The Science of Everything Podcast - Episode 134: Hormones and the Endocrine System

Episode Date: February 13, 2023

An introduction to hormones and the endocrine system, including a discussion of the definition of hormones and their production, storage, release, and mechanisms of action. I also consider the mechanisms of control and regulation of hormone production, focusing on the role of the hippocampus and the pituitary gland. I conclude with an overview of major endocrine glands in the human body, including the gonads, adrenal glands, thyroid gland, and the thymus. Recommended pre-listening is Episode 38: Neurons and Synapses. If you enjoyed the podcast please consider supporting the show by making a PayPal donation or becoming a Patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

you're listening to The Science of Everything podcast episode 134, hormones and the endocrine system. I'm your host, James Fodall. So in today's episode, we're going to tackle another of the systems in the body, the endocrine system. This is one that receives a fair bit of popular attention, although usually not under the name of the endocrine system. I think people typically just talk about hormones more commonly, but the endocrine system is effectively the system that synthesizes, distributes, and kind of modulates the effect of hormones. So in this episode we're going to be covering that. We're going to be talking about what hormones are, different types of hormones and how they act. We'll talk a bit about pheromones, which are a special type of hormone that is of interest.

We'll talk about how hormones are regulated and we'll focus on the role of the hypothalamus and the pituitary gland as two of the major body regions which are responsible for regulating hormones by generating hormones themselves. We'll then talk about many of the major glands within the endocrine system, including the thyroid, including the thyroid, adrenal, thymus, and reproductive organs, which are also glands of the endocrine system. Recommended pre-listing is episode 38 neurons and synapses. You may also be interested in checking out episode 118 on cell signaling, as that is very closely related to the mechanisms of hormone action, which is one of the topics we'll be talking about. But without further ado, let's get started and start by talking about what are hormones and what is the endocrine system.

So the endocrine system is the bodily system that regulates the activity of target organs at distant sites by means of special chemical messenger molecules that are called hormones. A hormone is a type of signaling molecule in a multicellular organism that is sent to a distant organ that helps regulate the activity and biochemical processes of that organ so as to maintain homeostasis, regulate physiology and behavior. So the key thing is that a hormone is like a messenger molecule, but it's a long-distance messenger molecule. Hormones act across long distances within an organism, like they travel between organs. They don't act within, I mean, they do act within a cell ultimately, but they're not localized to within a cell like other types of

messenger molecules are. So hormones are used to communicate between organs for the regulation of all sorts of physiological functions and behaviors, including digestion, metabolism, respiration, sensory perception, sleep, excretion and urination, lactation, regulation of stress, growth and development, as well as movement, reproduction, and psychological states like mood and emotions. So hormones are ubiquitous in multicellular organisms and particularly humans, and that will be the focus of the episode here, human hormones. So there are different types of hormones depending on where they are produced, how they're released, and how they act on their target.

So remember, the key thing about hormones is that they are long-distance chemical messengers, and therefore there's always a question about where they, where are they released from, and where are they targeted to, because those are always going to be different locations. That's sort of the point of a hormone. In order to understand the distinction endocrine glands and endocrine signaling, which is the type of signaling done by endocrine glands, It's useful to contrast that with two other types of signaling that are found in humans and other organisms. Exocrine glands are glands that secrete hormones outside of the body.

So these include like the cellarvary glands and sweat glands. So they excrete outside. Parachrine glands or paracrine signaling done by those glands is sort of similar to endocrine signaling, but acts over a fairly short distance. So that's kind of localized signaling between neighboring cells. So an example of paracrine signaling, a neurotransmit is found in the nervous system because they are released by at the post-synaptic membrane of one neuron and then travel across the synaptic cleft, the gap, the small gap between two neurons, and then they act on the post-synaptic membrane of the post-synaptic neurons. So they only travel a very short distance, the gap between one cell and the other that are directly adjacent. So hence they're paracrine signaling because they act very much. locally. In this episode we're focusing on endocrine signaling and endocrine glands in the endocrine system more generally that facilitates that. So the key thing here is that the hormones,

the signaling molecules, target distant body sites, often entirely different organs located at a great distance in the body. Now the properties of a hormone depend on its chemical structure, unsurprisingly, and there are a few major different types of hormones. So just to keep us on track, I've just explained the different types of signaling. Here we're focusing on endocrine signaling. That's long distance signaling. We talk more about paracrine signaling when we talked about the nervous system. So here we're focused on endocrine long distance signaling. Now we're talking about different chemical subtypes of hormones. And the chemical type of the hormone is, as I just said, what primarily determines its functional properties and kind of what it,

what types of cells it acts on. So the major classes of hormones are steroid hormones, peptide hormones, and polypeptide hormones. There are some others as well, but those are the main three that we're going to focus on. So steroid hormones, hormones that follow a structure that's sort of similar to the steroid molecule. So you probably heard about steroids as growth hormones and the masculine sex hormone. We'll talk more about those in a bit. But steroid hormones as a class are broader than that. So it's a little bit confusing because the term here is used differently in sort of a biochemical physiological context than it is in an everyday context. When we talk about steroid hormones, we're not just talking about like anabolic growth

steroids. We're talking about a chemically defined class of signaling molecules. And what defines steroid hormones is that they have a set of four carbon rings and there's some various functional groups that come off of those like hydroxels and so forth. We don't really need to discuss that here, but just understand that there is a particular chemical structure, like a template, that steroid hormones follow. And each steroid hormone is sort of like a variation on this underlying structure. So the key point is that they follow this sort of four carbon ring structure with an additional elements around the edges. And depending on exactly which functional groups, like which bunches of atoms, depending on which ones are present, hormone will have different properties and be able to affect different types of receptors.

Now, steroid hormones are lipid soluble. That means that they can move through the plasma membrane. So lipids are a particular class of macromolecules. You may recall that there are lipids, carbohydrates, nucleic acids, and proteins. And so lipids are essentially fats, and cells are surrounded by a bilipid membrane structure, which consists of two sort of stacked layers of the special lipid molecules. Think of it as like a double layer of fat. So the point about steroid hormones being lipid soluble is that they can pass through the plasma membrane of target cells because they're basically fat soluble, right?

