This is the lecture material about our last organ system, that's the endocrine system. Like the nervous system, the endocrine system functions to communicate signals throughout the body and to help maintain homeostasis, that is, relatively constant internal conditions. The endocrine glands are ductless organs that secrete hormones into the interstitial fluid, where they find their way into the bloodstream. A lot of places will report that endocrine glands release their products "directly into the bloodstream". This is not correct! Don't you dare say that on the test! They secrete their products into the interstitial fluid that surrounds the glands, and from there they make their way into the blood vessels that are often extensively distributed around endocrine organs and endocrine glands. Just a little reminder about exocrine versus endocrine glands. Both of them are epithelial in origin, both of them are secretory tissues. Exocrine glands have a duct, an opening to an exterior or interior surface, that allows them to secrete products locally, in that local environment around the gland. Endocrine glands are completely ductless, they're cut off, they secrete products, and these products are hormones. Hormones are infochemicals similar to neurotransmitters. These get secreted into the interstitial fluid, the fluid outside the cell, and those products make their way into the bloodstream. By ultimately ending up in the bloodstream they can act on tissues throughout the body, whereas exocrine glands just act locally. We'll compare the communication of the nervous system and the endocrine system. You'll remember that the nervous system is also a system of integration and control; the endocrine system is also a system of integration and control, and both nervous and endocrine system have cells that can act on target cells by way of releasing chemicals. In the nervous system, we have neurons as those cells that release the chemicals, and we have one neuron that acts directly on one or more cells that it's in very close proximity with. So this is a faster way of communication and it's more precise. With the endocrine system, we have endocrine cells producing hormones that are released into the interstitial fluid and make their way into the bloodstream, that's what's shown in these green triangles here, and because it's traveling through the bloodstream, it can act on target cells that are located throughout the body. The endocrine system can have more of a global response across different body systems. The nervous system would be more important for things like reflexes, which occur instantaneously. The endocrine system is a more prolonged response things like the hormonal flight or fight or flight response, which persists for minutes or more. If somebody jumps out at you, gives you a big scare, your heart starts to race, you're scared, and even after you realize that the threat is gone, your heart's still racing, right? That's the influence of the of the stress hormones epinephrine and norepinephrine that persists in the body for a few minutes. The organs of the endocrine system include endocrine glands, which are glands that have exclusively an endocrine function, and then we'll see some other organs that contain endocrine tissues that have other functions as well. You'll want to know these organs and these glands, you'll want to know the hormones they secrete, and you'll want to know the very general function of these hormones. We'll look at some of these structures, including the exclusively endocrine glands, which are shown over here on this side with the green, and then some of these organs and structures that contain endocrine cells that are over here on the right in blue. You already know about some of them. For instance, we've talked about the pancreatic islets, we've talked about the cell types in the testes and the ovaries, so this won't all be new material. Here's an overview of hormones: endocrine glands produce these chemical messenger molecules, these infochemicals that are called hormones. Hormones can only affect target cells or target organs that have receptors for that specific hormone. Cells that don't possess receptors for a specific hormone don't respond to that hormone. These hormones travel through the bloodstream. They travel to all of the tissues and all of the organs of the body. Not every tissue, not every organ, will respond to a particular hormone, it's just those that have a receptor for a particular hormone that will give a response. Now we'll look at the hypothalamus and the pituitary gland, the master seat of the endocrine system. The hypothalamus, part of the diencephalon remember, is the control center of the endocrine system. The hypothalamus produces regulatory hormones that either stimulate or inhibit other hormones that would otherwise be secreted from the anterior pituitary gland. So the hypothalamus releases hormones that then cause a secondary chain of events to occur at the anterior pituitary. The hypothalamus produces two hormones that are stored in and released from the posterior pituitary. These two hormones are oxytocin and antidiuretic hormone. Oxytocin targets the reproductive systems, it plays roles in social bonding and in childbirth, and in breastfeeding. It's sometimes referred to as the cuddle hormone, but it's also responsible for stimulating uterine contractions for instance. Antidiuretic hormone targets the kidneys, where it causes the reabsorption of water. If you've ever heard of people drinking a lot of alcohol, and then having to pee a lot, this is because alcohol is an inhibitor of antidiuretic hormone. So alcohol inhibits the water conservation efforts of antidiuretic hormone, and causes somebody who's been drinking alcohol to release more water from the kidneys, contributing to dehydration with drinking alcohol. The hypothalamus is also the overseer of the autonomic nervous system. The hypothalamus secretes hormone secretion...errr... it stimulates hormone secretion of the adrenal medulla by way of sympathetic