 Ulfaction is brought about by the receptors which are present in the olfactory epithelium and this olfactory epithelium is present at the roof of the nasal cavity and has a surface area of approximately 10 centimetre square. Now since this is present at the roof of the nasal cavity the odor answer that is the chemicals which are present in the air need to be sniffed to reach to this olfactory epithelium. Now this olfactory epithelium has special type of neurons whose epithelium has a receptors for these odorants and these neurons are known as olfactory sensory neurons. So let us see what are the different types of cells which are present in this olfactory epithelium. So this diagram here shows the olfactory epithelium right this is the olfactory epithelium and if we see the type of cells here this cell is the olfactory sensory neuron which is basically a bipolar neuron. So see the cell body of this neuron is present in the olfactory epithelium this is the cell body and the dendrite is projecting below this epithelium and here it ends in a olfactory knob okay olfactory knob and from this olfactory knob there are celia which project out of this olfactory epithelium and this part below the olfactory epithelium is mucus okay so the celia are projecting into the mucus below the olfactory epithelium so that is one type of cell. Other types of cells are the supporting cells so here this is the supporting cell also known as sustentacular cell then there is stem cell so here this is stem cell and this is responsible for proliferation and formation of these new olfactory sensory neurons so these are one type of neurons which regenerate actually their life span is approximately one to two months and when they die their regeneration occurs by the proliferation and differentiation of these stem cells. Then finally we also have bowman's glands so these are bowman's glands which are present in the olfactory epithelium and they are responsible for secreting this mucus which is lining this olfactory epithelium right now the receptors for this odorance is present on the celia of these olfactory sensory neuron and the odorants which are there need to cross this mucus and reach to the receptor which is present on the celia and that is brought about by certain proteins known as odorant binding proteins which are present in this mucus so these odorant binding proteins they kind of bind the odorants and concentrate it within the mucus and they can actually cross the mucus and reach to the celia so for the odorants to actually enter into the mucus and bind to the odorant binding proteins and reach to the mucus these odorants have certain characteristics first I have already told you that they need to be sniffed right then they should be partly water soluble right and they should be partly lipid soluble as well so if they are only lipid soluble then they will not be able to reach the celia the receptor okay so they should have some water solubility as well and some lipid solubility as well fine now let us see that how these odorants are able to stimulate these receptors and cause a generation of the action potential see the resting membrane potential of these neurons is minus 55 millivolt and at this resting membrane potential there is some action potentials are already there it is not that it is completely silent some action potential is there but it is very slow like there can be one action potential in 20 seconds right but it is not like completely silent now when the odorants bind to the receptor and sensory transduction takes place that is a conversion of chemical energy of the odorant to the electrical energy there will be change in the membrane potential and once this membrane potential changes to minus 30 millivolt that is the threshold and at that time there will be generation of lot of action potential maybe approximately 20 action potentials per second okay so how is it that this chemical energy of odorants is converted to electrical energy that is the change in the potential known as sensory transduction let us see the mechanism so the odorant binds on a G protein coupled receptor present on the cilia so this membrane is showing the ciliary membrane and there is presence of G protein coupled receptor which can bind with the odorant G protein coupled receptor as you might be aware that it has three units G alpha G beta and G gamma three units are there which are attached to the receptor and as the odorant binds to the receptor this G alpha unit gets separated from the other two G beta and G gamma unit and it goes and activates the enzyme adenylate cyclist which is also present on the membrane nearby this G protein coupled receptor once this enzyme is activated it converts ATP to a second messenger CAMP and this CAMP goes and causes the opening of a channel which is permeable to both sodium and calcium so there will be entry of sodium and calcium within the cell and this leads to depolarization okay so that is a simple method of sensory transduction that is via G protein coupled receptor and once the entry of sodium and calcium is such that it causes a change in potential to the threshold that is the minus 30 millivolt there will be generation of lot of action potentials now how many smell sensations can we actually perceive so actually before there was a concept of primary sensations of smell primary sensations of a smell like we see in color vision that there is red blue green primary sensation then in taste also we say that there are five primary taste sensations similarly in olfaction before it used to be said that we have seven primary smell sensations but now this concept is old it doesn't exist because what we have found that we can perceive thousands of a smell sensations so how is that made possible so first thing is remember that there is no concept right now about the primary sensation of a smell being only seven right so let us see that how is it possible that we can smell so many different types of smell sensations so that is made possible because as I told you that there is presence of G protein coupled receptors on the celia of the olfactory sensory neurons now these G protein coupled receptors are too many actually it has been found that there are thousand different types of genes and out of this 400 are