Hypothalamus & Pituitary Physiology

Hypothalamus & Pituitary Physiology

okay continuing our discussion of the endocrine system I'm going to introduce the hypothalamus and pituitary gland now these are the considered the hypothalamus is considered what's called the master gland because it has control over so much of the endocrine system and we'll get into all the control systems in a few minutes but first I wanted to talk a little bit about the anatomy of the system so the hypothalamus actually so this here is a set of sidon picture of the view of the brain and this is a sagittal picture as if you know your face would be here if you were able to see your face but the skull is sort of cut in half and here you know the eyes would be you know somewhere around here indeed of you're here okay just to give you a view of where we are so this dark blue structure here is actually the lateral ventricles of the brain and I know we just talked about the neurosystem and but the brain comes in here as well because of the connection of the hypothalamus does light blue structure here would be the third ventricle and if you remember the third ventricle lies in the middle of the brain and drains into the fourth ventricle which would be down here okay so um now why is the third ventricle important well it's because it's actually the floor of the third ventricle that is the hypothalamus so there's a little bit of neural tissue down here and that little strip of neuro tissue is the hypothalamus and then hanging below that hypothalamus and connected to it as a pituitary gland okay so that's the location of it all right so let's get a close-up view I'm going to move up here to a picture and I have of the pituitary gland now you can see up here is a collection of nerves so this here is actually the hypothalamus and within the hypothalamus there are a number of nerve bodies some of which extend into the pituitary gland so we have some these are the nerves the soma of the nerve bodies and these are the axons you can't really see that very well but we have some axons here that extend down into the pituitary gland some of them extend down just to the to the stock of the tuatara gland and some extend down all the way these ones here okay so you can see here that the pituitary gland is roughly divided into two sections and these sections are called lobes so we have here the anterior lobe of the pituitary gland and we have here the posterior lobe posterior and we actually have here in the middle a third lobe that's called the medial lobe and we're not really going to get into that but just to be aware that it's there it's a very small part of the pituitary gland okay so I want to start by talking about the anterior lobe of the pituitary gland now the anterior lobe of the pituitary gland has sort of a unique anatomy with a unique vascular supply now you can see here I'm going to switch to some ink that you can see you can see here that there are neurons that communicate down into the socket of the pituitary gland and then stop and it stops at this at this cat at a capillary bed now those capillary bed is fed by an arteriole as our most capillary beds but you know most capillary beds in the body have an arteriole a turret arteriole that feeds them and then they break up into these capillary beds and then the capillary beds make their way back to a vein and the vein goes where it goes back to heart right particularly the right side of the heart but so you know in ordinary veins we'd go back to the heart but in a couple of places in the body it doesn't go back to the heart it actually opens back up into a second capillary bed okay now that happens as far as I'm aware this only happens in two places in the body it happens in the anterior lobe of the pituitary gland and it happens in the liver now so we have a first capillary bed and a second capillary but actually I think it happens in the kidneys as well but it's a little bit of a different situation in the kidneys so we have to cow anyways we have two capillary beds here and when when you have two capillary beds that are separated by a vein carrying the blood between them this vein is called a portal vein so that's the exact structure that we have here we have an arteriole that's feeding the anterior section of the stock of the pituitary gland and then actually this I think this should be blue in my mind so I'm going to change it we have a portal vein and why do we have a portal vein here well because we opened back up into another Kappa a second capillary bed why do we need a second capillary bed well that is because it is the way that the anterior pituitary communicates with the hypothalamus so what we have here is the hypothalamus is actually releasing hormones in right next to this capillary bed and the capillary bed is absorbing them and particularly it's releasing what are called releasing hormones and these releasing hormones are things like thyrotropin and Kannada trope and releasing hormone and corticotropin-releasing hormone so these are hormones that direct the pituitary to release other hormones okay so these releasing hormones get come from the axons from the hypothalamus and release into this capillary bed and then they travel through this portal vein down into the pituitary where they move out from the capillary bed into the anterior pituitary cells where they signal it where they signal the anterior pituitary to release specific hormones and response so what hormones do we have in the anterior pituitary we have all sorts of stimulating hormones that stimulate other endocrine glands to release their hormones so here we have stimulating hormones and examples of this would be thyroid stimulating hormone and thyroid stem the role of thyroid stimulating hormone is to release is to tell the thyroid gland to release thyroid hormone right and then similarly there is adrenocorticotropic hormone that stimulate the cortex of the adrenal glands to release cortisol and it communicates with follicle simulate stimulating hormone and luteinizing hormone to the testes and the ovaries to these testosterone and estrogen right okay etc and you can um you know I know the book has a lot of details about exactly which hormones so I'm not going to go into every single hormone but this just gives you an example of how this occurs so again just to go through it again step by step we have the hypothalamus sending neuropeptides so these are actually neurotransmitters that move down the axon actually they don't move down but we have an action potential in maipo thalamus that sends information down the axon to release a a neurotransmitter or neuro endocrine hormone and the neuro endocrine hormone is a releasing hormone like thyrotropin and that is released and absorbed into the capillaries it moves down the portal vein into the anterior lobe of the pituitary gland where it where it exits the capillaries and moves into cells within the anterior lobe of the pituitary and those cells are signaled to release another hormone so again if it was thyrotropin that was released the thyrotropin is communicating with cells and enter to terry gland to release thyroid stimulating hormone so now thyroid stimulating hormone is released in again in is absorbed by this capillary bed and it comes through the vein back into the body and circulates through the body until it gets to the thyroid gland and when it reaches the thyroid gland it tells the thyroid gland to release thyroid hormone okay so that's how the process occurs in the anterior pituitary in Prior classes is probably heard of the hypothalamic pituitary axis or HPA axis now I just wanted to talk a little bit just review it quickly now the HPA axis is essentially a negative feedback loop okay so what happens here is there is you know steady state then something changes in the hypothalamus right now we're using an example of luteinizing hormone releasing hormone and luteinizing hormone and estrogen in a woman's body okay so but this system is the exact same negative feedback system that controls all hormone systems in the HPA axis so what happens is the body decides that it needs more luteinizing hormone it communicates through the thalamus through the hypothalamus to send information to the pituitary gland and it does this by again releasing a neurotransmitter that functions as a neuro