And that means that they are typically not soluble in water. Remember oil and water don't mix, right, because they're different types of molecules. And because steroid hormones are lipid soluble, they can easily penetrate through the plasma membrane of target cells and bind to specific protein receptors inside that cell. So often hormone receptor complexes that are responsive to steroid hormones are located in the nucleus and act to either inhibit or enhance the transcription, so the amount of manufacturing essentially, of proteins from different genes. So the reason that's convenient, obviously, is because the steroid hormone can easily get inside the cell, across the membrane inside the cell to the nucleus where the genetic material is stored,

and thereby fairly readily bind to different receptors, which can change how much of a particular protein is produced from different genes. So many steroid hormones are responsible for changes in gene expression because of their easily ability to get into the nucleus. You don't have to have steroid hormones in order to change a gene expression because there's various mechanisms of transduction, which we've talked about in previous episodes, to go from a signal outside the cell to a signal within the cell, but steroid hormones can kind of bypass that, right? Because they can just go straight through. So you don't need to sort of transduce the signal from one protein to another and so forth. Anyway, so that's steroid hormones. That's a very big class of hormones.

I'll talk more about some examples of specific hormones that fit into that class later on. So the second class that we're going to talk about that are peptide hormones. So peptide hormones are made of, unsurprisingly, peptides. So a peptide is a building block, or a single peptide is a building block of polypeptides, which make up proteins. So these are effectively amino acids, or there are 20 amino acids that are found in, you in humans, so these peptide hormones are made up of one or a couple of these amino acids join together. I think mostly the peptide hormones, it's just a single amino acid. Now, peptide hormones are not liposoluble, so they can't cross the plasma membrane and then sort of go straight

into the cell. Instead, they bind to a receptor on the surface of the cell and then trigger a signal transduction process, like a cascade of reactions, that lead to some kind of changing activity with inside the cell. Again, I've talked more about that in the cell signaling episode, so go back to that episode if you're interested in how that process works. In this episode, I'm not going to talk about the signal transduction pathway so much because we've already covered that and it's a bit of a different thing. Here we're going to more focus on the relationship between the different components of the endocrine system and how they are regulated and also what they regulate in terms of physiological function. Now, the third main type of hormones are polypeptide hormones. I mean,

this is sort of the same as peptide hormones, but the difference is that instead of one or maybe a couple of amino acids, polypeptides are longer chains of amino acids connected together. And they tend to be, well, like bigger, obviously, for that reason. And also, therefore, bind two receptors on the surface of the target cell, but they're going to be different types of receptors that are responsive to the big polypeptide hormones compared to the much smaller amino acid derivative, the single peptide hormones. All right, so those are some of the major classes of hormones and the big distinction between the steroid hormones and then the peptide and polypeptide hormones. Before we talk about the process of hormone regulation and look at the hypothalamus and the pituitary gland in more detail, I want to address the sort of a subtopic here, which is pheromones. Now, pheromones are very interesting. A pheromone is a chemical that is secreted or excreted from the body that triggers a social response, like a behaviour.

cognitive response in members of the same species. So they act like hormones outside of the body, except they affect a different individual instead of affecting the behavior of the same individuals. So hormones are excreted within the body, travel to some distant location in the body, and then affect the physiological function and behavior of the same organism. They affect the host organism, right? Whereas a pheromone is excreted out of the body, technically the gland that excretes a pheromone is going to be an exocryne gland because it excretes outside of the body. And also, the other difference is that it has an effect on a different individual, not the same individual that produce the hormone.

So that's what a pheromone is. Now, what are they used for? Well, pheromones are found in many different species. They're used to mark territory, so that different individuals know where the territory of one animal ends and where the other begins, to signal alerts between members of the same species, like if a predator is nearby, to signal a trail or pathway that other members of the species should follow, for example, to a food source, or to attract mates.

So there's a sort of things you might expect, basically, like behavioural regulation of territory, resources, threats, or mates. Pheromones are found, as I said, in many different types of species, including many insect species. Pharamones are found in some species of reptiles, amphibians, and non-primate mammals. In many of these cases, so vertebrates like the reptiles and mammals and so forth, Pheromones are detected by both regular olfactory membranes, so basically the same receptors that detect molecules for transducing smell. Also, many of these vertebrate species detect pheromones through a specialized organ in the nose called the vomeronasal organ, which is also called Jacobson's organ after someone who studied it. And that lies at the base of the nasal septum between the nose and the mouth.

So it's kind of like at the sort of the back and middle of the nose. So this vomeronasal organ is basically just a series of receptors that are sensitive to particular types of molecules, like to pheromones, and that transduces certain signals that lead to behavioral responses in those animals. One of the main reasons that I wanted to talk about pheromones, apart from the fact that they're an interesting application of hormones, is the question as to do humans respond to pheromones? Now, I think that this is a misconception that's propagated through popular media that just sort of talks about pheromones in a bit of an imprecise way. So humans certainly respond to smell, but pheromones aren't the same thing as smell. If a human detects a smell, they may well, that may will trigger a behavioral change, but that is different to a hormone.

So if a smell triggers a behavioral change, that occurs through a process of the smell being detected by the receptors on the nasal memory. brain that is then processed by the brain at an increasing level of abstraction and then whether consciously or subconsciously it then may have an effect on the behavior of that organism through an effect on brain function essentially which is then you know manifested through some signals that are sent down to change some aspect of the skeletal muscle system like moving around or maybe the autonomic nervous system or whatever the point is that behavioral changes mediated through smell occur through the mediation of of the brain processing that information in a certain way to signal a threat or food or something else, like or a person

you're familiar with, you know, whatever. So certainly that happens in humans and that happens in, I mean, presumably all organisms that have both a brain and sense of smell, right? But pheromones are different to that. Ferramones are signaling molecules that have specific effects on specific receptor cells. So obviously it's going to depend on what the particular hormone is, what the particular pheromone is, but these will trigger specific behaviors in a more direct way that is not sort of mediated through the cognitive processing of the brain. I mean, you know, there's going to be links to that, but it's not sort of the same thing. It's probably better thought of as more, like, reflexive behavior that's triggered in a more direct way through action of the pheromone

molecules on receptors in both the olfactory membrane and the vomoroneasal organ. So responding to pheromones is not the same as simply responding to a smell. It's a different sort of chemical process or biochemical processes. Now, therefore, the question is to do humus respond to pheromones? Obviously, humerus respond to smell. It's not clear whether humus respond to pheromones, and I suspect that they don't. There has been research conducted on this, unsurprisingly, and there are some studies which have purportedly found results, although the methodology of these studies has been contested.

One big issue is that in order to test whether a pheromone is having a behavioral effect, you need to control for all other possible sources of influence of behavior. And that includes odour. Right. So in order to do these sorts of studies, they need to control for the odor and any effects on like taste or vision and anything else that could be, that could be sort of masking the actual effect or confounding the effect of the pheromone itself. So it's very difficult to do this, especially controlling for smell. There have not been any studies which have clearly established the effect of any pheromonal substance. on human behavior in a well-controlled study.

