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- [Voiceover] Smell perception in mammals involves the interactions of airborne odorant molecules from the environment with receptor proteins on the olfactory neurons in the nasal cavity. The binding of odorant molecules to the receptor proteins triggers action potentials in the olfactory neurons and results in transmission of information to the brain. Mammalian genomes typically have approximately one thousand functional odorant-receptor genes, each encoding a unique odorant receptor. Alright, part a, describe how the signal is transmitted across the synapse from an activated olfractory sensory neuron to the interneuron that transmits the information to the brain. Alright, so let's draw this out. So, this is part a, so let's, let me draw the activated olfactory sensory neuron, so, it might, so those are its dendrites, there, so, I'm not gonna draw a perfect, so those are its dendrites right over there, and this is its axon, it's axon, and then this is the axon terminals. The axon terminals, just like that. And I could draw other details if I want, I don't think you'd have to draw all of these details on the actual test. But, just to get a sense of things, we could also draw the well, we could do it in a different color, we could also draw the nucleus right over there. So this is the activated olfactory sensory neuron, so activated olfactory, olfactory sensory neuron. sensory neuron. And let me draw, let me draw the interneuron. So, the interneuron, so I'll draw one of its dendrites right over there. So this is the interneuron, drawing its dendrites, and then its axon. I'll draw it a little bit better than that, its axon just like that and it goes to the brain. And if we zoom in on the synapse, where we have to transmit the signal between the two, so if we zoom in, if we zoom in, right over there, we would see the axon terminal of the sensory neuron. So, maybe it looks something like that. And then the, the, the interneuron right over that. Just like this. And this is the synapse. So, this is the zoomed in of the synapse. And the activated olfactory sensory neuron, you have this activation potential that is going to go down the axon, and then when it gets to when it gets to the axon terminal right over here, it will trigger the release of neurotransmitters. So, these neurotransmitters are typically hanging out in, in these, these vessels right over here, but then when the action potential comes, they will get released into the synaptic cleft. And then, the interneuron is going to have, is going to have proteins that sense, that sense those neurotransmitters, and that activates that neuron. So, once again this is the interneuron, interneuron, and we could say that interneuron, interneuron gets activated, actually it could be inhibited as well, but activated by, by neurotransmitters neurotransmitters released into synapse, released into synapse by the sensory neuron, by sensory neuron due to action potential, due to action, action potential. And this is probably enough, but I went through a few more pains to draw it out a little bit, and you could draw other details. You could draw the myelin sheath and all of that, you could draw the nuclei of the different cells, but the basic idea is that the signal goes from one neuron to another with the release of these, these chemicals, these neurotransmitters, which are actually fairly, fairly small molecules, but then they trigger the next neuron, and then after that signal goes they get all they get metabolized by enzymes and things and there might be a few that just always stick around but for the most part they signal from one neuron to another. And they could activate the next neuron, or they actually could inhibit it as well, but in this case, they would probably activate. Alright, let's do part b. Let's do part b. Explain how the expression of a limited number of odorant receptor genes, you see there's thousands, receptor genes, can lead to the perception of thousands of odors. Use the evidence about the number of odorant receptor genes to support your answer. So, one way to think about it is, and this is this is kind of a theory, here, is that you don't necessarily have a one-to-one mapping between odorant, odorant receptors and odorant molecules, so don't, don't necessarily, necessarily have one-to-one relationship between, between odorant molecules odorant molecules and receptor proteins. And receptor proteins. Maybe, maybe one protein can, can detect multiple molecules, maybe one, maybe a given protein can detect multiple molecules. Or vice versa, or vice versa, whoops I keep having trouble with my little pen here. or vice versa, or vice, vice versa. And so then, you could have this could lead to many combinations, combinations being detected. So even though you could have, maybe these one thousand odorant genes, they code for one thousand receptor proteins, and so you might say well, well those'll only be able to detect one thousand different molecules, well, no, each of those could detect more than one, and then, and when they come in different combinations they might trigger the brain in different ways. And then, many, I mean I could even add that, this could lead to many combinations being detected which would be different, different smells in the brain, different smells as perceived by brain, perceived by brain. Now, there is some possibility that each of these genes, maybe they could be coded into proteins in different ways, but they do tell us each encoding a unique odorant-receptor, so I like, I like going with this one, you don't necessarily have a one-to-one relationship, each receptor could recognize multiple molecules, or one molecule could be bound by multiple receptors, and then the different combinations of all of the above means that you could have much more than one thousand molecules being detected, and especially different molecules in different combinations, you could have thousands upon thousands of smells quote, unquote, being detected.