've had trauma so we can't get secretion from the posterior pituitary. Or there is a tumor constricting the release from the posterior pituitary. Under these conditions you would have very low levels of vasopressin or anti-diaretic hormone. The individual is not able to concentrate their urine and inconsequently, they are secreting or excreting from the body 18 liters of urine per day. The second type of, of pathology actually is found within the kidney, and this is called a nephrogenic problem or a nephrogenic lesion. These, there can be two separate problems. One is where we have a receptor resistance. We have a defective vasopressin receptor present on the principal cells, which are lining the collecting ducts. So if the receptor is absent or not active, then vasopressin is present, the cells, the target cells, cannot recognize that the vasopressin is there, and they don't respond. So this would be receptor resistance. And the second type of defect that can occur, is that the aquaporin2 channels are defective. What, what vasopressin is doing in these cells, is that it's moving aquaporin channels, or water channels up to the luminal surfaces of the cells. and in, in, it is inserting these aquaporin channels into that plasma membrane. In the presence of aquaporin, water can move across these cells and into the blood. In the absence of aquaporin, no water can move from the lumen of these tubules, to the blood. So one of our general concepts, so the general concept, if you recall, that the pituitary then has two distinct lobes, anterior and posterior. They have different embryonic origins. They're regulated separately. And they produce different hormone products. The posterior pituitary has got two products, oxytocin and vasopressin. The anterior pituitary is regulated by these negative feedbacks from hormones from the target cells and we said there are four major feedback loops. The posterior pituitary secretes neural endocrines in response to changes in osmolarity of the blood, or the volume of the blood. Also to mechanical stimulation. So, the osmolarity and the volume we're going to be secreting vasopressin and to mechanical stimulation we'll be secreting oxytocin. So, the major feedback loops are not mediated by hormones coming back to regulate the release of these two neuropeptides. But instead, that the, that the feedbacks are either, by changing the osmolarity, changing the volume of the blood. Or by changing, by actually change, by stopping the mechanical stimulation, um,either at the breast, or within the birth canal. Okay. So the next time we come in, then, we're going to start talking about the hypothalamus-pituitary axis and how it regulates energy or basal metabolic rates within the body. So see you then.