As I said, there have been a few that have purported to find this effect, but it's been difficult to replicate them and their methodology has been suspect. So I would say that there haven't been any clear results to demonstrate this in humans. Furthermore, it's not clear that humans have a vomeronasal organ, or at least a functional one. So it's present, this organ is present in the fetus, but it appears only in an atrophied or shunken form, or completely absent altogether, in most adults. unclear whether it may still have some degree of function, even if it's in a sort of a shrunken or atrophied form, or whether perhaps only some people have a functional vomeronasal organ,

but at least it seems to be dysfunctional or non-functional in the majority of perhaps all adult humans. So given the inability of finding any clear examples of pheromones that have behavioral effects on humans, and also the anatomical evidence of the sort of absence or severe atrophy of the vomeronasal organ in humans. It seems pretty unlikely to me that pheromones play any very significant behavioral role in humans. We can't rule it out altogether. It is possible, evolutionarily, that there could be something there, but the evidence isn't strong. So, you know, this is just something interesting because often in, you know, movies or whatever, um, or popular culture that there's, um, portrayal of pheromones, um, as if, you know, it's just sort of an established

fact that humans are responsive to pheromones. And there are certainly, you know, colognes and deodorants and perfews and so forth that claim to have sort of a pheromone effects, but none of that's ever been demonstrated under controlled conditions. So you should take claims like that with quite a few grains of salt. But anyway, let's move on, and let's talk about the process of signal transduction in the endocrine system. Particularly, let's talk a bit about how hormones are spread across the body. So again, a hormone is like a long-distance signaling molecule. We talked about like the steroid hormones and peptides and polypeptides and so forth. So that tells us a bit about the structure of the hormone molecules.

Where do hormones come from? Well, they're synthesized by special tissues called glands or endocrine glands. We'll talk more about those in a little bit, but these are things like the pituitary gland, thyroid, adrenal glands and so forth. So you may have heard of those. And these are located in different parts of the body, which we'll get to. What happens is that a hormone is synthesized by specialized tissue within these glands and then released into the interstitial fluid.

So the interstitial fluid is the fluid that surrounds like pretty much all cells in the body, or at least the majority of them, and that sort of separates, that forms the kind of matrix that separates different cells from one another. So there's the cytosol, which is the sort of viscous liquid substance within the cell, like inside the cell membrane. Outside of cells, between cells, there's also a sort of liquid substance. it's like mostly water, but it's not completely water, there's all sorts of other soul use in there as well. But it's different in composition to the cytosol.

So within cells, it's the cytosol. Outside of cells, it's the interstitial fluid. It's just the fluid in between spaces, effectively between cells. So hormones are released by glands into the interstitial fluid, so they're basically dumped out of the cells that they're produced in, where they then diffuse into capillaries and thence into the bloodstream. So they're carried in the bloodstream from one part of the body to another, where they then will diffuse back into the capillaries and then into the interstitial fluid. And from the interstitial fluid, either they will diffuse across the cell membrane if they're steroid hormones,

or if they're not if they're peptide or polypeptide hormones, they will interact with receptor molecules on the surface, generally protein complexes on the surface of the target cells and have their effect of there. So endocrine signalling operates through the circulatory system, through the blood system, in that that's what transports them long distances. but they are released directly into the interstitial fluid, not into the bloodstream. Okay, so that's a bit on how the sort of transduction process works at a high level. In terms of how it works specifically at the level of like an individual cell,

we're not going to go into too much detail here because, as I said, we did a whole episode on this, and much of that intercellular signaling is sort of relevant here in the case of hormones. The basic idea, though, is that if you have a peptide or a polypeptide hormone, so one that cannot diffuse across the membrane, it will bind to the receptor, so a particular protein complex on the surface of the target cell. When I say bind to, that means that they come into physical contact with each other. And usually there's a certain shape that's required in order for the hormone to sort of fit into the receptor.

So this is often described as like a lock and a key. It has to just fit the shape correctly, like the physical three-dimensional confirmation. And often there'll be particular intermolecular bonds that form like hydrogen bonds or other types of mechanisms like that that ensure that it sort of fits in properly and that something with a different shape doesn't get in when it shouldn't. So once that happens, that binding to the receptor triggers a conformational change in the receptor, just a change in its shape, which then instigates a signaling transduction cascade inside the cell, because the idea is that the protein receptor complex extends across both sides of the membrane, and this is how a hormone outside of the cell can affect what's happening inside the cell,

because the protein complex sort of sticks out on both sides. And so when it changes in confirmation, when it kind of changes shape, that has an effect on attached or associated proteins inside the cell, which then produces a series of reactions and secondary messenger molecules and other things, which ultimately lead to a cellular response within the cell. So a change in gene expression or a change in the confirmation of a certain protein, or it may trigger a release of calcium or something like that, which then triggers vesicles to bind to the membrane and release something.

You know, different processes like that. Again, we're not going to go into the details here because that's for a different episode, but it's these sorts of signal transduction processes that are behind the actual process by which hormones produce their effects in the target cell. And so that's very much at the sort of biochemical level. Many hormones have as their target other cells that produce hormones. And so the specialized cells contained in endocrine glands that produce hormones will often store those hormones, or I think they always store them,

which is basically like a little bubble of membrane inside the cell, those vesicles that sort of wait around until the cell receives the right signal, which then triggers them to fuse with the membrane, the cell membrane, releasing them into the institutional fluid, and thereby they diffuse away and then eventually into the bloodstream and it transported to where they need to go. So that's how hormones are stored, released, and have their transport it and then have their effect on their target cells. So one thing about hormones is that unlike paracrine signaling, which is local and therefore fairly well targeted to nearby cells, hormones are not targeted. So once they enter the bloodstream, they're going to be transported across the entire body.

So the hormones will be available locally everywhere in the body. However, only the cells with the right receptors will respond to those hormones. This is kind of like the way that radio signals work, right? So radio waves are very long wavelength, electromagnetic waves, right? So they are emitted by some sort of signaling tower or antenna somewhere and travel across a wide area. So like everyone in the city or maybe a wide region of your country, everyone receives the same radio waves that they're everywhere. The reason that only some people hear that radio station is not because some people have the radio waves present and some don't. It's not targeted that way.

