 So there's a set of hormones that are derived from amino acids, and these are their own category. And actually, half of them have a mechanism that's similar to the steroid hormones, and half of them have a mechanism that's similar to the peptide hormones that we're going to talk about next. So they don't have their own independent mechanisms of action. So we're not going to talk about those. But I do want to talk about which hormones fall into this category. We have two primary amino acids that are the precursors that give rise to some specific hormones. One of them is tryptophan. Tryptophan is the one that erroneously we've all been told makes us fall asleep at Thanksgiving after we eat turkey. Apparently, this is not a true story. But tryptophan, if it is modified, it can be used to produce melatonin, which is a hormone. Now, serotonin is also produced from tryptophan, but serotonin is a neurotransmitter. Now, I'm going to tell you the next little piece, and that piece is going to show you that, you know what? Sometimes the edges around the definitions of neurotransmitter versus hormone, sometimes the edges are like super blurry. And, in fact, like really blurry right now. They're about to get really blurry. Tryptophan gives rise to melatonin. Tyrosine is an amino acid that gives rise to two groups of amino acid derivative hormones, and they have two different mechanisms. So one of the groups are... the whole group is called the catecholamines. And the catecholamines, hopefully you look at these guys and you're like, wait a minute, I've heard of those guys. Dopamine is one of them. Now, if you know anything about these chemicals, you probably are going, wait a minute, dopamine. Dopamine is a neurotransmitter. And they actually are referring to these guys as neurohormones because norepinephrine and epinephrine are also in this category. Both of these, all of these, can act like hormones because they can be dumped into the blood. Actually, I don't know if that's true about dopamine, but it is true about norepinephrine and epinephrine. If they're dumped into the blood, they act like hormones, but they can also act like neurotransmitters. And if they're transmitted across the synapse to a very specific target, then they're considered neurotransmitters. So technically the catecholamines are... I can do this. Technically, they are neurohormones. All right, their mechanism of action is not the same as a steroid hormone. And you could actually think about that, and that would be a phenomenal essay question on an exam. Why? If you had to guess. If you had to argue and defend yourself, why wouldn't these guys have the mechanism of steroid hormones? Probably the first thing you should think of is, dude, steroid hormones are slow. And epinephrine and norepinephrine, remember, these guys are involved in a sympathetic nervous response. This is basically adrenaline. And if their action was slow, you, my friend, would be eaten by the bear. That would be tragic. So that tells you right away that the mechanism of the peptide hormones that we're going to talk about next, is faster than the mechanism of the steroid hormones. However, all of our thyroid hormones are also built from tyrosine, and all of them have the mechanism of the steroid hormones. So a totally same precursor, but a totally different mechanism. Probably a good idea to remember both of those different categories. Now, let's look at our peptide hormones and learn about the mechanism of action for those guys and some characteristics that they have, and then consider again, why is it beneficial to have our adrenaline-type substances act in that manner?