today we're going to be discussing taste and smell both the reception of taste and the perception of taste and then the reception of smell and then the perception of smell um in general tastants are the chemicals that are tasted uh when you're tasting something and odorants are the structures that you're tasting when uh that you are smelling when you are going through olfaction when you are smelling something so remember that both taste and smell work through chemo recept receptors and the the actual chemicals we're referring to are tastants and odorants now both of these things happen in aquous environments they're you're going to have tastant that interact with neurons you're going to have odorants that interact with neurons but these can't happen in a dry environment so tastants must be dissolved in saliva and odorants must be dissolved in mucus so this is just a really good example of uh something that you should probably already know is that most physiological or all physiological functions only happen in aquous environments or water water filled environments so you cannot sense taste or you cannot sense smell unless you have an aquous environment for those things to happen in so tastants are just little tiny little molecular particles of the food that you're eating and odorants are just tiny little molecular microscopic particles of the thing that you're smelling all right so let's talk about taste what we're looking at here is a tongue of course and what we refer to as taste buds is probably not what we actually think they are and when you stick out your tongue you see all these dots on the tongue um these are not actually taste buds they're called pilli remember a pillis is a nipple-like projection so there are three different types of pilli on your tongue these ones out here there's some in the back here and there's some along the side here we're not going to spend a lot of time on the different types of py except to say that um the the taste buds exist along the sides of the py so for example right here we're looking at a vay p pillus and what you'll notice is that along the sides of it you'll have like a dozen of them along the side and you'll probably radiating along different edges of this structure so along the sides of the pillas you'll actually have a whole bunch of taste buds so here we see all these little blue dots here these are taste Buds and if you look at it um blown up it looks like this and it actually looks like a like a flower bud that's looking to flower so that's how it these got their names of taste buds all right so when you look at the taste bud it's along the the side here and remember that tastants are going to be dissolved in saliva and what's going to happen is that those dissolved structures those dissolved chemicals are going to make their way along the side of the pill and they're going to interact with the taste buds and the way they do that is the taste buds are actually buried within epithelial cells but there is one small hole here called A Taste pore so some of the tastants with some of the saliva are going to make their way down along the edge of the Pilla and make their way into the taste pour so each taste bud consists of um well actually three different types of epithelial cells there are stratified squamous epitheli that make up kind of the protective um walls around this entire thing this protective coating uh there are also gustatory epithelial cells look at all these things in here all these long purple looking things in here are gustatory epithelial cells living cells you can see the nucleus here that are actually the sensors of taste additionally there are basil epithelial cells basil epithelial cells now these are basic Bally the stem cells of these uh of these structures they are cells that are ready to differentiate into uh taste uh into taste bud gustatory epithelial cells whenever they're actually needed so you don't actually get to the neural structure until you see this one right here so here we have a cranial nerve and here are the dendrites sticking out of the end of the cranial nerve that are ready to send that information back to the brain and so these little yellow structures here they're part of the nervous system but what actually senses the food again are the gustatory hairs I'm sorry are the uh gustatory epithelial cells and the part of the gustatory epithelial cell that's that actually senses these are these structures here called gustatory hairs so the gustatory hairs are going to project from the tips of the gustatory epithelial cells and you'll see that they kind of um are you know looking towards or um sticking out towards that taste pour because this is the only place where they're actually going to be able to sense any kind of tastant coming into the mouth okay so these are vulnerable cells they're epithelial cells which is nice because epithelial cells in general are always regenerative these ones particularly so um these can actually regenerate every 7 to 10 days because taste buds are subject to friction and burn and they will need to be regenerating pardon me they will need to regenerate uh on a pretty regular basis okay so that's the job of the basil epithelial cells to just be there to grow up when these other guys these gustatory epithelial cells um die off because of friction or burn okay let me see where we're at hush Kitty um Okay so we've covered the basil epithelial cells the gustatory epithelial cells the gustatory hairs those those are the kind of the terms you should be paying attention to there um all right now the tastant that comes through here are actually going to cause these cells to depolarize when they come through here and they they they interact with the correct cell that they themselves are are able to depolarize then these guys depolarize and then this the signal is sent back to these um these wres and then it's sent back to the to the brain so we'll talk about that what happens next in a moment but let's talk in a little more detail about these um these uh taste bud cells so it turns out that there are some taste bud cells that are you know these gustatory epithelial cells that are sensitive to particular things so that we have what we call bitter gustatory epithelial cells salty sweet sweet Umami and sour gustatory epithelial cells and those particular ones will only be sensitive to the things that actually cause that particular sensation so let's talk about those if you have um there are five basic taste sensations the first one is sweet sugars saccharin alcohol and even some amino acids will depolarize the sweet gustatory epithelial cells so they will interact with these little gustatory hair cells and they'll cause dear ization of this gustatory hair cell which will then allow the signal to be sent back right salt metal ions so this cell here is what we call a salt gustatory epithelial cell it is going to be sensitive to metal ions such as sodium ions and chloride ions and potassium ions sour these green guys here are particularly