Today's is the first in a series of posts on the CRANIAL NERVES, the nerves that innervate a good chunk of your body, and which emerge DIRECTLY from your brain. We're going in order from the "front" of the brain, "brackward". So today's post is about the cranial nerve that's the closest to your face, the olfactory.
Take a nice, BIG sniff. What do you smell? Sci smells coffee, some nice spring air outside, car exhaust (ah, city living), my deodorant...you know, the basics. Whatever it is YOU smelled...well I hope it wasn't gross.
The reason you can smell what you're smelling is entirely due to the glory of your olfactory nerve. So today's post is, in a way, a salute to your nose. 'Cause it KNOWS.
While it's not as important to us as it is to say, dogs, who rely on smell a lot more than we do, the olfactory system is still a thing of awesome. Humans can tell the difference between THOUSANDS of different scents, as high as 10,000 or more in people who are trained for things like perfume, wine, or cheese (or coffee. Anyone wanna hire Sci as a sniffer of coffee??). And the olfactory system is also interesting because it's one of the two cranial nerves (the other is gustatory) that rely heavily on chemosensing, or the direct sensing of chemicals in the environment. In this case, the chemicals you're sensing are the ones in the air (and yes, this does mean that when you smell poo...you are smelling chemicals coming DIRECTLY from the poo. Gross, huh?!).
For the olfactory nerve, we're going to start on the outside and move in. And the outside means your NOSE.
You're looking here at a cutaway of the human schnozz, with the nostrils toward the bottom right corner. Air (and the chemicals in it from that dog poo you just stepped in) goes up into your nostrils and nose, and hits the olfactory epithelium, a little patch of cells on the roof and wall of the inside of the nose, represented here in grey. This olfactory epithelium is made of three types of cells: olfactory receptor neurons, support cells, and basal cells. It's rather self-explanatory what the support cells do, so we're gonna let them finish, but basal cells and olfactory receptor neurons are really where it's at. Olfactory receptor neurons are true neurons, and, unlike most of the neurons you will generally encounter, can and do regenerate frequently. So when you burn out your nose hard on that pixie stick you inhaled (what, you've never done that?), don't worry, it'll come back. Honest. And the way it comes back is through the basal cells, which form the membrane of the olfactory epithelium, and which can divide and become olfactory neurons and supporting cells when they are needed.
Here's a microscope view of the olfactory epithelium:
On the right you get a good view of the olfactory receptor neurons. These are typical bipolar neurons, with a single dendrite coming out one end and poking through the epithelium into your nasal cavity, a cell body, and then a LONG axon which joins up with others to become...your olfactory nerve.
The dendrites here are super cool, though. They extend out to this little bulb called the olfactory vesicle, and from there all these little cilia spring out, protected by a layer of mucus. These cilia are actually pretty visible, or at least the layer covering them is.
The chemicals that are in the air around you will then diffuse across the mucus membrane and hit the cilia, stimulating the olfactory receptor neuron. Each olfactory receptor neuron makes a specific kind of olfactory receptor protein (we think it's just 1:1, but we could be wrong), and we have over 1000 genes for olfactory proteins to chose from (srsly it makes up more than 1% of our genome. The nose KNOWS). Because each neuron only makes one (though there can be more than one olfactory receptor neuron with that protein, in fact there are usually thousands), this means our sense of smell can be extremely specific. All the receptors here are G-protein coupled receptors (something which I will HAVE to blog about sometime, because they are really cool). Each one binds a limited array of odorants, and no two are entirely overlapping. So this means that by stimulating a few of them, you can get enough overlap to determine just WHAT that is that you're sniffing. The receptors, when stimulated, will start a cascade of proteins which will finally produce an action potential, stimulating the neuron.
When the neuron is stimulated, an action potential will go up the axon. And it goes kind of slowly. Olfactory receptor neurons have no myelin to help conduct the signal, and the axon fibers themselves are long and thin, which means the action potentials move slowly (fatter axons produce faster conduction). So the good news is that, if it's dog poo you're smelling, you get a little bit of reprieve. But since the conduction is still at 0.1 m/s, it's not THAT much (maybe half a second).
