 Smell is another chemical stimulus, and so it's picked up by chemoreceptors found in your nasal epithelium. The olfactory receptor molecules' cells are found embedded in the olfactory epithelium that covers the superior portion of your nasal cavity. So they're actually covering the superior nasal concha. This is a nice view to give you perspective of where we are, and now I'm going to draw you a picture because I know that shocks you. I never draw pictures. And I'm going to draw it upside down so that you know that this is my nasal cavity down here. Do not be confused. I put my mouth at the top, but now I'm putting my nasal cavity at the bottom, and I'm just doing that because, you know, it makes it a little bit easier to visualize. Just like with the tongue and taste, we have cells, and these are specialized neurons. And in fact, this cell, I'm going to cut it off there, but it has an axon. It is a neural cell, but it's like a modified neuron. And it actually originated in the central nervous system. So this is kind of a central nervous system born cell that's reaching outside of the central nervous system. If you call your nasal cavity outside of your central nervous system, it gets a little messy. Remember how your eyes are actually extensions of brain tissue? So your eyes kind of are a piece of central nervous system. Kind of the same thing with these cells embedded in the olfactory epithelium. I'm just going to throw a nucleus in there so you don't forget. It looks like a little Pac-Man ghost with an anglerfish lure on top of its head. Guess what? We have on our olfactory epithelium. But of course, we must have sensory receptors. And these sensory receptors are going to bind to smell molecules that have entered your olfactory epithelium. And just for the heck of it, so you don't forget that you are a mucus-y pile of mucus. There's so much crazy mucus in here. You even have specialized cells. They're not goblet cells, but they're special mucus barfers that are in this olfactory epithelium and their only job is to make this beautiful mucus layer. So if the molecule binds with the receptor, I'm going to tell you the mechanism of action. First, molecule binds with receptor. Second, what happens? Second, messenger cascade happens, which is going to amplify our ability to pick up very small amounts of odor stimulus. And third, I have to write this down for you because it's going to be harder to draw this. Our third event in this series is that we're going to end up with sodium channels opening. And guess what happens if sodium channels open? Of course, sodium channels are going to open. Na, na, na, na, na, na. Sodium is going to rush in, and what does that stimulate? All it's going to do is stimulate an action potential that's going to travel down the axon of this modified neuron, and it's going to barf out a response. Here's my axon terminal. There's also some neurotransmitter out there, and it's going to barf out to, who do you think this is? Cranial nerve number one, that's my olfactory nerve. Of course. So you can see that we actually have a little sequence here, but this is how we get the information to the olfactory nerve, and then ultimately to the brain for processing. There are, just like we had five primary sensations of taste, guess how many smell sensations we have. In your mind, 100, 100 possible sensations of smell, and there is smell blindness, just like there is taste blindness with PTC paper. I'm sure you all did this in general bio. You tasted, you ate the paper, and either you were like, oh my God, and wanting to barf, or you were like, pfft, it tastes like paper. And if it tastes like paper, like it does for me, that means you're taste blind. You can be smell blind too, and color blind. I'm smell blind to formaldehyde. What? That's like the worst thing you could possibly be smell blind to, because if you can't smell formaldehyde and you happen to be in a room with lots of it, that's not a very good plan. All right, that's it, you guys. Let's talk about hearing next. Let's give them another mechanical sensation.