Instead, the reason why some people hear the radio waves is because they have a receiving. which has its own antenna and then which is the resonant frequency of that antenna is modified by changing the dial so that it basically picks up that signal and then amplifies it and plays it as and converts it into a sound. So in other words, it's on the receiver end that the specificity is determined by basically having the radio set to the right station. That's kind of similar to how it works for the endocrine system as well. It's on the receiver end that the system is targeted. So only cells with the right receptors will respond to the hormones. others that will just won't do anything. It'll just, the hormones will be there, but they won't

have any effect because there's nothing to bind to them. And so the way it works is that cells in particular parts of the body express different receptors, either on the surface membrane or like internally for steroid hormones that cross the membrane. And therefore, it's which receptors are present in that cell that determine what hormones it's responsive to. So hormones are not targeted to specific locations. They go everywhere. And then it's the, it's the receptors on the receiving end that determine whether that those, those, particular cells are responsive to that particular hormone. Now, hormones are typically regulated in terms of like how much is released, how much is produced. They're typically regulated by negative

feedback loops. So a negative feedback loop is a very simple idea. It's where doing something causes a result, which then reduces the amount of the initial thing done. The simple example here is if you produce, say there's a certain cell which produces a hormone, that hormone may act on a second type of cell, which produces a second type of hormone, that second hormone has an effect on the first type of cell, reducing the amount of the first hormone that it produced, and therefore reducing the amount of the second hormone that was produced. So the more of the first hormone that's produced, the more of the second hormone that's produced, and hence the less of the first one that's produced. Hopefully that's not too unclear, right? But the basic idea is that

in a negative feedback loop, doing something has an effect that reduces the amount of the first thing that you did. So it's self-limiting. It doesn't get sort of out of control. If you do too much, it will have a negative feedback that balances, like reduces the amount that's done. Or vice versa, as well, if not enough is done, then that will reduce the inhibition and increase the amount of the first thing that's done. Now, I sort of indicated that as if there's two steps, but there can be many steps in a negative feedback process. And in the endocrine system, there are many of these different negative feedback loops, which operate at sort of different scales, different timescales and like different numbers of intervening steps. But in many cases, that's

the amount of hormone is regulated. There's sort of an amount that we want that's going to be varied depending on the circumstances and often that's influenced by things like the nervous system which we'll talk about later. But basically in order to keep that in balance there are these negative feedback loops. So if we have too much of a hormone that's going to send a negative feedback by that hormone itself directly reducing the amount of its like predecessor hormone that was produced and so then less of it will be produced down the stream. We'll go through some examples of this but that's the basic idea that

I wanted to mention at the outset by the um introducing the idea of a negative feedback loop. I also mention that control is also exerted by the nervous system through the hypothalamus, and that provides a good transition point to talk about some of the specific organs or glands within the endocrine system and what their particular roles are. Now, I want to emphasize before we do that, that there are many names and terms here, which are confusing, and certainly I don't remember them all. The important point here, however, is not the names,

of particular hormones. The point is to understand the overall process and some of the major pathways. So I will mention some of the names of hormones when I think it's irrelevant or when they're names that are more likely that you've heard of before. In other cases, I won't necessarily mention the name of the hormone. I'll just sort of say that there is one or, you know, that there's a few steps along the pathway. And that's just to make it a bit easier to follow, rather than trying to throw like literally dozens of complicated names and acronyms at you. as usual in this podcast, we're focused on sort of conceptual understanding and not all of the precise details. Okay, so that being said, let's start with the hypothalamus. So the hypothalamus

is a small region of the brain, which is located below the thalamus. And funnily enough, that's exactly what hypothalamus means. It just means below the thalamus. So the thalamus is kind of in the middle of the brain. It's like below the cortex, but above the brainstem. So it kind of sits like right in the middle, loosely speaking. And the hypothalamus is kind of like below that and in the center. The hypothalamus produces a number of hormones which control other endocrine glands, particularly the pituitary gland. There are a number of different nuclei, which basically means like just clumps of neurons, different nuclei that make up the hypothalamus, and each of these nuclei contain cells that are specialized for the production of particular hormones. So, you know,

there's a nuclei that produces this hormone and one that produces that hormone and so forth. I'm not going to go through the names of those. That's not super important for our purposes, but just understand that there's one brain region in the hypothalamus, and then it has different specialized sub-remoner. regions that produce different hormones each. The hypothalamus is very highly interconnected with other parts of the central nervous system, including the brain stem and the particular formation, which is part of the brain stem. The hypothalamus is also part of a brain circuitry or series of structures called the limbic system, and therefore it has connections to other limbic structures, including the amygdala and the septum, and also is highly connected with other areas of the

autonomic nervous system. So you may not know what all of these brain regions. So you may not know what all of these brain reasons. Don't worry sort of too much. I'm just mentioning these in case you've heard of them. The basic idea there is that the hypothalamus is quite central, not just in terms of location, but in terms of interconnectivity as well. I mean, most brain regions are connected to many other regions, but, you know, the hypothalamus is sort of on the pathway between the lower parts of the brain, which are responsible for more basal metabolic functions like breathing and sleep and so forth, that's part of the brain stem. And then the higher parts of the brain, which are responsible for like control of skeletal muscles, sensory perception, executive function,

decision-making and so forth. And the hypothalamus kind of links those two. It's also connected to the limbic system, which is associated with emotions and decision-making and motivation. So it's quite central. And that's important because the hypothalamus is kind of the link between the nervous system and the endocrine system. And the endocrine system controls many aspects of physiology and behavior. And so it needs to have input from the nervous system. You know, if I see a threat, if I see something that's scary or that's enticing, that is going to have effects not just on my behavior, like through moving my skeletal muscles, which are under control of like the central nervous system, but it's also going to have effect on things like my heart rate and sweating and other responses, which are

partly under the control of the autonomic nervous system, that's the sympathetic parasympathetic nervous system, like the fight and flight versus rest and digest systems that we've talked about in previous episodes, but also through hormones. So these kind of fight and flight slash rest and digest are mediated through both the autonomic nervous system and through hormonal control. And therefore, there needs to be a linkage between the nervous system, which is how we perceive these threats or opportunities or other things in the environment. And there needs to be a connection between that and the hormones that regulate these longer-lasting physiological responses. And so that occurs through the hypothalamus, which links the two together.

The hypothalamus, as I said, contains a variety of nuclei, which have special cells for the production of specific hormones. Some of the cells in the hypothalamus are specialised types of cells, well, I guess extra specialised type of cells called neuroendocrine cells. As the name indicates, neuroendocrine cells kind of bridge the gap between the nervous system and the endocrine system. And they do that by receiving neuronal input through neurotransmitters released by other nerve cells. So they can basically receive input through neurotransmitters, like a neuron can, but then as a consequence of that input, they release messenger molecules, so hormones, into the blood. So the difference here is that whereas a regular neuron receives

input through neurotransmitters and then releases neurotransmitters to propagate that input to other cells in the nervous system, neuroendocrine cells receive input in the form of neurotransmitters, just like a regular neuron, but the output is in the form of hormones instead of, or I think sometimes in addition to neurotransmitters. So that's how they integrate information and they bridge the gap between the nervous system and the endocrine system as they receive input from the nervous system and then output it in a way that the endocrine system can kind of process if you like like that is in the form of hormones this process is called neuroendocrine integration appropriately enough so the cell bodies of these neuroindicine cells are