active and they lead to formation of so many different types of olfactory receptors which can bind to different odorants so for example this is a schematic diagram showing these are the receptors and these are the different types of odorants so you see here this odorant is able to bind to a particular odorant receptor protein on the other hand if you see this particular odorant has a specificity for two different types of odorant receptor proteins then another one again it can bind to two other different types of receptor proteins so you see that there are first of all so many different types of receptors that is 400 receptors and that is also not you know one is to one relationship with the odorant that it is not that only 400 odorants can bind to 400 receptors you see because of this different combination of binding there will be a lot of permutation combination by which odorants can bind to multiple receptors and it is this combination of the receptors which are stimulated ultimately lead to the perception of different smell sensation we will see little bit more about this when we talk about the processing of the olfactory stimuli so for now let us limit here that one odorant can stimulate a combination of receptors fine so this is what is happening at the level of the olfactory epithelium that is binding of the odorant to the odorant receptors which are present on the dendrites of the olfactory sensory neurons and there will be generation of the action potential now the axons of the olfactory sensory neurons they cross a cribriform plate which is basically present in the ethmoid bone and this cribriform plate actually separates the nasal cavity from that of the cranial cavity so there are perforations in the cribriform plate let us see in the diagram see here in the diagram this is the cribriform plate and there are perforations in the cribriform plate and these are the axons which are crossing this perforation and reaching to the cranial cavity and in the cranial cavity there is a region just above the cribriform plate known as the olfactory bulb right it is seen in the previous diagram so here you see this is the cranial cavity and just above the cribriform plate we have olfactory bulb now let us see that what happens within the olfactory bulb so here within the olfactory bulb we have certain structures known as olfactory glomerulus okay and it is in the olfactory glomerulus that the axons of the olfactory sensory neurons are making contact with the dendrites of the other neurons that is the mitral cells and tufted cell so this is the olfactory glomerulus and in the olfactory glomerulus you see there is synapse formation so this is the cell body of the tufted cell and this is the cell body of the mitral cell which is outside the glomerulus but their dendrites are projecting into this glomerulus where these olfactory sensory neurons are making contact with them and the axons of this tufted cell and mitral cells are going into the cortex then there is certain processing going on here first of all you see the number of the mitral cells is quite less compared to the number of the olfactory sensory neurons even the number of the tufted cells is quite less there are like only 25 large mitral cells and approximately 60 tufted cells but the olfactory sensory neurons are in too much number there are like 100 million olfactory sensory neurons so you see that how so many of them are making contact with the mitral cells and tufted cell dendrites then there are other cells as well there is periglomerular cell and there is granule cells now if you see this periglomerular cell what it is doing is that the they are making contact with the olfactory sensory neurons but their axon is going and inhibiting the dendrites of the mitral cells understanding so information from one glomeruli is inhibiting the information from the nearby glomeruli what is this actually this is lateral inhibition as is presented for all other sensations as well and this lateral inhibition is responsible for contrast enhancement okay contrast enhancement which is very important for acuteness of the sensation which we feel so it increases the sharpness of the sensation which we feel by inhibiting the other sensation so the odorant which is stimulating this particular receptor say suppose will not be filled to that much since the mitral cell which is carrying its information is being inhibited by the nearby glomerulus connections and obviously there will be one periglomerular cell here also which will inhibit this particular dendrite of the mitral cell however since the information from this particular neuron is not that much so the inhibition will also not be that much isn't it so there will be enhancement of the contrast which is happening because of these periglomerular cells then coming to granule cells granule cells actually receive efferent information from the cortex so they are bringing information from the cortex and they are causing inhibition of these mitral cells and this is responsible for adaptation we'll see when we are talking about the mechanisms of adaptation of the smell sensation so they inhibit the mitral cells so that the information which the mitral cells are carrying to the cortex that is decreased and that is responsible for adaptation now as i told you before that we'll come back to the combination of the odorant with the receptors how that is happening in the olfactory epithelium with the olfactory receptors this combination actually continues forward and there is a creation of 2d map in the glomeruli how let us see certain example say suppose there is an odorant one which is stimulating two receptors two different receptors which are present in olfactory sensory neurons just remember here that each olfactory sensory neuron has only one type of receptor okay there are different celia there will be receptors on all the celia but there will be only one type of receptor in one olfactory sensory neuron so say suppose odorant one stimulates the receptors two different receptors present on two different olfactory sensory neurons now this information will go to two different glomeruli you see in the previous diagram that here i have shown two different types of olfactory sensory neurons with different colors one is green and