Welcome to part three of our series on senses! Today is a pretty fun one: smell.
Smell and taste have always particularly interested me because of how related they are. They have also always particularly interested me because of their relation to food, one of my favorite parts of life. So, while today’s story will focus on smell, remember that all of this is related to taste, which I’ll talk about next week.
Alright: what is smell?
At a very basic level, smell is caused by a bunch of different molecules that float around in the air. These volatile molecules are, fittingly enough, called odor molecules. For reasons we’ll get into in a moment, we can’t smell every molecule that exists, but we do have a decent olfactory repertoire (though not compared to some other animals).
But how does an odor molecule translate to an odor? Let’s imagine you’re one of these molecules, the fictitious odor molecule Flibbizine that lives inside potatoes.
Naturally, you are not alone on your potato. You have many other Flibbizine friends, some of whom live on the surface, others of whom live inside. Let’s say you live inside, too.
Now, let’s say some ill-advised person decides to bite into a raw potato. They bring the potato up to their mouth and take a bite. What happens?
First, all your surface friends (Flibbizine, but also any other odor molecules that happen to live on potatoes) waft in the air around the potato. Some of them get breathed in through this genius’s nose, where they disappear (temporarily. You’ll see them again soon).
Next, as the potato is cut open, all the interior odor molecules are released, yourself included. You find yourself in this person’s mouth, getting mashed up by their teeth.
But! You’re a volatile compound, not a soluble one that would stay in their saliva. Instead, you also get breathed in, this time through the mouth. But because bodies are weird, the back of the mouth leads to the throat, which in turn leads up to the nasal passage. You’re back with all your buddies!
Now, the nasal passage is very close to the brain, but no matter how hard someone sneezes, it is very difficult for their brain to come out through their nose. This is in large part thanks to the cribriform plate, which is essentially a bone barrier between the brain and the olfactory epithelium (the lining of the nasal passage).
As you drift along the nasal passage, you notice a bunch of little cells sticking out. These are the olfactory cells, which have lots of little tendrils sticking out, each one with an assortment of proteins that make up a particular shape.
Suddenly, you see one of these protein assemblages that is exactly the reverse of your shape! As you get close, you decide to snuggle into it, and you fit perfectly. The olfactory cell seems rather happy with this, because your snug embrace causes it to trigger a series of signals that shoot all the way up the cell. You notice that the cell is actually quite a bit longer than you’d realized, and it snakes its way up one of many holes in the cribriform plate until it reaches something called the olfactory bulb.
So, this series of signals (called an action potential, the bane of my high school biology existence) gets sent all the way up to the olfactory bulb. But it’s not just your signal that gets sent there. Every other olfactory cell that has come into contact with a Flibbizine molecule sends a signal to the same place, called a glomerulus. If you were to look around the olfactory bulb, you’d see a different glomerulus for every kind of scent molecule humans can sense (which is over 400, I believe). Each glomerulus sends a signal to its friend the tufted cell, and all the tufted cells send their own signals to the brain. Then the brain does its brain stuff to turn those signals into a given smell, but that’s a story for another day.
What happens to you, the Flibbizine molecule, after the signal gets sent? I actually don’t know. If anyone does know, please say something in the comments, because it will solve a very important question that I may someday turn into another Science Story.
But in the meantime, that’s how scent works! Next week we’ll look at its partner in crime, taste. Until then, remember that most of what you’re tasting, you’re actually smelling.
As usual, thank you to Khan Academy for explaining things simply and clearly.