Welcome back, we're going to finish the senses with the chemical senses, We're going to talk about the last of the special senses. So we're going to be dealing with chemoreceptors, molecules that will bind certain chemicals and then eventually lead to a graded potential, which will lead to an action potential. So we'll be obviously talking about taste and the sense of smell. But keep in mind, as we've mentioned before, there are going to be other chemicals that our body is also monitoring and responding to. Such as detecting osmolarity in the brain which we'll talk about when we talk about the kidney. And then talking about the amounts of oxygen and CO2 and the pH of the blood, which we'll be talking about when we get to the respiratory system. Let's consider taste first where you're going to have along the surface and kind of in recesses of your tongue, you're going to have taste buds. These are going to be made up of taste cells. Which you can think of these like segments in an orange. They form this sphere kind of around a central channel, which is called the taste pore. You can see that because this taste pore is recessed from the surface of the oral cavity and because it's really small, the substances, the molecules that we taste are going to have to be dissolve in liquids. That's fine because we pretty much constantly have saliva in our mouth and certainly we have it when we're eating. But that doesn't mean that in order just taste something, it needs to be dissolved. So, that it can enter this little taste pore and expose the tops of these taste cells to the molecule. These taste cells will form graded potentials, that if they're strong enough will affect the afferent neurons leading from the taste bud. These action potential are then sent into the central nervous system. We can taste different flavours or different tastes modalities. We've already said how these substances are going to have to be dissolved in saliva. And no matter what we're tasting,the results is that we're going to increase intracellular calcium. That is going to cause the release of neurotransmitters by these taste cells. They cause graded potentials that if they are great enough or strong enough will cause action potentials in that post-synaptic neuron. That neuron leads away from the taste bud. We have a couple of major types of channels that are going to be activated. When we're tasting something that's sour it's because we are detecting protons, we're detecting something that's acidic. Often when we're tasting something that's salty what we're detecting is sodium. It can be potassium as well. But in any case, these are going to be ions. They're going to act on ion channels that are then going to cause an increase in intracellular calcium in the cell versus sweet tasting substances, which are going to be sugars. Bitter, which are compounds which are often going to be plant alkaloids. Then, a fifth taste modality is umami which is meaty flavor. It makes meat taste like meat but, also causes mushrooms to taste like meat. What we are sensing are amino acids like glutamate. Sugars, bitter molecules and amino acids like glutamate are going to be detected by G protein-coupled receptors. These tastants will bind the G protein-coupled receptors and then the G protein-coupled receptors will cause a signal transduction pathway to initiate that will increase intracellular calcium and cause the release of neurotransmitter. We're going to finish up with smell, which is going to be unique compared to most of the other systems that we've talked about, especially of the special senses. Here the receptors are actually the neurons themselves. The primary afferent neurons themselves, are going express olfactory receptors that actually bind to the odorants. In the epithelium covering the nasal cavity is where we have these neurons that send out cilia that have receptors on them that can bind the odorants. These receptors are again G-protein coupled receptors. So each neuron will express a specific type of G-protein coupled receptor that can combine a certain type of molecule. We have several hundred olfactory receptor types. So, we're going to have several hundred neuron types that are expressing a certain receptor, but yet we can discriminate about 10,000 odors. So when these odorants bind they're going to cause a graded potential which when great enough or big enough can lead to an action potential. It's going to be which neurons are activated and how much they are activated by binding a certain molecule that let's us convert that to a smell. So if we have a certain molecule. Let's say that there are two or three different odorant neurons that combine different portions of that odorant molecule. So which neurons are activated and how much they're activated is what we translate into smelling that molecule. Again it's very similar to our ability to see color where we don't have a different receptor type and a different neuron type for each color or for each molecule we can smell. But instead it's the matter of which neurons are activated and how much in combination that allows us to detect so many colors and so many different different odors. So we've talk about the two special senses that involve chemoreception. Taste, where we're going to have taste receptors in the tongue that are associated with our five basic taste modalities. Umami, salt, sour, sweet, and bitter. Again, when we're sensing a taste, it's going to be combinations of taste receptor activation that's going to give us certain tastes. And we're going to be interpreting those in terms of perceiving what we are tasting. And then in smell, we're going to be activating olfactory receptors, which themselves are going to be the primary afferent neurons which send information into the central nervous system.
