 This video will cover the following objective from the endocrine system, discuss how the hypothalamus and the pituitary gland are structurally and functionally related. Describe the structural and functional differences between the anterior and posterior pituitary gland. The hypothalamus regulates the activity of the pituitary gland by producing hormones that are either secreted from the posterior pituitary gland or regulating the production of hormones that are released from the anterior pituitary gland. The hypothalamus and pituitary gland are connected through a stock called the infundibulum. The infundibulum contains the axons of neurosecretory cells that are located in the paraventricular nucleus and supraoptic nucleus of the hypothalamus. These neurosecretory cells are neurons that release hormones called neurohormones. Oxytocin and antidiuretic hormone are examples of neurohormones that are released from the posterior pituitary gland by the neurosecretory cells of the hypothalamus. Oxytocin stimulates a positive feedback loop during childbirth in response to stretching of the cervix when the baby's head is pushing against the cervix, stretch receptors, relay information in through sensory nerves. This information is relayed into the hypothalamus in the brain and stimulates the activity of the neurosecretory cells that release oxytocin from the posterior pituitary gland. Oxytocin then binds to receptors on smooth muscle cells in the myonetrium, stimulating contraction of the uterus, pushing the baby towards the cervix, which creates an even greater stretching of the cervix, activating even more secretion of oxytocin in a positive feedback loop until the baby is born. Oxytocin is also involved in a positive feedback loop that stimulates the release of milk during suckling. When an infant suckles, it stimulates sensory receptors in the ariola in the nipple that relay information into the hypothalamus, stimulating the neurosecretory cells that secrete oxytocin from the posterior pituitary gland. Then oxytocin binds to receptors on myoepithelial cells in the mammary gland, stimulating contraction that releases milk as milk pulls in lactiferous ducts and then in the lactiferous sinus. This milk will be secreted out of the mammary gland and this will reward the infant that will continue suckling as the infant continues suckling. This will stimulate more secretion of oxytocin in a positive feedback loop until the infant is satiated when the infant is full and stops suckling. Then the stimulus is removed, ending the positive feedback mechanism for the release of oxytocin. When there is insufficient water in the body, that is, you're dehydrated, this leads to an increased blood osmolarity or increased concentration of solutes in the blood and also an increased solute concentration in the extracellular fluid. This is detected by osmoreceptors in the hypothalamus. This stimulates thirst, which will then stimulate you to go find some water and get a drink in order to restore the osmolarity or the concentration of solutes in the blood and extracellular fluid in your body. However, these osmoreceptors in the hypothalamus will also act to stimulate the kidney to increase reabsorption of water, which causes the volume of the urine to decrease, leading to a more concentrated small volume of urine. As I'm sure you're familiar with, when you're dehydrated, the color of the urine becomes darker, and this is a result of the action of the hormone antidiuretic hormone. So here we can see the location of the neurosecretory cells in the super optic nucleus of the hypothalamus that function as osmoreceptors and release antidiuretic hormone in response to a high osmolarity, a high concentration of solutes when you're dehydrated. Antidiuretic hormone will be released from the posterior pituitary gland, and then it will have actions to stimulate the kidney to increase water reabsorption. Antidiuretic hormone is produced in the hypothalamus and released by the posterior pituitary gland, then it travels through the blood to reach target cells in the kidney, as well as in the arterioles, the small blood vessels, the small arteries that carry blood into capillaries. In the arterioles, antidiuretic hormone stimulates constriction, stimulates vasoconstriction, that is the smooth muscle in the wall of the blood vessel contracts, and this will have an effective increase in the blood pressure. A synonym for antidiuretic hormone is vasopressin. Vasopressin and antidiuretic hormone refer to the same hormone, but the name vasopressin comes from the fact that it has this effect of increasing blood pressure by stimulating contraction of arterioles. The name antidiuretic hormone comes from its function in the kidneys, where antidiuretic hormone increases the reabsorption of water, causing the volume of the urine to decrease. A diuretic is a drug or a chemical that causes the volume of the urine to increase. For example, alcohol is a diuretic, and alcohol works by inhibiting the secretion of antidiuretic hormone by disrupting the signaling of antidiuretic hormone, which would normally work to decrease the urine volume. Alcohol acts as a diuretic, increasing urine volume. The hypothalamus also produces hormones that are secreted into the blood in the infidibulum and travel into the anterior pituitary gland. These hormones are known as releasing hormones because they will stimulate the anterior pituitary gland to release other hormones known as tropic hormones. The tropic hormones will then stimulate yet another endocrine gland in the body. Let's look at a specific example where the hypothalamus makes the thyrotropin-releasing hormone, TRH, that will travel through the infidibulum and the hypofysial portal veins down into capillaries of the anterior pituitary gland. The TRH will bind to receptors on cells in the anterior pituitary gland and stimulate those cells to release another hormone known as TSH. So TRH stimulates TSH to be released and TSH then stimulates the thyroid gland. TSH thyroid stimulating hormone stimulates the thyroid gland to produce other hormones known as the thyroid hormones T3 and T4, which have a function in the body increasing the metabolic rate. Another example would be the regulation of the adrenal gland or the regulation of the gonads. And so we'll go through the hypothalamic pituitary adrenal axis where the hypothalamus makes CRH, the corticotropin-releasing hormone, the anterior pituitary makes ACTH, the adrenal corticotropic hormone, and the cortex of the adrenal gland makes glucocorticoid hormones to help the body respond to chronic stress. And the HPG axis is another example