located in these different nuclei of the hypothalamus and that's where these hormones are synthesized but the hormones are transported down the axons of these cells and are released into the capillary beds surrounding the pituitary gland. So the way you can think about it is that you have, in the middle of the brain is the thalamus, which I mentioned is kind of like the sort of big structure that sits at the top of the brainstem. Just below the thalamus, and kind of in the center, is the hypothalamus, which is where these neuroendocrine cells, cell bodies are located, and where the neuroendocrine integration happens most directly. Then there's kind of this stalk that connects the hypothalamus to the pituitary gland. It's almost like it's, um,

a bunch of grapes growing on a vine or something like that, although I guess there's only sort of two grapes, the anterior and the posterior pituitary, which we'll get to that in a minute. But basically there's this sort of this sort of small lump, which is the hypothalamus, just below the thalamus, and then there are these sort of stalks, this sort of thin neck region that then connects the hypothalamus to the pituitary gland. And this stalk consists largely of these axons of the neuroendocrine cells. So the neuroendocrine bodies are up in the different new. clay eye of the hypothalamus, and their axons connect down and project into different regions

of the pituitary gland. And this forms the bridge between the nervous system and the endocrine system. So that leads us then to talking about the pituitary gland. We'll come back and talk about some of the specific hormones that are produced by the hypothalamus, but we'll talk a bit about the anatomy and function of the pituitary gland first, to give us some context to that. So I've just described a little bit about the sort of location and structure of the pituitary gland. It's a P-sized gland that sits at the base of hypothalamus, connected to it by sort of this small neck, which consists of the predominantly the axons of the neuroendocrine cells. There are two lobes or parts of the pituitary gland. Well, I described this as sort of like a bunch of grapes. Well, there's a bunch

of two grapes, and there's two of them, right? There's two lobes. Anteria and posterior. Basically, it just means like front and back. The functions of the anterior and posterior pituitary gland are pretty much the same. Their function is to receive input from the hypothalamus, and based on that input to synthesize and release various hormones, which then target different target organs or systems. Now, that's done a bit differently in the posterior versus the anterior pituitary gland. We'll come back to that. But before we sort of go through that in detail, let me explain some terminology about different classes of hormones. Now, earlier I introduced you to different types of hormones. defined structurally in terms of their structure and the types of receptors that they interact with.

So these are the steroid versus your peptide versus your polypeptide hormones, right? The steroid hormones are lip solubolibule, the other ones are not. So that's a structural characterization. Here I'm going to give you another way of thinking about some of the different classes or types of hormones, but it's not defined structurally, it's more defined functionally or in terms of like their position in the hierarchy of control. Think of it as if the hypothalamus sits on top, the pituitary gland is kind of in the middle, and then at the bottom are the other glands and also organ systems within the body,

like the thyroid gland, adrenal glands, thymus, reproductive organs, and so forth. And there's a hierarchy of control. The hypothalamus controls the pituitary gland, the pituitary gland controls the other glands, and then those other glands in turn may control other functions in the body. So this leads us to then explaining some of the terminology here that we use to describe this hierarchy of control of hormones within the endocrine system. Releasing and inhibiting hormones are hormones whose main purpose is to control the release of other hormones, either by stimulating or inhibiting their release. Now, releasing hormones and inhibiting hormones are primarily produced by the

hypothalamus. There are some that are also produced by the pituitary gland, because the only, you know, the requirement of being a releasing or inhibiting hormone is just that you primarily affect another hormone, right? So a hormone that affects. affects another cell, which in turn is a cell that produces hormones. So that's how you can have an effect on the production of another hormone, right? So in that sense, there are releasing and inhibiting hormones in both the hypothalamus and the proteratory gland. The way I'm going to use the term is I'm going to sort of reserve that term of releasing and inhibiting hormones to just the hormones from the hypothalamus, just because I think it's a little bit simpler for our purposes. That's not strictly

true, but I think it's good enough for our purposes here just to simplify things a little bit. So releasing hormones and inhibiting hormones, think of those as hormones that regulate other hormones and are released, that are produced and then released by the hypothalamus. So they're at the top of the hierarchy, the releasing and inhibiting ones. Now, a tropic hormone is a hormone that has another endocrine gland as its target. Most tropic hormones are produced and released by the anterior pituitary gland, some also by the posterior pituitary gland. So this is what's confusing, right? Because technically, releasing and inhibiting hormones are also tropic hormones, because releasing and inhibiting hormones have, as their target,

another endocrine gland, namely the pituitary gland. But I'm going to simplify things a little bit and talk about tropic hormones as if they are released only by the pituitary gland, just so we can kind of link up the different levels of the hierarchy with these different names. But technically, I'm massaging the definitions a little bit, but I think it will be helpful for our purposes here. So a tropic hormone is one which has its target an endocrine gland. Non-tropic hormone directly stimulates some target cell to produce an effect, not through a different endocrine gland. So we'll go through some examples in a moment.

So the way you can think about it is that there's a hierarchy of control. It starts at the hypothalamus with the releasing and inhibiting hormones, and these affect the pituitary gland. The pituitary gland, in turn, releases tropic hormones, which have a... as their target, other endocrine glands lower again in the hierarchy. And so these are glands like thyroid, adrenal, thymus, and so forth. At the very lowest level will be the non-tropic hormones, which directly stimulate some target cell to produce an effect.

So non-tropic hormones do not regulate some other hormone. They just regulate some process directly. So that's the overall framework. Let's now go through an example to kind of flesh that out a little bit. And then we'll also, in discussing this, we'll talk about some of the major glands that I've, talked about that are regulated by the pituitary gland. But just remember at the top, hypothalamus, at the middle, pituitary gland. Next layer down are the other major endocrine glands, and at the very

bottom level are like the target systems. So that could be like digestion or perspiration or something like that. So things that are not themselves part of the endocrine system. So let's talk about this now giving an example and talk about the HPA axis or the hypothalamic pituitary adrenal axis. An axis basically in this context just refers to like a line drawn between two things. So we talk about an axis because there's sort of a direct linkage between them and a direct functional and causal relationship between these different endocrine glands. The chain starts with the hypothalamus. Remember, that's at the top of the hierarchy, and it's in turn influenced by the nervous system.

