What happens to volatile molecules that reach the main olfactory epithelium in the nasal cavity after they bind and the neural stimulus fades?

To what extent do such factors as receptor kinetics and diffusion (or any others) direct where these molecules end up?

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    $\begingroup$ Hi Trancot, I edited your question to make it more biochemical in nature and less biology. I think I captured the underlying questions that you were really after, but please feel free to make any changes so that it represents what you are interested in. If it is more the biology angle that you are after, flag the post and we'll move it to Biology.SE for you, but I think this has some interesting aspects that can be answered in the physical chemistry realm. $\endgroup$ – jonsca Mar 3 '13 at 19:46

Simply, the odorant particles/molecules never actually enter the cell through endocytosis. Instead, these olfactory receptors, belonging to the G-protein-coupled receptor family, are transmembrane receptors whose structure passes through the cellular wall (several times actually) to functional complexes on the other side, and they use simple and reversible ligand binding as their primary activation mechanism.

Odorant particles are dissolved in the mucous lining the nasal cavity, and in this form come into contact with the receptors in the cell membrane of the epithelial cells. These bind as ligand complexes to receptor sites, triggering a change in the receptor's structure which in turn causes the coupled G-protein to release its bound guanosine diphosphate, and instead bind guanosine triphosphate from the interior cell environment. When that happens, the G-protein is "activated"; it decouples from the receptor, splits in half into Ga and Gbg subunits, and those two halves bind to effector enzymes which trigger the cell signalling. As a result of the protein decoupling, the charge balance of the receptor changes, releasing the complexed ligand from its binding to the receptor, back into the extracellular environment.

To reverse all this, an enzyme called RGS (Regulator of G-protein Signalling) binds to the Ga subunit, triggering hydrogenation of the GTP molecule it still has bound into GDP, causing the protein's subunits to recombine and recouple to the receptor.

The released ligand, meanwhile, is washed away again by the mucous flow from the Bowman's glands. It may complex with other receptors along the way, but eventually it makes its way past the nasal membrane to the pharynx at the back of the throat, where the mucous is swallowed and the odorant broken down in the stomach. This is the final fate of virtually all odorant agonists.

A select few, however, bind more strongly, and are not released by the structure changes of the receptor. When this happens, the receptor is stuck in the "on" position. One of two things then happens; either the receptor continues to activate, causing neural impulses which your brain eventually ignores (odor fatigue), or mechanisms inside the cell notice the faulty receptor, bring it into the cell in a process similar to endocytosis, and attempt to break down its components and fix the damage to reuse it. More often than not, the bound chemical ends up poisoning the cell by binding to something more vital than an ordinary G-protein receptor, eventually triggering programmed cell death. That's fine; the basal cells behind the surface of the olfactory epithelium divide fast enough to replenish every cell in the nasal membrane about once every two days.

Sources: Wikipedia - Olfactory epithelium, Olfactory receptor and G protein-coupled receptor.

  • $\begingroup$ Great answer, very informative! Could you provide some (scientific) references to your story as the OP requested? @Trancot I think one place to look is the wiki page on Olfaction and references on that page $\endgroup$ – Michiel May 1 '13 at 6:39
  • $\begingroup$ I got most of this from Wikipedia. Look up the pages on olfactory epithelium and G-protein-coupled receptors. $\endgroup$ – KeithS May 1 '13 at 14:26
  • $\begingroup$ Why is Wikipedia so ubiquitous as a reference... Is it that people don't have access to alternatives? I encourage all to seek out alternative sources, and to not rely too much on Wikipedia. $\endgroup$ – Trancot May 7 '13 at 0:42
  • $\begingroup$ @Trancot - well, as long as the wiki article is properly referenced I don't see a problem with it. I typically look up the references in the wiki article, but usually the explanation on the wiki is much more straightforward and easy to grasp. Looking at the wiki pages that KeithS used: they are full of scientific references so by checking out those you don't have to worry about reliability (at least not too much) $\endgroup$ – Michiel May 7 '13 at 5:07
  • $\begingroup$ The resources are sparse. You must admit. I do, however, very much appreciated the answer. The question came up during dinner when my friend wanted to know why the smell of spicy food was so ephemeral some times and more lasting on other occasions. $\endgroup$ – Trancot May 7 '13 at 5:20

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