2015 AP Biology free response 7

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 1,000 functional odorant receptor genes, each coding a unique odorant receptor.

Alright Part A, describe how the signal is transmitted across the synapse from an activated olfactory sensory neuron to the interneuron that transmits the information to the brain. Alright, so let's draw this out for Part A. So let me draw the activated olfactory sensory neuron. So it might... so those are its dendrites there. So I'm not going to draw a perfect... so those are its dendrites right over there, and then this is its axon, its axon. And then this is the axon terminals, the axon terminals just like that. I could draw other details if I want, and 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 nucleus right over there.

So this is the activated olfactory sensory neuron, so activated olfactory sensory neuron. And let me draw the interneuron. So I'll draw one of its dendrites right over there. So this is the interneuron and its dendrites, and then its axon—draw a little bit better than that—it's axon just like that, and it goes to the brain.

If we zoom in on the synapse where we have to transmit the signal between the two, 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 interneuron right over that, just like this. And this is the synapse. So this is a zoomed-in view of the synapse, and the activated olfactory sensory neuron has this activation potential that is going to go down the axon. Then, 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 these vesicles right over here, but when the action potential comes, they will get released into the synaptic cleft. Then, the interneuron is going to have proteins that sense those neurotransmitters, and then that activates that neuron. So once again, this is the interneuron. We could say that the interneuron gets activated—actually, it could be inhibited as well—but activated by neurotransmitters released into the synapse by the sensory neuron due to 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 chemicals, these neurotransmitters, which are actually fairly small molecules. But then they trigger the next neuron, and then after that signal goes, they get all metabolized by enzymes and things. There might be a few that just always stick around, but for the most part, they signal from one neuron to another. They could activate the next neuron or actually 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—see, there's a thousand 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 kind of a theory, is that you don't necessarily have a one-to-one mapping between odorant receptors and odorant molecules.

So you don't necessarily have a one-to-one relationship between odorant molecules and receptor proteins. Maybe one protein can detect multiple molecules. Maybe a given protein can detect multiple molecules or vice versa. Oops, I keep having trouble with my little pen here, or vice versa. And so then this could lead to many combinations being detected. So even though you could have maybe these 1,000 odorant genes that code for 1,000 receptor proteins, you might say oh those would only be able to detect 1,000 different molecules.

Well, no. Each of those could detect more than one, and then when they come in different combinations, they might trigger the brain in different ways. And then, many combinations can lead to different smells as perceived by the brain. Now there is some possibility that each of these genes could be coded into proteins in different ways, but they do tell us each encoding a unique odor receptor.

So 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. The different combinations of all the above mean that you could have much more than 1,000 molecules being detected, and especially different molecules in different combinations. You could have thousands upon thousands of smells, quote-unquote, being detected.