So the hypothalamus releases a releasing hormone called corticotropin releasing hormone. So it's one of these hormones at the top of the hierarchy, which regulates the control of a different hormone. Well, okay, what hormone does it regulate? Korticotropid releasing hormone stimulates the anterior pituitary, so next layer down, and causes it to release tropic hormone called adrenocorticotropic hormone. So it's a tropic hormone because it targets another endocratrobin gland. In this case, well, what gland is that? It's the adrenal glands, as you might gather by the name,

adrenocorticotropic hormone, it affects the adrenal glands. So what are the adrenal glands doing? Okay, they're being controlled by, you know, adrenocorticotropic hormone, but what are the adrenal glands doing? Well, the adrenal glands produce a non-tropic hormone, so one that directly stimulates some target cell to do something, but that's not another innercline. Cortisol is a non-tropic hormone, which is secreted by the adrenal glands, entering the bloodstream, where it has widespread effects on organs and tissues, and it's involved in stress response and other things. We'll get into a bit more what cortisol does in a moment,

but it is involved in regulation of stress responses. The important point that I want to make here is in relating the different levels of control. So at the top level, you've got corticotropin releasing hormone, released by the hypothalamus, that's your releasing hormone. That controls the production and release of the tropic hormone, adrenocorticotropic hormone, by the anterior pituitary. That, in turn, controls the production and release of cortisol by the adrenal glands, which is at the third level of the hierarchy.

And then at the very bottom level is the, basically tissues that are affected by cortisol, and those are quite diffuse, actually. So some hormones have effects on specific tissues, others have effects on many different tissues depending on which cells have the appropriate receptor for that hormone. So this HPA axis here is an example of the layered hierarchical control of many hormones in the endocrine system. And also, hopefully some of the names that I've mentioned give you an indication as to why I'm not going to go through all of the names, because many of them are very long and confusing. But that HPA axis is just one of many of these axes that exist as part of the endocrine

system consisting of essentially a hypothalamic hormone, a pituitary hormone, and then a target hormone, which produces a product which itself may be a hormone, or just maybe some sort of modulatory behavior response. So we talk about the HPA axis. There's also the HPT axis, the hypothalamus, pituitary thyroid thyroid, thyroid axis, so that relates to the thyroid gland. There's the HPG axis, so hypothalamus pituitary and gonads, so these are testes or ovaries. This relates to particularly reproduction and secondary sexual characteristics. There's also the HPP axis, which is hypothalamus,

pituitary, and prolactin, which is a hormone involved in lactation, so that has the effect on the mammary glands. So there's a few of these axes that exist as part of the endocrine system, and then there are other aspects as well that don't quite into this framework here, but it does highlight the importance of understanding the relationship between the hypothalamus and the pituitary gland. Okay, so let's talk about some of the different hormones that are produced, particularly by the posterior and the anterior pituitary gland, and then we'll talk about the major glands that they affect, so these different axes that I talked about. Most of the hormones that are released by the pituitary gland are released by the anterior

pituitary gland. So this is the front part of the pituitary. There are, in addition, a couple that are released by the posterior pituitary. Let's start with the posterior pituitary gland, so the sort of back part of that. The posterior pituitary gland is the part of the pituitary gland that consists largely of these axonal projections from the hypothalamus. So remember I said that there are these neuroendocrine cells in the hypothalamus, which extend across the like little stalk, neck thing that connects the hypothalamus to the pituitary. And then they terminate. in the posterior lobe of the pituitary gland. So it's these sort of bundles of axons,

as well as the blood vessels that surround them, which comprise the majority of the posterior lobe of the pituitary gland. So the hormones released by the posterior pituitary are actually produced in the hypothalamus, just to be confusing, but they are stored and released from the posterior pituitary because of the fact that you have these axons extending from the hypothalamus down to the posterior pituitary.

So I'll talk about them as if they're produced and released in the posterior pituitary, even though technically, you know, the cell bodies are actually located in the hypothalamus, but that's a complication that we don't need to worry about too much. So the two main hormones that are released by the posterior pituitary are antidiarrated hormone and oxytocin. You probably heard of both of these because they're fairly readily discussed. So antidiarratic hormone controls urine production, sweat and blood pressure. More antidiarratic hormone in the bloodstream increases the permeability of the ducts in nephrons. So these are specialized cells in the kidneys, which basically control water retention.

But basically, more ADHD in the bloodstream increases the permeability of duxin nephrons, which leads to a greater re-uptake of water by the kidneys. And hence, more salty urine. So more ADHD basically means more water retention, and hence less dilute urine. So it's involved primarily in regulation of blood pressure and water control. So in this case, there's no releasing hormone, because the hormone itself is technically produced in the hypothalamus. The pituitary hormone is the antidiaryngormone,

and the target cells here are like kidneys and sweat glands and other parts of the circulatory system. So this is an example of part of the endocrine system that doesn't really have this axis structure because there's really just one hormone, antidiarrotic hormone, and then it's target tissues or systems, like the kidneys and sweat glands and so forth. This is different from like the HPA axis,

where you had three different hormones, and then in addition, the targets to that final hormone, because every hormone has a target. So some aspects of the endocrine system are sort of more layered than others, right? And so antidiaryary hormone is an example of one where it's basically, it's just produced by, technically the hypothalamus, but we'll just say, but by the posterior pituitary, and then it's released, and then it affects its targets. So antidiarytic hormone is not a trophic hormone. It's a non-trophic hormone. It has a direct effect on target cells. It does not target other endocrine cells. The other major hormone released by the posterior pituitary gland is oxytocin. It is also a non-tropic hormone because it affects the female reproductive system directly.

It doesn't, to my knowledge, affect any other hormones, although perhaps it does in some cases, but primarily it affects the female reproductive system, stimulating contraction of the uterus during childbirth, and also of the mammary glands during breastfeeding in response to infant sucklings. So oxytocin is produced by the posterior pituitary gland, and then it's, it's released into the bloodstream and has its effect on the target organs of the female reproductive system. So it's also a nontropic hormone. So those are the two main hormones of the posterior pituitary. Let's now talk about the anterior pituitary. And this is where it gets more complicated because most of the hormones in the anterior pituitary have a much more complicated

regulatory structure. So as far as I know, all of the hormones released by the anterior pituitary, are regulated by releasing, and in addition, sometimes also inhibiting hormones released by the hypothalamus. So an example that we mentioned before would be the corticotropin releasing factor, which is produced by the hypothalamus and has an effect on certain cells in the anterior pituitary. So for all of the hormones that I'm going to discuss, there is a corresponding in the anterior pituitary, there is a corresponding releasing factor produced by the hypothalamus. And some also have, in addition, an inhibiting factor. but I won't go into details about that.

