 So now we're going to look at different ways that our actual receptors and signaling molecules can basically fine-tune the response so that we can be a little more nuanced. It's not just like if I send out this receptor, if I send out this chemical and you have this receptor, then this is the reaction that's going to happen. And the example that I have for you is using epinephrine as my signaling molecule, epinephrine. Epinephrine is dumped into the blood. It's basically adrenaline, and it's dumped into the blood during a sympathetic nervous response. You can imagine that... I'm going to give you a little example here. Epinephrine binds with receptors in blood vessels, and it causes either vasoconstriction, so the effect is vasoconstriction or vasodilation in blood vessels. Now, think about that for a second. It's not like I can say, oh, every time epinephrine binds with a receptor, vasoconstriction is going to happen. Every time vasodilation is going to happen. But how can one molecule, one chemical, cause multiple responses? So it has to do with specific receptors that can bind with the same signal. So epinephrine can come in, and it can bind with this guy, which is going to be an alpha receptor, or the epinephrine can bind with this guy, which is going to be a beta receptor. Two different flavors. You can see that they look... I gave them different colors, but you can see that they both will fit epinephrine. And then we can think about, okay, here I'm going to cause vasoconstriction. Now I have given the message of vasoconstriction, think about a place where, if you're having a sympathetic nervous response, you might want to have vasoconstriction. You might want to deliver less blood. How about your guts? How about your intestines? If you're running away from the bear, please don't spend time digesting the food in your stomach. Like, that's a waste of time. Vasoconstrict the gut so that there's not as much blood going there. Where would you rather have your blood going if you are running away from the bear? Let's vasodilate in the skeletal muscles. I want to vasodilate in skeletal muscles, but vasoconstrict in the guts, and I want one molecule to stimulate all of that. I'm going to use different receptors. I'm going to have a unique receptor specificity to allow different responses to the same stimulus. That's fantastic. All right, there's another way that we can fine tune our response.