sensitive to sour and this is going to be sensing acids and especially H um hydrogen ions in particular bitter the bitter ones will be sensing um alkaloid such as quinine and nicotine and Umami is the one that a lot of people haven't yet heard of it's only really been um discovered I don't know these particular cells have only been discovered in the last 15 years or so and The Sensation of umami really didn't come about until about 110 120 years ago um or or I should say the the manufactured version of the Umami taste Umami is a uh is um a Savory meaty flavor and this is elicited by the amino acid glutamate so if you hear of monosodium glutamate the infamous MSG um this causes particular cells these Umami sensitive cells to depolarize um okay generally speaking it is thought that every taste bud has all of these cells represented within it although it seems as though different localities on the tongue are sensitive are particularly sensitive to particular taste so it could be that the makeup of the taste buds themselves may have a higher proportion for example of sweet you know sweet um gustatory hair cell or gustatory epithelial cells um or they may have a higher proportion of the bitter sensitive ones um but it seems as though all taste buds have representatives of all five of these taste within them all right um let's talk about the the um perception of taste so there are two there are two cranial nerves that actually send the information back to the brain cranial nerve seven which is the facial nerve cranial nerve 9 which is the gloss ofial nerve these carry the impulses directly from the taste buds in the tongue to the solitary nucleus the medulla so here we have your medulla aong and that is the first place where information is carried back to and that makes sense if you know where the position of the facial nerve and the glossop ferial nerve are in addition the vagus nerve seems to carry some information from deeper in the throat this is not specifically a taste sensitive um structure but it may have some chemosensitive properties where it may be sensing danger um or if you just tweak the vagus nerve it actually causes an edic response or regurgitative response so um vegus nerve is cranial nerve 10 so all three of these nerves carry the information to the solitary inclus of the medulla and then the uh it is carried to the thalamus where a lot of information is sorted out actually not just about taste but about a lot of things and from there it is sent to the gustatory cortex to be tasted to the hypothalamus and to the lyic system remember we talked about having an emotional response to your food amongst all kinds of different other parts of the brain so your taste is going to be sensed at all the is going to be perceived at all these different places so that is the perception of taste all right let's move on to smell then olfaction again we're going to be talking about dissolving odorants if you smell bread baking what's actually happening is tiny little microscopic pars particles of that bread are making their way through the air dissolving themselves in your mucus right at the to top of your nose let me show you a picture that shows the mucus itself here is your mucus so here we have little bits of bread that are dissolving themselves in the mucus making their way to the old factory cyia part of the old factory Sensory neurons and uh then they will be sent you know up through the axons to old um cranial nerve uh one the uh the olfactory nerve the olfactory bulb and then sent back to the brain anyway so the O the organ of smell is going to be the olda factory epithelium which is this entire structure in here which covers the superior nasal conka so here we have uh the nasal coni here are what basically these are bones or parts of bones that are set up to kind of wh whiff um uh swirl air around so that it makes its way upward to the roof of the nasal P nasal cavity because these olfactory neurons are only in the roof of the nose many smells actually pass right through you know into the throat but some of them will make their way up there very sensitive organ so those are them that do and if you sniff if you sniff you kind of are you know you know that you'll be able to smell things better if you sniff them and the reason you can smell things better is because you're kind of forcing the air upward to the roof of your nose Okay so this is the organ of smell is the olfactory epithelium the olfactory receptor cells are really unusual if you look at these guys here's you know here's one right here here's its dendrites it's body or Soma and then it makes its way so it's a bipolar neuron if you know what that is and it makes its way it sends its axon up here to um interact and eventually send the signal to a mitro cell well Al Factory receptor cells there are going to be millions of them in your nose and if you take a if you take a nice scanning electron micrograph such as this one of the olfactory epithelium what you'll see are tons of cyia these are called radiating cyia so this is one of the unusual characteristics of these neurons these radiating cyia are very weird um these are cyia that don't really move very much they just kind of sit there in the mucus and wait for odorant to come to them so as far as cyia go most cyia don't work this way most silia in your body or in any animal's body will work by moving um moving something around or helping the animal to move or something like that so these radiating ol Factory cilia are really unusual olfactory neurons themselves are regenerative they can regenerate about every 30 to 60 days that's there's only two or three other neurons that are regenerative on a regular basis but these guys are extremely regenerative they they can regenerate every 30 to 60 days and the reason for that is that they are the only neurons the only neurons that are actually exposed to the outside of the body so they are buried in mucus here but you see those radiating old factory cyia are actually quite vulnerable in their position they're basically exposed in the in the nose um they are also very sensitive they're very sensitive you only need a few odorants to actually cause um an olfactory neuron to depolarize so when we take a look at well first of all um these are odorants you know we're kind of showing that this particular flower most most smells are very complex they'll have like you know dozens or hundreds of different odorants within them that the body kind of kind of mixes together and identifies as one particular odor but the odorants themselves tend to be very separate and very discreet so here we're showing just a an example of three different types of neurons that are sending out projections um we have red ones and purple ones and green ones and what you'll notice about these is that at the level of these olfactory um uh cyia um and at the level of their you know of all of their dendrite complex all of their protein receptors the things that actually sense um o uh actually sense odor