where the hypothalamus makes gonadotropin-releasing hormone to stimulate the anterior pituitary to make LH and FSH, luteinizing hormone and follicle stimulating hormone, which will then travel through the blood to the gonads, the testes and ovaries, stimulating the production of the sex hormones testosterone, estrogen, and progesterone. Growth hormone is secreted by the anterior pituitary in response to a releasing hormone known as GHRH or growth hormone releasing hormone that is secreted from neurons in the hypothalamus into the hypothesial portal system. When GHRH binds to receptors on cells in the anterior pituitary, those cells release growth hormone. Then growth hormone will bind to receptors on cells throughout the body. When growth hormone binds to receptors on adipose sites within adipose tissue, it stimulates the breakdown of fat releasing nutrients, fat molecules that can be broken down to release energy that will fuel growth. Growth hormone will stimulate the uptake of amino acids from the blood and these amino acids are building block molecules that are used to build proteins. Growth hormone will also stimulate cellular proliferation and reduce apoptosis. Cellular proliferation is a cell division to make more cells and apoptosis is a programmed cell death, so growth hormone will stimulate the production of more cells to increase the growth of a tissue. This is especially important at the epithesial growth plate where it will stimulate the elongation of the bones. In the liver, growth hormone will have a diabetesogenic effect, that is it increases the breakdown of glycogen, a polysaccharide, which functions as a storage molecule storing glucose. As glycogen is broken down, glucose is released from the liver into the blood, increasing the blood glucose level. This glucose can be taken up by cells and used to fuel growth. The liver also produces a hormone known as insulin-like growth factor 1 or IGF1 in response to growth hormone. IGF1 has similar effects in cells all through the body. It will stimulate growth, so similar effects to growth hormone in that it will stimulate cell division, reduce apoptosis and help stimulate tissue growth. But in the hypothalamus, IGF1 will bind to receptors on the cells that produce growth hormone releasing hormone. It will decrease the production of the growth hormone releasing hormone and will also stimulate the production of another hormone known as growth hormone inhibiting hormone. The hypothalamus will produce more growth hormone inhibiting hormone in response to IGF1, and growth hormone inhibiting hormone will inhibit the release of growth hormone from the anterior pituitary. This works as a negative feedback mechanism where as the levels of growth hormone rise, levels of IGF1 rise, and IGF1 then inhibits the production of growth hormone by stimulating the production of growth hormone inhibiting hormone, which as the name suggests inhibit the release of growth hormone. Abnormally low levels of growth hormone during childhood produce a condition known as pituitary dwarfism, seen in the image on the left a full grown adult may be very short stature the size of a child with the overall normal proportions because growth hormone signaling was disrupted, this led to a decreased growth rate. In the opposite example shown on the right, an excess of growth hormone signaling during childhood leads to increased growth rate, especially stimulating the elongation of bones causing a very tall, statured human, as we can see in the example here of gigantism, someone who has a really large body size as a result of excessive growth hormone during childhood. Growth hormone is just one of the hormones that is produced by the anterior pituitary gland. The anterior pituitary gland is secreting several different hormones in response to releasing hormones that are being produced by the anterior pituitary and secreted into the hypophysioportal vein traveling into the anterior pituitary gland. Here we can see at the top the GnRH that stands for gonadotropin releasing hormone is produced by the hypothalamus and stimulates the anterior pituitary to release two tropic hormones, one known as luteinizing hormone abbreviated LH and the other known as follicle stimulating hormone abbreviated FSH. LH and FSH both have actions in the reproductive system. They will stimulate the gonads in order to lead to the production of the sex hormones and also stimulate the production of the gametes, the sperm and eggs. Next you see TRH that stands for thyrotropin releasing hormone. So TRH is the releasing hormone from the hypothalamus that stimulates the anterior pituitary to make TSH, thyroid stimulating hormone. So thyrotropin releasing hormone TRH stimulates the anterior pituitary to make thyroid stimulating hormone, TSH, that will then stimulate the thyroid gland to release the thyroid hormones T3 and T4. T4 stands for thyroxin and T3 stands for triiotothyronine. But these thyroid hormones T3 and T4 regulate metabolism increasing the metabolic rate. TRH stands for the prolactin releasing hormone. Prolactin releasing hormone stimulates the anterior pituitary to release prolactin. Then prolactin is a hormone that stimulates the mammary glands to produce milk. So during the end of pregnancy and following pregnancy prolactin is stimulating the production of milk but this is different than the action of oxytocin that stimulates the release of milk. Prolactin prepares for that action of oxytocin by stimulating the mammary gland to produce milk and then in response to suckling oxytocin will stimulate the positive feedback mechanism for the release of milk. So next we see GHRH which stands for the growth hormone releasing hormone. So growth hormone releasing hormone stimulates the anterior pituitary to release growth hormone then growth hormone stimulates growth throughout the body especially in the bone and muscles but also stimulates the liver to produce insulin like growth factor another hormone that stimulates growth. Last we see CRH is produced by the hypothalamus. CRH stands for corticotropin releasing hormone. So CRH stimulates the anterior pituitary to release ACTH. ACTH stands for adrenocorticotropic hormone. ACTH stimulates the adrenal cortex the outer region of the adrenal glands and it stimulates the adrenal cortex to produce glucocorticoids which are hormones that help regulate metabolism in order to respond to chronic stress. So glucocorticoids will have the effect of increasing blood glucose levels and that's why they're called glucocorticoids. So glucocorticoids are produced by the adrenal cortex and they'll stimulate an increase in blood glucose levels.