Hypothalamus-Posterior Pituitary Axis

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Из курса от партнера Duke University

Introductory Human Physiology

1804 оценки

Duke University

Introductory Human Physiology

1804 оценки

In this course, students learn to recognize and to apply the basic concepts that govern integrated body function (as an intact organism) in the body's nine organ systems.

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The Endocrine System

IIn this module, we return our attention to the endocrine system and its role in the maintenance of homeostasis. In particular we consider the hypothalamus-pituitary axis, which integrates signals from the nervous system and from the blood to regulate most homeostatic functions, including growth, ion balance, fluid balance, response to stress, and energy use. The first lesson gives an overview of the hypothalamus-pituitary axis and its actions in regulating growth of the body. In later lessons we consider how this complex negative feedback loop governs the body’s energy use and its response to stress. Then, later in lesson 3, we turn our attention to the simple reflex loop by which the endocrine pancreas regulates metabolism in both the fed and fasted states and the failures of this system (diabetes mellitus). The hypothalamus-pituitary axis and its control of reproduction in both males and females are considered in the next module (Module 9). The things to do this week are to watch the 6 videos, to answer the in-video questions, to read the notes, and to do the two problem sets (endocrine system and fuel homeostasis). Please note that each module can stand alone, however, it will be most effective if you do the first two videos (H-P-axis) before any of the others. The notes provide a more detailed summary of each topic and again we encourage you to use the resource (videos and/or notes) that works best for you. Please do try the problems sets for self-review. Both your understanding and retention will increase with application of the new learned information.

Hypothalamus-Anterior Pituitary Axis- GH23:38

Hypothalamus-Posterior Pituitary Axis9:40

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Jennifer Carbrey
Assistant Research Professor
Department of Cell Biology
Emma Jakoi
Associate Research Professor
Department of Cell Biology

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.

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