…and this is a 
Today in class we learned about the functioning of olfactory nerves. It was really quite interesting, especially to find out how the olfactory system is organized. Let's begin in the nasal cavity. Here, present in the mucus layer, are projections of the olfactory receptor cells. Each receptor cell is only capable of binding to one specific type of odorant molecule. These receptor cells travel through the porous bone separating the skull from the nasal cavity, and feed into a specific glomerulus. Glomeruli are located in the olfactory bulb, and have multiple receptors feeding into them. However, each glomerulus receives input from only one receptor type. In the glomerulus, the receptor neurons make excitatory connections with other cells, whose own axons project into the olfactory cortex in the brain. What is cool here is that there are about 1000 different types of olfactory receptor neurons, which each have their own proteins. This means that there are most likely 1000-2000 genes encoding for olfaction. It has been shown that this is not a combinatorial system like that developed for immunity.
Another cool thing I learned was that every time someone blows their nose, they are losing part of their brain. Seriously though, it's not quite as bad as it sounds. What is really happening is that you are losing the olfactory receptor cells that protrude out into the mucus layer inside the nasal cavity. The neat thing is that these cells are able to regenerate, which is unusual for other human neural cells. I was wondering, if perhaps, due to the organization of the olfactory nerves, regeneration is able to happen because of the interaction between the olfactory receptor neurons and the glomeruli. Perhaps the protruding parts of the cells are able to be regenerated because they are actually only part of the cell, not the entire cell. If this is true, then if the glomerulus were destroyed, would that permanently destroy the sense of smell? Or would the glomerulus be capable of regeneration, like the olfactory receptor cells that feed into it?
Comments
Posted by: Spaulding | December 3, 2007 4:11 PM
Very interesting stuff! How would this compare to the olfactory structures and genes of other mammals?
Posted by: John Emerson | December 3, 2007 4:24 PM
Off topic but interesting.
Posted by: Ichthyic | December 3, 2007 4:28 PM
Another cool thing I learned was that every time someone blows their nose, they are losing part of their brain.
damnit! I knew those allergies of mine were slowly rotting my brain.
now I have the mechanism...
:p
Posted by: Drerio | December 3, 2007 5:16 PM
You also can regenerate your taste buds. And in addition to the receptor cells being regenerated the olfactory bulb where the receptors send their signal also regenerates very well in mammals (it's one of the few sites of major adult neuron incorporation... they are born deep inside the brain and migrate through the rostral migratory stream out to the olfactory system where they functionally integrate).
It turns out that we arerather unusual (as are all mammals) at being such bad regenerators of the rest of our nervous system. PZ's fish can regenerate their eyes and ears in response to damage. They can grow back the sensory cells, the interneurons, motorneurons. They can reform complex networks. We just seem too evolutionarily concerned with stopping cancer to have such great regeneration everywhere (for all intents and purposes the cells that underlie regeneration in fish and amphibians are identical to cells we have, we just have much tighter cell cycle control).
Posted by: ddt | December 3, 2007 5:20 PM
As an anosmic (no sensa smell), I always like learning about how this strange, foreign sense you people have works. And what may have caused my condition, which I barely notice. I don't get a handicapped parking permit out of it, either.
Posted by: John Emerson | December 3, 2007 5:51 PM
DDT: There are certain jobs which would be easier for you. Unfortunately, they're not very good jobs. (E.G., cleaning up week-old human remains).
Posted by: TheBlackCat | December 3, 2007 6:06 PM
@Drerio: Taste buds are epithelial cells, not neurons. They behave in a similar manner to neurons in many way but are not true neurons. Olfactory receptor neurons, as their name suggests, are neurons. Birds are able to regenerate the hair cells in their inner ears, although they are no where near as specialized as the hair cells in the mammalian inner ear.
What I find most interesting about the olfactory system is actually the organization of the olfactory cortex. At least in rats it has a very specific, consistent map where certain smells cause certain patterns of excitation across the bulb. Further, there is lateral inhibition so smells that cause certain areas to become excited can block perception of smells that cause neighboring areas to become excited.
Posted by: Hazard | December 3, 2007 6:47 PM
I had a psychology professor that lost her sense of smell after she was in a car accident. She hit her head in such a way that the bone seperating the olfactory bulb from the receptors in the nose shifted forward, and sliced the two apart. Weirdest car-accident handicap I've come across.
Posted by: Rachel I. | December 3, 2007 7:15 PM
DDT: Me too! And I was so bummed that I couldn't find any scholarships for anosmic people.
This info makes it even harder for me to understand how anosmia works, though... My sinuses are shaped about right, so it's not a lack of airflow or anything. If there's 1-2k genes for receptor proteins, then that ain't it either. Maybe my receptor cells don't regenerate, and I blew them all out as an infant? Or some other gene failed to put receptors there in the first place?