one is blue so suppose these three neurons have a particular type of receptor only one type of receptor they are going to one glomeruli only and here the green ones are going to one glomeruli only so information from one type of receptor goes to one glomerulus coming back to our example here again you see there are i have shown these are different olfactory sensory neurons with different types of receptors so information is going to two different glomerulus now suppose there is another odorant odorant two which stimulates this type of receptor and other two receptors as well from there again information will go to different glomeruli so you see depending on which glomeruli are being stimulated there is a pattern which is being created within the olfactory bulk of the stimulation of the glomeruli and this pattern is going to project to the cortex so whatever pattern of neurons are stimulated in the cortex based on that there is a perception of a particular kind of smell fine so this is a new concept in olfactory physiology nowadays there is no concept of seven primary sensations of smell fine with this let us now go on to the olfactory pathway now one thing important here is that olfactory pathway is the exception in the sensations where it directly reaches to certain areas of the cortex without passing through the thalamus so here if you see that the axons from the mitral cell and tufted cell reach via the olfactory tract olfactory tract is nothing but the axons of the mitral cells and tufted cells from the olfactory tract and olfactory nerve is basically this the axons of the bipolar olfactory sensory neurons all together from the olfactory nerve okay so then the axons of mitral cells and tufted cells from the olfactory tract which reaches to various areas in the cortex so one is the primitive area primitive olfactory system and that is responsible for the basic olfactory reflexes which reaches basically to the hypothalamus and that is actually a direct connection to the hypothalamus and like basic reflexes licking of the lips salivation these are innate reflexes not learned reflexes okay and that is the most primitive olfactory pathway and that does not pass via the thalamus second is that the pathway also goes to the pyriform cortex and amygdala and from there to the hypothalamus and that is responsible for liking and disliking of the food aversion to the food which foods we don't like which foods we like so this is learning depending on the experience and that is happening because of the passage of information first to the areas of the cortex pyriform cortex then cortical portion of the amygdala and from their information reaching to the hypothalamus so this is also a primitive pathway but it is less primitive than that of the basic innate reflex pathway and finally there is a newer pathway which passes through anterior olfactory nucleus or factory tubercle via the thalamus it reaches to orbital frontal cortex or directly reaches to frontal cortex okay so this newer pathway is actually passing via the thalamus the dorsomedial thalamic nucleus and this is important for conscious perception of the order and analysis of the order that what is the type of the order what is the strength of the order that is by this newer pathway which reaches to the orbital frontal cortex and to the frontal cortex via the thalamus then the information from olfactory pathway also reaches to the memory area that is via the entorhinal cortex to the hippocampus and that is important for the memory of the order so that I was talking talking about the liking and disliking of food so from hippocampus information is reaching to the hypothalamus only when memory is there then only we can remember that this order is linked to a particular food and whether we like it or don't like it so that was about the olfactory pathway coming to the final part that is adaptation of the smell sensation well there are two mechanisms of adaptation of a smell sensation one is the peripheral mechanism which is happening at the level of the receptor and the other is the central mechanism so at the level of the receptor as I told you that it is a G protein coupled receptor right so what happens that we have seen that how G alpha unit gets separated from G beta and gamma unit after some time that G alpha unit comes back and joins G beta gamma unit that is the basic action of the G protein coupled receptor for details you can watch my video on G protein coupled receptors I have given the link in the description section as well so this G protein coupled receptor action stops and there is again decreased activity of adenyl cyclase as well so the sodium calcium channel which we were talking about actually it closes okay so there will be peripheral adaptation however the adaptation is not complete okay in fact there is approximately 50 percent adaptation and after that adaptation is very very slow so there is another mechanism of adaptation that is central adaptation we know actually from our experiences that there is complete adaptation of a smell sensation if we are sitting in a room full of certain odor we will feel a strong sensation for some time but after that maybe even within a minute there will be complete adaptation of the sensation and that is brought about mainly by the central mechanism and this is because of the granule cells which I told you before that granule cell is receiving efferent from the cortex so this granule sense in the olfactory bulb is stimulated which causes inhibition of the mitral cells and tufted cells so that the information doesn't reach to the cortex about a particular odorant just before finishing we'll talk about certain terms that there is something known as anosmia anosmia is absence of perception of smell sensation then there is hypoosmia hypoosmia is a decrease in olfactory sensation we perceive it less then there is hypoosmia hypoosmia is less common but commonly seen in pregnant women it is enhanced olfactory sensation and then there is dysosmia dysosmia is altered olfactory sensation or whatever is the sensation we don't feel that we feel some other perception of the smell so that is dysosmia so that was all about olfaction physiology thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel physiology open thank you