odorants they're all the same so this purple guy here only has one type of protein receptor that senses odors in all of its dendrites and along the entire length of its dendrites so it's only going to be sensitive to the one odorant and I guess that's probably why there are millions of different uh of different neurons in there is that they have to be sens to many different types of smells could be you'll have um a neuron that is sensitive to a particular odorant that you never actually encounter or you don't encounter over the life of that cell you know and it'll never actually be used it'll never actually be depolarized but it's there in case the odorant comes along um okay so one of the another point that I want to make about ol Factory receptor cells is that they are very very sensitive it only takes a few odorants to interact with these particular receptors receptor proteins in order for the cell to depolarize now the cell is going to depolarize and this is going to be on your test this picture I'm going to show you a blank of this and ask you to uh label it just like it shows up in the um practice exam with a little bit of extra information um uh with a little bit of extra question there's going to be a bonus question on there um what we're looking at here is a g protein coupled receptor a metabotropic receptor what we've studied in the past uh they're called ionotropic receptors because there's a a receptor that receives we've always talked about neurotransmitters um and that neurotransmitter binding to the receptor is going to cause it the opening of a channel that is buried right within the receptor itself now take a look at the difference here um it kind of works the same way because when you look at the start of this what you'll have is an odorant binding to a receptor you notice there's no channel here it's just a solid protein buried in the membrane look all the way over here A whole series of things is going to happen that's eventually going to open a channel um and at the end of the day that's the whole point of the thing there's a receptor that b there's odorants that bind over here and there are there is a an ion channel that opens over here so this is called a metabotropic receptor system or a g um a g protein coupled receptor system cuz look at this here we have our receptor and here is a g protein so it's coupled or tied to this G protein as soon as the receptor receives a couple of odorants it's going to um cause this G protein to uh move down the membrane to um interact with this enzyme here called aate cyclas aate cyclas is kind of um a general structure that causes things to happen especially this which is the conversion of ATP to cyclicamp cyclic then goes on to interact with this Channel and cause it to open from the inside um kind of a lock and key situation from the inside that then causes sodium and calcium to flow into the cell this so I'm going through this very quickly I would like you to take a closer look which is why I'm asking you about it on the uh on the exam um well in general terms metabotropic receptors or decoupled protein receptors are usually in place because they amplif a signal it's one of the reasons why these olfactory neurons can be so sensitive you can have just a few odorants causing a depolarization of the cell because the signal is Amplified so that is usually the case with g-coupled protein it's kind of like a Cascade of things happening within the cell um it doesn't necessarily have to happen in in the membrane itself it could be happening like it could be um mediating a number of uh structures to be phosphorated for example within the cell and so on anyway um uh so the point that I'm making is that you don't necessarily need a channel to be attached to make something happen within the cell sometimes the decoupled protein receptors can actually be very very powerful much more powerful than a standard you know ionotropic receptor attached to a channel sort of a thing that we've studied in the past all right so as far as the the way that uh smell is perceived here we have so all that stuff I was talking about a moment ago is happening at the level of the olfactory cyia all those depolarizations and so on so SE cell is depolarized it's going to send the signal onto the mitro cell and from the mitro cell then the signal is going to be sent back to the brain this is the olfactory uh uh the olfactory neuron cranial nerve one and some would say that it's not a typical or it's not actually a cranial nerve at all because it never actually leaves the brain you'll notice here this is the before PL of the ethmoid bone remember this guy um with the olfactory famina that allow those axons from those epithelial cells to move up into the cranial nerve here um so uh um what happens is everything that uh that is outside of the central nervous system is down here once you get up through these olfactory foramina this is actually kind of the brain this is actually the brain this is the frontal lobe up here this is the prefrontal cortex up here this is the frontal lob so cranial nerve one is right under that it's basically attached to it so these are not typical cranial nerves at all some would argue they're not actually cranial nerves okay so the mitr cells will then send impulses through the cranial nerve one or the olfactory nerve to the olfactory cortex and from the olfactory cortex additionally signals are sent to the hypothalamus to the amydala remember the amydala is responsible for emotional memory and so and also to the lyic system the lyic system is responsible for the um feelings of emotional involvement but the amydala is very interesting because this is the part of the brain I believe that is responsible for the fact that when you smell something it will make you remember some the sense of smell has been renowned to be very closely tied to memory and especially an emotional memory so the fact that smells are sent directly to the Amala probably has a lot to do with that so if you remember something if you get a scent of something obscure you're walking through a park one day and you smell a flower that you haven't smelled in 20 years and all of a sudden you're brought back to that Garden that you walked through when you were 10 years old with your parents or something like that um you really it this can be that strong you can actually put yourself right back in those situations so smell is very closely tied to memory because of the amydala oh my gosh I almost forgot this is what I was going to say about this I do apologize I'm sorry I'm out of order here when the sodium and calcium come into the cell the sodium causes depolarization of the cell the calcium causes the cell to be become habituated remember that calcium uh remember that olfactory neurons habituate easily and quickly that you can become very insensitive to a smell very quickly and that's the job of calcium so when calcium flows into the cell its job is to adapt the cell to the smell and the job of sodium is to depolarize the cell