Posted by: Don Wilson | December 3, 2007 7:34 PM
BlackCat - just to clarify there is a nice stereotyped spatial pattern of neural activity in the olfactory bulb for each odor, but not so much in the olfactory cortex. The receptors and patterns in olfactory bulb represent odor features, not "odors" as we perceive them. Thus, there is no receptor or glomerulus for coffee. Rather, coffee odor is composed of literally hundreds of different features, each recognized by different receptors and thus evoking a unique pattern of activity in the olfactory bulb. The olfactory cortex is a site for convergence of all these features. The cortex learns frequently-experienced patterns (e.g., coffee, merlot, feces) and uses those memories to match new patterns as we smell them. I had a recent short "ask the experts" article on some of this in Sci Am:
http://www.sciam.com/article.cfm?id=experts-olfactory-neuron-turnover
Good post
Posted by: cm | December 3, 2007 8:22 PM
Yet two other hip facts about the olfactory system:
1) The glomerulus has some "dendrodendritic" synapses, meaning the dendrite of one neuron makes a synaptic connection onto the dendrite of another neuron. Usually we think of axodendritic or axosomatic connections (axons -> dendrite or axon -> cell body).
2) The mitral cells of the olfactory bulb were named that by the "father of neuroscience", Santiago Ramon y Cajal, because they looked to him like the special hats that bishops wear called mitres (mitras in Spanish).
Posted by: ddt | December 3, 2007 9:55 PM
Innnnteresting about all the other anosmic people! (Not that I use that term often; usually I just say "I can't smell".)
I did have some sinus surgery when I was very young (and more when I was an adult), but no-one can remember asking me if I could smell things before the surgery. So this could be the result of the surgery, or could be congenital.
One cool side effect of all that surgery: once I got a head x-ray and it totally baffled both the radiologist and the internist. They put it up on the light board thingie, traced where my sinuses should be, hmmmed about it, turned it over, looked at it again, turned it back, then sloooowly turned to look at me, as though they'd found proof I was an alien impostor. Who knows?
Posted by: Drerio | December 3, 2007 10:14 PM
@BlackCat
You are right that the taste buds and hair cells are epithelial cells, but in fact all of the nervous system is derived from epithelial cells. For example, the retina is both an epithelium and full of neurons. Many animals can regenerate the photoreceptors, the interneurons, and the output neurons.
The olfactory receptor neurons, like the taste receptors, are routinely traumatized by environmental factors and need to be replaced. The mechanisms of their turnover are very similar, as in fact, are the mechanisms for hair cell regeneration, retinal regeneration, etc.
Posted by: Drerio | December 3, 2007 10:16 PM
Oh, and while the birds auditory hair cells aren't as specialized as ours, their vestibular hair cells are very similar. They regenerate those too. We don't.
Posted by: MyPOV | December 3, 2007 11:49 PM
I had anosmia for a year due to a concussion (scary when my boys were at the stage of lighting matches and I couldn't smell it) and my net research turned up the idea that nerves severed above the bone structure cannot "regrow" correctly because the holes (and therefore the paths) through the bone are the tiniest in the body.
Additionally, there is a new line of evidence about the lack of regeneration of brain cells affecting smell first.
http://www.npr.org/templates/story/story.php?storyId=7431209
FYI - We know a tongue can taste sweet, salty, sour, bitter, and umani. But it also can perceive mint and pepper. This is important to geriatrics, as older people lose their sense of smell and therefore, their desire to eat.
Posted by: David Marjanović, OM | December 4, 2007 8:30 AM
Birds generally get less cancer than mammals. That's also why they don't all have the same number of neck vertebrae.
BTW, umami. (Probably a typo, but the term is rare, so I prefer posting it the right way.)
Menthol is perceived by a cold receptor, and capsaicin by a pain receptor; no taste involved.
Posted by: FutureMD | December 4, 2007 5:54 PM
Not sure if this was addressed, but olfactory nerves are part of the peripheral nervous system and we can regenerate quite a lot of our peripheral nervous system. Neural bodies are difficult to replace, but mitosis has been shown to occur in peripheral nerve cells. Axons are more easily regenerated provided that the trauma to all the surrounding tissue is not too bad. Regeneration in the central nervous system (brain, spinal cord, and eyes) doesn't happen because the cells that support the neurons scar over and prevent axon migration.
Posted by: Escuerd | December 4, 2007 10:49 PM
I seem to remember learning (back when I was a lab gofer for a biologist who studied olfaction in fruit flies) that humans had on the order of 250 odorant receptors compared to something like 54 in the flies (a quick googling tells me it's more like 400 so far). It was kind of cool to take out and view transgenic fly brains that displayed GFP only in the cells of a particular glomerulus.
I'd also like to add that many animals (like C. elegans) differ from humans in that each olfactory neuron expresses multiple odorant receptors, though each only expresses a small subset of all that are present.
The only experience I can remember in which someone's anosmia was relevant was from even farther back, in high school, when I worked at Subway and one such fellow always had about two thirds of a full bottle of mustard on his sandwich.
Posted by: Keith Douglas | December 5, 2007 8:12 PM
What's the status of knowledge of the human vomeralnasal (sic?) organ? Do we have one?