The anterior pituitary is quite different structurally to the posterior pituitary. The posterior pituitary is essentially just a bundle of axons and a conduit from the hypothalamus. But the anterior pituitary has a glandular structure, so there's a bunch of glands that produce and store different types of hormones locally in the anterior pituitary. And it's surrounded by blood vessels, so when the hormones are released into the cysticial fluid, they diffuse into the capillary bed and then then it's out into the pituitary bed, then it's out into the bloodstream and are carried around the body. Cells in the anterior pituitary gland are innovated, so they receive input from the hypothalamus, but they're not just made up of axons

from the hypothalamus in the same way that the posterior pituitary is. So it is sort of more structurally distinct. So let's talk about some of the major hormones that are synthesized in the anterior pituitary. Remember, the synthesis and release of all of these is regulated by releasing hormones from the hypothalamus, but I'm not going to go through the names of those because they're often related to the names of the pituitary hormone. So we've already seen one of them that I mentioned before. That was adrenocortico-tropic hormone, which targets the adrenal glands. There's also prolactin, which promotes milk production by the mammary glands. So that's different from oxytocin, which is involved in the letdown reflex, releasing milk as opposed to producing it.

Thyroid stimulating hormone, which stimulates the synthesis and secretion of thyroid hormones by the thyroid gland, so that's an example of a tropic hormone because it has as its target another endocrine gland, follicle stimulating hormone, which stimulates a development of eggs in females and sperm production in males, lutanizing hormone, which stimulates ovulation in females and testosterone production in males, and growth hormone, which acts on many different organs to stimulate and promote tissue growth and protein synthesis. So you can see there's a wide range of functions that are mediated by hormones in the anterior pituitary. Some of them are tropic hormones, so that they have, that they act on other endocrine

glands. Examples there would be thyroid-stimulating hormone and adrenocorticotropic hormone. Others are non-tropic hormones, so they do not have an effect on other endocrine glands. They have an effect on some other part of the body. So growth hormone would be an example of that in addition to prolactin. So hopefully, even if you don't quite remember all of those names, you get a sense of the relationship between the different components here, that there's regulation at the level of the hypothalamus, which controls what the, or influences what the anterior pituitary gland does,

and the anterior pituitary gland in turn releases a number of hormones that affect other endocrine glands throughout the body, and also has direct effects on certain systems, like the growth hormones and prolactin and so forth. Now, to finish out the episode, I wanted to talk a little bit more about some of the other endocrine glands that are kind of at the receiving end of input from the anterior pituitary and discuss a little bit more about how they work on sort of what they're what they're doing. Let's start with the thyroid gland, which we've already talked about before. That's involved in the HPT axis, involving the hypothalamus pituitary and the thyroid glands. So the thyroid gland is located in the neck, kind of in the lower neck,

and consists of two lobes on either side of the body. It produces iodine-containing thyroid hormones. There's a few different hormones, T3 and T4 or tyroxene, but I'm not going to worry too much about their names. I'll just call them thyroid hormones. These hormones, in turn, stimulate metabolism and are important for development and maturation in vertebrates. There is a negative feedback mechanism that controls the production of thyroid hormones. So low levels of thyroid hormones stimulate the hypothalamus to secrete the corresponding releasing hormone. The releasing hormone in turn stimulates the anterior pituitary to secrete.

to secrete the tropic hormone, thyroid-stimulating hormone, which in turn stimulates the thyroid gland to produce more thyroid hormones, which in turn then has effects on other parts of the body. So you've got three laser regulation there, and then the final target, which are just tissues throughout the body. You may have heard or seen of a goiter. A goiter is a swelling of the thyroid gland around the neck, and so it looks like someone has some kind of growth or expansion around their lower neck.

It can be caused either by hyperthyroid. which means too much production of thyroid hormones, or underproduction hypothyroidism of thyroid hormones. And this is a condition that can fairly readily be treated these days by supplementation with iodide, like iodine containing salt and salt. Thyroid hormones increase the basal metabolic rate, and so they have an effect on pretty much all tissues throughout the body. So they affect appetite, absorption of nutrients,

and also motility, so movement throughout the gut. So they're all influenced by thyroid hormones. In addition to the main thyroid gland, there are also parathyroid glands. So these are much smaller glands that are small around and kind of sit on the back surface of the thyroid gland. The parathyroid hormone produced by the parathyroid glands is the main regulator of the amount of calcium in the blood. So it's secreted when calcium levels drop, which causes bone cells to digest more bone

tissue and that releases calcium back into the bloodstream. That's important for homeostasis, maintaining the concentration of calcium in the the bloodstream. So we talked about the thyroid and parathyroid. Let's now talk about another one of the key plays, which is the adrenal glands. So this is the same glands that we discussed before in the HPA axis. The adrenal glands are two glands, one on either side of the body, that sit just above the kidneys. They kind of sit on top, almost like a hat that each kidney has. Each gland has an outer region called the adrenal cortex, and an inner region called the adrenal medulla. and they're functionally distinct, so they do different things.

The adrenal cortex produces the steroid hormones, whereas the inner medulla produces a different set of hormones. Here we'll focus on the adrenal cortex. So there are three main steroid hormones that are produced by the adrenal cortex. Aldosterone, cortisol, and epinephrine, neuropronephrine. Aldosterone promotes the secretion of protons, so hydrogen ions, into the bloodstream and therefore regulates basically the pH of the blood and the concentrations of other ions like sodium and potassium. So it's important in homeostasis. Cortisol, which I mentioned

before, is one of the hormones that's regulated by the PTA axis, and it acts in opposition to insulin. So it inhibits glucose uptake. So insulin you may know is the hormone that promotes glucose uptake by cells. Cortisol inhibits glucose uptake. And it's also involved in, as I mentioned, in the stress response. Now I'll talk more about cortisol in hopefully the next episode when I talk about emotions. Epinephrine and neuropronephrine are two closely related hormones which produce effects that enhance those of the sympathetic nervous system. So sympathetic nervous system is involved in the fight and flight response and epinephrine and neuropronephrine are sort of like the pairing within the endocrine system. So they're the hormones that also promote similar

fight and flight responses, but in a sort of more prolonged way. So having an effect over a longer period of time than the nervous system, which is sort of very fast acting, but also fast acting, but doesn't last as long, whereas endocrine system is more slow acting, but longer lasting. There are other hormones that are produced in the adrenal glands as well, but for now, I'll move on to talk about the reproductive organs. So the reproductive organs are a very important component of the endocrine system, well, I suppose they're endocrine and reproductive systems, and these are the sites of production

of probably the most well-known hormones in humans, so testosterone and estrogens. So we will talk much more about the effects of these hormones when I get to do reproduction, like the reproductive systems and the development of secondary sexual characteristics and so forth. So I'm not really going to go into too much of the detail here because it gets quite involved. But I'll just give an overview of some of the major hormones here. So testosterone are produced by the male testes and in smaller amounts by the ovaries. So a common misconception is that only men produce testosterone and only women produced estrogens. In fact, both men and women produce testosterone and estrogens. There are different

types of estrogens. But in men, the testosterone concentration is much higher and women have higher amounts of estrogens. But it's not like it's one or the other. So testosterone is the hormone that is responsible for stimulating development of male secondary sexual characteristics. So this includes things like more body hair, a deeper voice, broader chest, things like that. It also stimulates sperm production, obviously is important for male sexual function. Estrogens are a class of hormones that are produced in the ovary. They promote the development of female secondary sexual characteristics and also have other important effects on inhibiting the production of other hormones.

