Hi, this is Dr. Claire, and this is our lecture on the endocrine system. You're probably familiar with the endocrine system. The endocrine system is a system of hormones that control cellular processes in our body. And most people are familiar with hormones, at least in a popular culture kind of way.
You can talk about people being hormonal or whatnot, and it's something that we frequently discuss in popular culture. But actually, the endocrine system is one of several... ways in which cells can communicate with each other chemically so cells can communicate chemical chemically if they have direct contact with each other also synaptic signaling is a type of chemical communication as well as a pheromone signaling and then endocrine which is hormonal signaling and also paracrine signaling so we're going to take a look at some of these in a little more detail direct contact just an example of direct contact is if two cells are actually touching each other they may have passages that allow for compounds to pass back and forth between those cells and so cells that have direct contact can communicate directly through those passages. So for example, gap junctions in animals allow for chemicals to pass back and forth between cells and allow cells that are adjacent to each other to coordinate information. Synaptic signaling, we talked about that when we were talking about the nervous system.
That is when nerve cells have a synapse or a connection with another cell. It can be another nerve, it can be a muscle cell, it can be a number of different things. And the neuron actually communicates chemically with the next cell over by releasing neurotransmitters, which then stimulate a response in the adjacent cell. So again, it's a cell that is close to another cell.
Pheromones are actually pretty interesting because pheromones are chemicals that are released by one individual that cause a response in a different individual. So many animals use pheromones to communicate between animals. So for this example here you have a male goat who is releasing a pheromone and that's triggering a physiological response in the female goat so causing her to become reproductively susceptible. and a lot of insects also use pheromones to attract mates and to communicate with other individuals so pheromones are another chemical communication signal endocrine signaling and and paracrine signaling are very similar to each other they both involve releasing chemicals from a particular cell the difference between endocrine and paracrine is that paracrine only affects cells that are near the cell that's secreting the chemical so they're they're affecting cells in a close distance So that's paracrine signaling.
Endocrine signaling, the chemical is actually released into the bloodstream and it travels throughout the body and it is picked up by different cells in various parts of the body and can trigger responses in those cells. Now you can think of one of these chemicals which we call hormones in the endocrine system. A hormone being released by one cell, let's say, let's say corticosterone, which is a stress hormone.
Let's say that that one is traveling in the blood. Not every tissue in your body may respond to that hormone, but the tissues that do respond, what they have is they have a receptor for that hormone. So if it's a type of tissue that needs to respond to corticosterone. then it's going to have a corticosterone receptor on the surface of the cell.
Okay? All right, so let's take a closer look at the different types of hormones, different classes of hormones. You can classify hormones broadly as either being lipophilic or hydrophilic. So lipo means fat, and philic means loving.
So it's a fat-loving or a fat-soluble hormone. Hydro means water, and again, philic is loving, so it's a water-soluble hormone. Okay?
So... Basically it's talking about the polarity of those molecules so remember Something like water molecule is polar it has positively charged areas and negatively charged areas Generally things that dissolve well in water are also polar okay, whereas things that don't dissolve oil and water which dissolve in fats Are nonpolar okay now within those larger classifications In the hydrophilic hormones, you have hormones that are large proteins, peptides and polypeptides. So these are very large, complex molecules made up of a string of amino acids.
There are also some hormones that are smaller but still hydrophobic that are derivatives of amino acids, so the things that make up larger proteins. So there are some of those as well. An example of the peptides and polypeptides are things like oxytocin which is a pleasure producing hormone and also prolactin which is a A hormone that encourages parental care. In the amino acid derived hormones, things like epinephrine. Epinephrine is a stress related hormone.
So those are the types of things that are hydrophilic. And the hydrophobic side or the lipophilic side, you have the steroid hormones. And you think steroids and you think about some big sports star. taking a bunch of drugs and making their muscles really big. Not all steroids have that effect.
There's a number of different steroids, steroid hormones. The term steroid actually has to do with the structure of the molecule. There's a set of interconnected rings of carbon, and that's what makes it a steroid. So it's not actually how we view steroid in popular culture that determines whether it's a steroid or not. Those steroid hormones are fat soluble.
They include things like testosterone, estrogen, cortisol which is again a stress hormone there's a number of different steroid hormones now these different types of hormones actually behave differently in the body a lipophilic hormone because it dissolves well in fat it doesn't dissolve well in water what's our blood mostly made up of water right so it's not going to dissolve very well in the watery part of your blood so it can't be transported by itself through the bloodstream so usually the lipophilic hormones the steroid hormones are actually attached to a protein that will allow them to transport through the blood and then when they get near the cells they can disassociate from that poor protein and because they are lipophilic because they like fats that the cell membranes of each of our cells are actually made up of a lipid bilayer so they can dissolve really easily in that membrane and they'll just pass right through so if the cell has a receptor for a lipophilic hormone that receptor will actually be on the inside of the membrane and the hormone will pass through the membrane to get to the receptor Once it binds to that receptor that receptor then will become activated and it'll trigger some response in the cell whatever that response is, okay The hydrophilic hormones on the other side, they can dissolve just fine in the blood because they're hydrophilic and our blood is mostly made out of water. So they dissolve just fine in the blood, they transfer around. But when they get to the cell membrane, that double lipid bilayer that's fatty, they can't dissolve into the membrane. So they can't get through the membrane to affect the cell. So because of that, the receptor is actually embedded in the membrane of the cell with a little bit that sticks out on the outside.
and the hormone will bind to that little bit that's stuck on the outside of the cell. Okay, so rather than going into the cell like a lipophilic hormone, the receptor has to be on the outside of the cell. All right, so most of these hormones are actually working through negative feedback. You're probably, hopefully, familiar with the negative feedback from when we talked about homeostasis. So you have some sort of stimulus that causes a hormone response.
whatever that may be. So let's say you're under some sort of stress, your body picks that up, it sends a signal to the effector, that effector is going to release a stress hormone and then usually that stress hormone feeds back and actually turns off the sensor that would continue to send the hormone. So hormones are usually maintained at a relatively balanced level. So when you increase a bunch of hormone, the presence of that hormone actually will turn off the signal to increase more and that way you don't get things that are kind of Out-of-control levels all right because they most of these hormones need to be in balance for things to work Well a few of these hormones have a positive feedback cycle instead So one example of this would be oxytocin during labor So oxytocin is the hormone that is used to stimulate the contractions within the uterine wall in order to birth the baby and so when you are Going into labor you have a stimulus to produce oxytocin and then the presence of oxytocin in the body actually has a positive feedback where the presence of oxytocin then Creates the production of even more oxytocin and that's what allows that Level to build up to the point where you're actually able to deliver the baby So this is a positive feedback cycle because the presence of the response Increases the response so you have more response. So you actually It amplifies it and dials it up more rather than having a negative response where it dials it back.
Okay? All right. That's the lecture on hormones, and we'll catch you guys later.