 The first thing we should definitely do before moving any further is define a hormone. Hormones. Hormones define the endocrine system. So an endocrine organ is something that produces a hormone. So we should definitely have a sense of, dude, what is this hormone you speak of? So we're going to play a little Pictionary because you know that's how I roll. What did I just draw? Oh, it's a little bit... Pictionary skills are phenomenal, but perhaps not perfect. A hormone. I didn't draw a hormone yet. The hormone is a chemical dumped into the... Now you know what I just drew. Blood. That's a blood vessel. This end looks super wonky. Like, I don't even want to pretend that I'm trying to figure out what it looks like, but the hormone is any chemical dumped into the blood that acts on a distant target. Compare this, my friends, to a paracrine or an autocrine chemical, and those are chemicals produced by cells that are dumped out into the universe, but they're not dumped into the blood and they don't act on a distant target. They're dumped into the interstitial fluid and they act autocrines on themselves or paracrines on their neighbors. So a hormone has to be dumped into the blood. And hormones? Okay, I just spent the entire last section telling you that hormones were really general and that their action was really general, but now I'm going to tell you something a little bit different because I want you to appreciate the fact that, yeah, they're general. It's not like the neuron, which... Okay, I can't help it. I just have to draw you a picture here. Here's my neuron. Here's the axon. And here's the axon terminal. This is where the neurotransmitter in a neuron is dumped into a synapse and it affects or causes action at a single cell. And that single cell is part of an effector organ. Do you see how specific this is? If I were to draw another cell over here, I'm sorry, that little axon terminal is not touching this cell. It doesn't matter that this cell totally has a receptor. It's not going to be affected. This is very specific. Don't get confused. This isn't like anybody will be affected by the hormone. A distant target that might be affected must have the receptor. So if this is my target, this is only a target if it has the proper receptor embedded in its cell membrane. Its neighbor, who is lacking the receptor, is not going to be a target. So in that sense, the hormone, the endocrine system can be very, very specific. And only folks who are expressing the genes that produce these specialized receptors will be able to make something happen. So the hormone is going to bind to the receptor and then signal transmission is going to happen. We're going to have some kind of message received by the cell. The cell is going to ultimately initiate some sort of action. And keeping in mind that these actions might take us a while to make it happen, but eventually we will definitely get some sort of action happening. The thing that happens, how are we going to get action from here? We get binding and then what happens? Well, that depends on the kind of hormone that we're dealing with. And there are actually three, that would be three. So there are three kinds of hormones that we're going to look at, three types of hormones that we're going to look at. And all the hormones that we're going to talk about in this class will fit into one of those categories. And so we can basically learn the general categories, okay, this kind of hormone acts like this, this kind of hormone acts like this. And then once we know, oh, well, progesterone is this kind of hormone, then we kind of know the action that that hormone is going to take. Now, with steroid hormones, coming at you.