innervation. It, by exciting the sympathetic nervous system, the hypothalamus stimulates the adrenal gland, specifically the adrenal medulla, to secrete hormones. It changes the activity of that adrenal gland and causes the release of epinephrine and norepinephrine, the fight or flight hormones. So it's this overseer of endocrine function by producing hormones that travel into the anterior pituitary and changes its activity. We're about to find out how important the anterior pituitary is. It produces hormones that are stored in the posterior pituitary, oxytocin and antidiuretic hormone, and it controls the activity of the adrenal gland. Here's the anatomy of the pituitary gland. Up here is the hypothalamus with its mammillary body and the infundibulum that extends down into pituitary gland, here's the optic chiasm for cranial nerve number II. The pituitary gland is located just inferior to the hypothalamus, connected by the infundibulum which means it's located in the sella turcica of the sphenoid bone. So it's already inside of the bones of the cranium and then it's further protected by the sella turcica, telling you that this is a really important structure, and it's important because it's controlling many of the functions of the endocrine system, directed by the hypothalamus. It's divided into anterior and posterior lobes, and there's a section in the middle that's called the intermediate pituitary or the pars intermedia, and that pars intermedia is the middle section. It's a small rudimentary structure that secretes melanocyte stimulating hormone. Melanocyte stimulating hormone stimulates the production and the release of melanin by melanocytes in the skin and the hair. And you don't need to know the other names for the anterior and posterior pituitary, but we'll take a quick look at them now. The posterior pituitary, remember, stores and secretes hormones that are produced by the hypothalamus, that's oxytocin and antidiuretic hormone. The posterior pituitary is the neural part of the pituitary gland. It's of neural origin, it's the same tissue as the brain and the hypothalamus. As shown with these axons here, there is a neural connection between the hypothalamus and the posterior pituitary. There are neurosecretory cells up here in the hypothalamus that have unmyelinated axons in a tract traveling through the infundibulum and down into the posterior pituitary. When they fire impulses, they release hormones from these endpoints, their terminals in the posterior pituitary, into the interstitial fluid and into the bloodstream. They release antidiuretic hormone, which is also known as vasopressin, and oxytocin. These are gray clusters of cell bodies, so they're called nuclei. You don't need to know their names; just know that the posterior pituitary is neural tissue and it's releasing hormones that are produced by these cells of the hypothalamus. The anterior pituitary is not neural in origin, it's epithelial in origin, and it's epithelial tissues that are controlled by regulatory hormones that are released from the hypothalamus. These regulatory hormones travel through a blood vessel network of capillaries, very small blood vessels in the hypothalamus, and that's where these hormones get picked up, they get picked up at this..err.. this capillary bed in the hypothalamus, and they travel through the bloodstream a very short distance into the anterior pituitary, and they affect the anterior pituitary, causing it to release a second set of hormones. So this is a two-step process. I have hypothalamic neurons, illustrated in purple, that release one hormone, illustrated with these red dots, that gets picked up by a capillary bed in the hypothalamus, and these red dots travel a very short way into the anterior pituitary, where they affect target cells in the anterior pituitary and cause them to release a second hormone, illustrated with those black dots there. These black dots, this hormone released by the anterior pituitary, now travels through the body and can affect tissues globally. This would be another portal system where we have capillary bed and then another capillary bed, because we have capillaries here at the hypothalamus and then we have capillaries in the anterior pituitary. The blood carrying this second hormone is now entering into veins and these veins are traveling throughout the body. So we have a capillary bed up here and another capillary bed here in the anterior pituitary. That's the portal system. It's capillary bed of the primary plexus and capillary bed of the secondary plexus. The hormones that are released by the anterior pituitary under control of the hypothalamus are numerous. We'll take a look at some of those. There are some that are tropic hormones. Tropic hormones is everything in this box up here. These tropic hormones are hormones that stimulate other glands to secrete other hormones. So we have the tropic hormones that are released from the hypothalamus itself and then there are some tropic hormones that are secreted from the anterior pituitary. Um, one tropic hormone would be thyroid stimulating hormone. The thyroid stimulating hormone stimulates the thyroid gland, and causes it to release thyroid hormone, which stimulates metabolism. Prolactin acts on mammary glands to stimulate milk production, it tells the body to make breast milk. Follicle stimulating hormone and luteinizing hormone act on the development and function of the gonads (the testes and the ovaries), it stimulates um, the development of the gametes, and luteinizing hormone also triggers ovulation in the ovaries. Adrenocorticotropic hormone acts on the adrenal cortex and causes the release of corticosteroids, stress hormones, things like cortisol. Growth hormone stimulates growth throughout the body, it stimulates action at the liver, at the muscles, at the bones, and adipose connective tissue to stimulate growth. Melanocyte stimulating hormone coming from that intermediate portion of the pituitary acts on melanocytes in the epidermis and stimulates melanin synthesis.