endocrine hormone called luteinizing hormone releasing hormone and this is released into that capillary bed inside the stock of the pituitary gland and that travels down the portal vein we shall make that portal vein blue in to the capillary loop and is secreted into the pituitary gland and is picked up by the cells there and the cells are stimulated by the luteinizing hormone releasing hormone to secrete luteinizing hormone and follicle stimulating hormone okay so they are going to secrete these things and these things are going to be picked up in the ovaries and stimulate cells in the ovaries to produce estrogen okay going to worry about progesterone we're just going to worry about estrogen now what happens in the body is there's actually two loops here the estrogen the presence of you know we're going to have an increased amount of estrogen now the increased amount of estrogen is is going to be pumped into the bloodstream and it's going to flow through the capillary system in the anterior pituitary gland and it's going to tell the anterior pituitary gland to produce less luteinizing hormone and follicle simulates stimulating hormone okay and at the same time the estrogen as it's traveling through the bloodstream is also going to travel through the hypothalamus now remember in one of my previous videos I talked about the hypothalamus is one of the only areas of the brain that does not have a blood-brain barrier so and one of the reasons why is because it has to be aware constantly aware of all the hormone levels inside the body and some hormones cannot cross through the blood-brain barrier except it can cross through into the hypothalamus so the hypothalamus detects this higher level of estrogen and it has a negative it provides negative feedback and tells the hypothalamus to produce less luteinizing hormone okay so it has a sort of double feedback loop a short feedback loop goes right from the you know ovaries to the pituitary gland and a longer feedback loop that goes from from the ovaries all the way up to the hypothalamus and both are causing a reduced production of these hormones which will eventually tell the ovaries to slow down their production of estrogen and that will just prevent the body from over producing estrogen okay the next thing i wanted to talk about is is the posterior lobe of the pituitary gland now the posterior lobe of the pituitary gland is very different from the anterior lobe both in anatomy and in function so the posterior lobe of the pituitary gland is in a way an extension of the hypothalamus itself the hormones are actually created in the hypothalamus nerve soma of the hypothalamus because these are neuro peptides and they travel they work their way down the stock where they're stored in the bouton of these long axons and released into the posterior pituitary and these are the hormones themselves these neuro peptides are the hormones and the hormones of the posterior pituitary gland are ADH and oxytocin now interestingly enough these hormones actually have a very similar chemical structure so they almost sort of cross react with each other so you know sometimes ADH can actually stimulate you know oxytocin receptors and vice versa although the cross reactivity is relatively low okay so what do these hormones do let's talk about ADH first so again ADH is a neuropeptide that is released from the axon bouton and into the capillaries of the posterior pituitary and from there into the general circulation okay so ADH what does ADH do well the receptors of ADH are in the kidneys some reason kidneys are always supposed to be orange now inside the kidneys you know we have a glomerulus and we're going to talk about how the glomerulus works but let's let's kind of expand our view here and create a little glomerulus and bonuses bones supposed to be Bowman's capsule and it drains into the collecting ducts and then you know the collecting ducts are these sort of tortuous things and then we have the loop of Henle and actually sorry about that I called these these are actually the proximal convoluted ducts and then the because from the loop of Henle they drain into the collecting ducts now I'm not going to go into the whole process here but essentially what happens with urine is the distal the hairpin turn of the loop of Henle actually functions the entire loop of Henle actually functions to provide a really really high concentration of solutes solutes in this area of the kidney and the osmolarity of this area is like 1,500 the osmolarity of blood on the other hand was around 300 so we have a huge concentration of solutes primarily sodium so we have this huge concentration gradient from the collecting ducts in the collecting ducts back into this area all right so water wants to travel from these collecting ducts back into the tissues around the glomerulus so but it generally doesn't because there are no all the water channels here are closed now remember there's water pores let me draw that in a different color everywhere in the body there are pores where water can flow freely from one membrane to another across cell membranes but in the case of the collecting ducts these water pores are closed so when these water pores are closed we actually have very dilute urine going down to the bladder and getting peed out okay so there's the platter but ADH works by signaling these water pores to open up so it opens them up and then water gets sucked down the concentration gradient into this really highly into this area that has a very very high osmolarity so this can concentrate the urine to an osmolality of near 1500 right so through osmosis water free water gets pulled through these open pores with the signaling of ADH back into this highly concentrated area near the near the hairpin turn of the look of loop of Henle and then the urine will become very very concentrated and not very much water will make it down to the bladder okay so it conserves water and this water ends up going back into the bloodstream and back into systemic circulation so that is why anti diuretic hormone has an anti diuretic effect because it sucks the water back into the systemic circulation and makes very concentrated urine okay now just a little anecdote those of you who have imbibed alcohol from time to time it's interesting to note I'm sure that everybody has noticed that when you drink alcohol you tend to urinate a lot and the reason for that is remember alcohol is generally works as an inhibitor in the brain and alcohol actually is remember the chemical symbol for alcohol is he um Ito or ethyl alcohol it actually works as an inhibitor for these neurons so these neurons are inhibited and it stops them from producing actually it stops them from producing both ADH and oxytocin that's one one reason if there's a woman that's having difficulty breastfeeding it's probably a good idea to encourage them not to drink alcohol because you're also going to suppress the the production and secretion of oxytocin as well but drinking alcohol decreases the production and secretion of antidiuretic hormone by this inhibitory effect in the hypothalamus so what happens is you have less antidiuretic hormone these pores remain closed so you dump a lot of undiluted urine or lots of free water into the bladder and then you know after finishing your you know second bottle of wine actually I shouldn't say bottle if your second glass of wine and you're rushing off to the bathroom down to your bladder and that is why because you've inhibited the secretion of your antidiuretic hormone now oxytocin oxytocin actually communicates with with tissue in the breast and they're actually mild inside the breast tissue there are myoepithelial cells and it stimulates these cells to to let down milk during breastfeeding and it also stimulates the the uterus itself to stimulate strong contractions and those of you who have worked in OB know that pitocin which is essentially synthetic oxytocin is actually giving sometimes to stimulate stronger contractions in the uterus and also women that have significant bleeding after after giving birth are oftentimes encouraged to breastfeed their baby because it'll actually increase the strength of contractions in the uterus and and that extra strength of contraction may help to sort of tamponade tamponade the bleed so the bleeding doesn't continue