The axons collect together and head up toward the brain, passing through something called the cribriform plate, the bone above the roof of your nasal cavity. This place is full of little holes and looks kind of like a sponge. The holes allow the axons of your olfactory epithelium fibers to pass through, and then they synapse on to your olfactory BULB, which is the big bulb labeled in the cranial nerve drawing. Believe it or not, that bulb, which looks like a big feature in the human brain, is RIDICULOUSLY small. Seriously, look at this.
That's a sheep brain, the front is to the right, and those TWO HUGE PIECES on the front are the olfactory bulbs (labeled B). To give you an idea of scale, the sheep brain is about 1/8 of the size of the human brain. Those olfactory bulbs alone are 2-3 times bigger than yours. Our sniffers are NOTHING.
In the olfactory bulb the olfactory receptor neurons will synapse onto the dendrites of mitral cells, these pyramid shaped things whose axons make up the olfactory tract (isn't it SO COOL to look at that photo above of the sheep brain and realize that whole piece of tissue coming from the olfactory bulb is made of AXONS?! IT"S ALL AXONS!! Doesn't that BLOW YOUR MIND!?). The mitral cells have their dendrites arranged into large trees called glomeruli which get a TON of information, you can get thousands of olfactory receptor neurons (all of which express the same kind of receptor protein) synapsing onto only one or two glomeruli (though the glomeruli can be made of many mitral cells). The olfactory receptor neurons with a particular odorant receptor protein will synapse on to only ONE kind of glomeruli, meaning that you get identity of the odor in which mitral cells are activated.
The mitral cells (and smaller tufted cells, which are dinky but do basically the same thing), send their axons in a bundle called the olfactory tract to the olfactory cortex. Now, if you're familiar with neuroanatomy, you'll note that I MISSED something there. That something is the thalamus, the great relay center of the brain. That's because the olfactory system doesn't HIT the thalamus, and this is the ONLY cranial nerve which misses it (though in higher level processing, correlating smells with memories, etc, it will go through the thalamus). This means three things:
1) The olfactory system misses the thalamus because it arises from a tissue called the telencephalon, the developmental tissue that will give rise to the forebrain (and your olfactory bulbs!) in development.
2) Without the thalamus, the olfactory tracts do not CROSS each other. This means that a smell in the right nostril will go to the right side of the brain, and a smell in the left nostril will go to the left side. This is called ipsilateral (same side) connectivity, and is a lot more rare than you'd think (wait til we get to the optic nerve...).
3) The final thing that this means is that the question about which cranial nerve bypasses the thalamus (or ends up ipsilateral, etc), is a VERY popular exam question. Exam takers, take note!
SO. We have olfactory receptor neurons synapsing onto mitral cells which are bunched together in glomeruli, and which send their axons along to the olfactory tract. This heads into the brain, and projections heads off to the orbital frontal cortex, while the main projections head off as the lateral olfactory tract. This tract will head under and around the brain until it ends up at the uncus. There, the fibers are going to split off and terminate into the primary olfactory cortex and the amygdala. The final termination point is here:
There are several bits of cortex that make up the primary olfactory cortex, and not everyone agrees on what they all SHOULD be...but anyway. There you have connections to the primary olfactory cortex and the amygdala (known mostly for emotional processing). Though we think of cranial nerve 1 as "ending" there, these area will take that information from your nose and project it to other areas for processing, like the hippocampus, the rest of the amygdala, or back through the thalamus for higher processing, as you wonder "what IS that smell?" and think to check your shoe.
Now, smell isn't AS important to humans as it is to other animals, but it's important in one area you wouldn't think of so much: it's an integral part of TASTE. The way something tastes to us is made up largely of how it smells, and when people have major problems with smell, they usually complain of being unable to TASTE things. Think of how, when you have a cold, most things taste kind of bland? It's cause your nose ain't at its best.
So take a deep breath, and appreciate your olfactory system. Or, if you just stepped in dog poo...maybe not.