But I'm not going to get into the details of that here. Again, we'll cover that in another episode. There are some other hormones that are also produced in the reproductive organs, including progesterone, inhibin, and relaxin, and these have effects on different aspects of reproduction, particularly many of these are related to the development of the placenta and the growing fetus in women, obviously. But again, we're not going to get into all the details here, because that's better situated in a discussion of reproduction and pregnancy, which we'll do in a different episode. To conclude today's discussion, I'll talk briefly about two more major endocrine glands,

so the thymus and the pancreas, or pancreatic islet. So first the thymus. The thymus is a fairly small gland, which is kind of located in like the centerish of the chest, and it is responsible for the development and differentiation of T lymphocyte. So basically it's involved in the production of T cells involved in the acquired immune response. And I talked about these when we discussed the immune system. So refer to that for a bit more discussion there. And finally, the pancreatic is So these are regions of the pancreas that contain cells that produce hormones. The hormones produced by the pancreatic islets are secreted directly into the blood flow, which makes it a bit different from other hormones,

which are typically released into the interstitial fluid and thence enter the bloodstream. There are two main types of cells that I want to discuss here. There are actually at least five types of cells, I think possibly more in the pancreatic islets, rather. But the two main types that I want to focus on are the alpha cells and the beta cells. Alpha cells secrete a peptide hormone called glucogon, whereas beta cells secrete a hormone called insulin. So I mentioned insulin before. Insulin is a hormone that promotes the uptake of glucose by cells. Glucagon, by contrast, has exactly the opposite effect.

It increases the concentration of glucose in the bloodstream by breaking down glycogen in the liver, and thereby releasing the constituent parts of glycogen, which are individual glucose molecules, into the bloodstream. They're by increasing the concentration of glucose. The relative amount of glucose versus insulin in the bloodstream is regulated by glucose levels. So high blood glucose levels stimulate the release of insulin, which increases cellular uptake of glucose, thereby promoting the formation and storage of glycogen in the liver. So that's sort of storage of excess energy effectively. Conversely, low blood glucose levels trigger glucagon release,

which increases blood glucose by stimulating the conversion of glycogen to glucose in the liver, and also increasing the breakdown of fats and proteins. So basically, both of these hormones are regulated by blood glucose levels, and they sort of stimulate opposite responses. If there's a lot of glucose, then insulin's going to remove that from the blood and encourage cells to take it up and use it for energy. If there's not very much glucose in the blood, glucagon is going to increase the amount of glucose available in the blood

so that cells have it as an energy source by converting a glycogen in the liver into glucose, which can then be released in the bloodstream. Now, this will be discussed in more detail when we go through the digestive system and the biochemistry and nutrition and so forth, where we'll discuss these processes in more detail. I'll just briefly mention type 1 and type 2 diabetes, which I'm sure you've heard of. This is a type 1 diabetes is an autoimmune disorder resulting from lack of insulin due to the destruction or most large or complete destruction of beta cells, so you can't produce insulin. Type 2 diabetes has a similar effect, except it's not three.

the absence of insulin. Instead, it's the lack of responsiveness or loss of responsiveness of the target cells to insulin. So it's also called insulin resistance, and it's sort of, for that reason, much more difficult to treat. But both of these type 1 and type 2 diabetes are effectively endocrine disorders because they affect the endocrine system and the effects of the hormone insulin. So before we finish out, let's just briefly summarize what we've talked about in this episode. We talked about hormones, what they are, what their effects are, and how they transmit signals across different parts of the body through being released by the cells that they're produced at into the interstitial fluid. They then travel some distance in the bloodstream

where they're released and have an effect on their target cells depending on whether those cells have the appropriate receptor for the particular shape and structure of that hormone. We talked about the difference between steroid and non-steroid hormones. And then we talked about the control of the endocrine system through the different levels of the hierarchy of control. the hypothalamus at the top, which connects it with the nervous system, then the pituitary gland, including the posterior and anterior regions of the pituitary gland. Below that are the target glands, like the thyroid adrenal and thymus and other glands. And then at the very bottom level are the sort of target tissues or organs, like the reproductive system

or just general tissues in the body in the case of growth hormones. So whatever it is that the tissues that are being targeted and affected at the bottom end by those hormones. And that relates to the different axes that we talked about, which refers to the direct sort of linear chain of control of different components of the endocrine system. So the HPT axis, controlling the thyroid glands, the HPA axis, the adrenal glands, the HPG axis, controlling the gonads and so forth. And so the major glands in the endocrine system, in addition to the control centers in the hypothalamus and the pituitary gland, would probably be the thyroid and parathyroid glands. are located sort of around the neck, then the adrenal glands located on the top of the kidneys, and then the various components of the reproductive system, so testes, uterus, and ovaries in females. There are other glands as well, but those are probably like the biggest ones and like the most

significant ones. So that will finish out this episode, I think. Hopefully you found that interesting. If you enjoyed the show, please consider leaving a favorable review on iTunes, Spotify, or the preferred aggregator of your choice. You can also support the show financially. you can make a one-off donation via PayPal, or you can become a Patreon supporter and donate some number of dollars every time I release an episode. I really appreciate all those who are

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The audio podcast isn't going anywhere. But I want to make some versions of some of the episodes, which have still images also added to them, to accompany the audio. I've already got a few people who've expressed willingness to help with that, and already some episodes in the pipeline. So that's really exciting. If you would be interested in helping, basically this would involve you finding images to go along with a certain episode

and then adding them in and editing them together and a little bit of other work relating to that. I am offering to pay a modest, but still not trivial amount for each episode that you do. Feel free to get in touch. Again, my email address is Fodzwar atgm.com. So I've already got a few people helping out there, which is excellent. I'm happy to get more people, though,

because there's quite a large backlog of episodes, and so there'll be more than enough to go around in terms of working on. I don't anticipate releasing any of those episodes onto YouTube until I've got at least half a dozen or so, so I can kind of put a few out at once, instead of just like one or two. Once the channel gets up and running, I will let you all know and invite you to go and check that out

and give some love to those videos there. But yes, I just thought I'd give you a bit of an update about how that project is going, so it is coming along slowly but surely. And thanks again for everyone who's expressed interest or willingness us to help out with that. I appreciate that. And I'm hopeful that this initiative will be a way of bringing the podcast to a new audience and we'll get this up and running over the course of 2023. All right. So thanks once again. Take care and I'll talk to you next time.