28 thoughts on “Hypothalamus & Pituitary Physiology

  1. thank you for taking time out of your life to help people understanding stuff that most professors dont know how to explain. you did a great job. thank you very much. have a lovely day mr andrew wolf. sincerely . mohamad alsharif. med student in istanbul

  2. Good day sir!

    I apologize. If I am correct, it is the pituitary gland that may be, in the common vernacular, titled "the master gland."
    Please inform me… I am correct.

    Thank you and please enjoy the remainder of your day. 🙂
    And, thank you for your services regardless. 🙂

              -Sincerely, Mr. Guckenberger

  3. awaiting third attempt of tumor removal from piturity gland have condition for over 20 rs and wish to discus mentality of cushings
    the condition has pros and cons would like to discuss research issues on its findings

  4. Great videos as always. One small suggestion. Around minute 7:00-15 you mention that the portal is how the "Pituitary communicates with the Hypothalamus." This can be misleading and confusing to some viewers since the hypothalamus communicates with the pituitary for most RH's. The Pituitary never really communicates with the hypothalamus. PIF/Prolactin may be and exception but Prolactin stimulates the Hypothalamus via systemic circulation. Correct me if Im wrong. Thanks so much YOu are The MAN

  5. I have to say thank you!!! Very informative and broken down nicely. My professor has us watch a lot of your videos and they are easily digestable mentally, making it a great way to learn.

  6. I have one question Mr. Wolf. Isn't the pituitary gland the master gland not the hypothalamus. The hypothalamus is only part of the brain it is not a gland i thought?

  7. Great, now how can I get these babies active – From what I understand, it's the hypothalamus and pituitary gland that are responsible for growth? I wanna grow 4 inches… what do I do? 🙂

    thank you kindly!

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