In this video, we're going to touch really briefly on both taste and smell I'm putting these two special senses together because I'm not going to go into a huge amount of detail about either one. I'm also putting them together because they both rely on chemoreceptors. So vision relies on photoreceptors that detect light. Hearing and equilibrium both rely on mechanoreceptors, sensory receptors that are physically opened up. They're activated by physical deflection of a cell membrane. Taste and smell rely on chemoreceptors, sensory receptors that respond to specific molecules that bind that receptor and activate the senses. We're also putting them together because taste and smell are related in our experience of the world. If you've ever noticed that things kind of taste a bit funny or taste a bit off if you have a cold or if you have a sinus infection. Smell is a huge component of taste, actually. If our ability to smell our food is impaired, it just doesn't taste right. First, we'll talk about smell. With smell, of course, we pull air in into our nose. The molecules in the air dissolve in the fluid that covers our nasal mucosa, and then those dissolved molecules move across the olfactory epithelium as we draw air in. The movement moves the mucus across our olfactory epithelium. And in our olfactory epithelium we have numerous nerve endings attached to olfactory receptor cells. Each receptor cell carries a single type of receptor, and there are at least 1,000 unique receptor proteins in the nose. Each receptor protein responds to one molecule or a small family of molecules. And each receptor cell carries one type of receptor. It's been estimated that we have about five million olfactory receptors in our nose. And if you think that sounds impressive, it's really nothing compared to many other types of animals, animals that rely a lot more on their smell. A dog, for example. It's estimated that dogs have more than 220 million olfactory receptors. That's why dogs are much more sensitive to smell, and also why dogs can have much finer, much better, smell discrimination than we do, like being able to smell cancer on someone's breath. Now the recognition and the identification of odors comes from activation of combinations of receptors. So it's really rare that when you're smelling something, you're going to be smelling one single molecule, and have one single type of receptor being activated. Usually it's a combination, and we learn as we grow up and we experience new smells, we learn to identify and attach names to combinations of receptors. So we learn the fresh-baked-cookie smell. We learn the garlic smell or the onion smell. This combination of activated receptors is relayed neurologically through the olfactory tract, the olfactory nerve, to the brain. Some people develop what's called anosmia, a lack of smell. Usually happens due to damage to the olfactory nerve or the olfactory epithelium. Sometimes you can get damage to the olfactory epithelium through chemical exposure or potentially burns. The olfactory epithelium can regenerate itself though, so anosmia due to damage to the epithelium is usually pretty temporary. If you have traumatic damage to the olfactory nerve however, it doesn't really regenerate itself. So blows to the face can sometimes sever the olfactory nerve. And anosmia can be problematic because, as I mentioned, smell and taste are closely associated. So people that have anosmia often lose their appetite because everything tastes weird. It just tastes really bland. So let's move on to taste. Your sense of taste relies on molecules from food being dissolved in saliva and then binding to receptors on taste cells. So people that have dry mouth, either because of medication they might be on or some kind of issue, people with dry mouth oftentimes also experience a decrease in appetite. Things don't taste right and may not taste as good because they don't have as much saliva to dissolve the molecules. Both the olfactory receptors and the taste receptors, they need to detect molecules in solution. And with smell, its molecules are dissolved in the mucus of your nasal epithelium. With taste, molecules are dissolved in saliva. So these taste cells, or gustatory cells-- gustation is the more technical term for taste. So the gustatory cells have microvilli on their surface referred to as gustatory hairs. And these microvilli serve a purpose similar to their purpose in the small intestine. They increase the surface area. More microvilli means more surface area for taste receptors to detect molecules from your food. These taste cells with their gustatory hairs are collected into taste buds. And taste buds are located primarily in the bumps, the papillae, on the tongue. So here we're looking at the taste cells. You can see they have the nerve fibers, they have the nerve endings attached to them so that when the gustatory cells are stimulated by molecules in your food, they can relay that stimulation onto the nerve endings. And then there's a little taste pore sitting above the taste bud allowing dissolved molecules to enter in. There are five basic tastes that have been identified, a sweet taste, sour taste, salty taste, bitter, and something called umami. Salty taste is detection of sodium ions in your food. Sour taste is a detection of hydrogen ions, so foods that are acidic stimulate that sour taste. The sweet taste comes from activation of receptors that bind sugars. So foods that contain glucose or fructose stimulate the sweet taste. Artificial sweeteners like Splenda or aspartame, they work as sweeteners because these molecules also stimulate our sweet receptors even though our sweet receptors evolved to recognize natural sugars like glucose and fructose. There are a variety of molecules that trigger our bitter receptors. Many of these molecules accumulate in-- many of these molecules you'll find in fruits or vegetables as kind of a deterrent to eating. And many of these molecules can be toxic in high quantities, which is why we have kind of an ingrained dislike of bitter tasting foods. The umami taste is a response to the amino acid glutamate. So umami is found in protein rich foods. That's why it's sometimes referred to as the beef taste or the protein taste. MSG, a flavoring common in a lot of Asian cuisine, monosodium glutamate, also stimulates the umami taste. And similar to identifying odors, we identify a taste based on the combination of activation of these taste receptors. So when you have a piece of candy, for example, you may get a lot of sweet taste, not a lot of the other ones. When we have a beef broth, we might get a lot of the umami taste and probably a fair amount of the salty as well. We learn over time to associate these different patterns of taste with particular foods. Like I said, I'm not going into a huge amount of detail about either the anatomy or the physiology of smell and taste. But you should be able to describe some of the mechanisms behind smell and taste and some of the anatomy behind smell and taste in a general way, including describing the role of chemoreceptors, describing generally how we detect and identify smells, describing the structure of a taste bud, and again, generally how we detect different tastes.