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  • Case study of the day: Olivia, she was a healthy 35-year-old woman.

  • Until one spring day, when she got into a bad bike accident, and suffered serious head

  • trauma. The doctors patched her up, but after a couple of days in the hospital, she noticed

  • something strange was happening.

  • Or, rather, something wasn’t happening - she could no longer smell.

  • Not the flowers in her room, not the nurse’s rubber gloves, not even the horrible hospital food.

  • In the weeks that followed, she blackened a batch of cookies because she couldn’t

  • smell them burning. She couldn’t smell the lilacs blooming, or her husband’s aftershave,

  • or her car overheating. She drank expired milk because she couldn’t taste that it had gone sour.

  • The world got a lot less interesting: eating wasn’t very exciting, and Olivia started

  • getting depressed. Life felt sterile and unfamiliar.

  • Olivia had anosmia -- a partial or complete loss of the sense of smell (and with it, most

  • of her ability to taste).

  • This unfortunate condition is caused by things as diverse as head trauma, respiratory infections,

  • even plain old aging.

  • And I sayunfortunatebecause, what we sense informs who we are.

  • But how we experience our six major special senses all boils down to one thing: sensory

  • cells translating chemical, electromagnetic, and mechanical stimuli into action potentials

  • that our nervous system can make sense of.

  • This process is called transduction, and each sense works in its own way.

  • Our vision functions with the help of photoreceptors, cells that detect light waves, while our senses

  • of touch, hearing, and balance use mechanoreceptors that detect sound waves and pressure on the

  • skin and in the inner ear.

  • But our sense of taste, or gustation, and smell, or olfaction, are chemical senses.

  • They call on chemoreceptors in our taste buds and nasal passages to detect molecules in

  • our food and the air around us.

  • These chemical senses are our most primitive, and our most fundamental. Theyre actually

  • sharpest right at birth, and theyre so innate that newborns orient themselves chiefly

  • by scent. They can not only taste the difference between their mother’s milk and another

  • mom’s, but they can even smell her breasts from clear across the room!

  • Tastes and smells are powerful at activating memories, triggering emotions, and alerting us to danger.

  • They also help us enjoy the small things that make life worth livinglike pizza.

  • All right, I’m about to perform a superhuman feat and sit here with this amazing slice

  • of Hawaiian pizza WITHOUT EATING IT, so that I can describe it to you how we smell things.

  • So if it sounds like I’m going faster during this episode, it’s not like I don’t enjoy

  • our time together; I just want to get to the part where I actually get to eat the pizza.

  • Now, the process starts as I sniff molecules up into my nose. This means that for you to

  • be able to smell something, the odorant must be volatile, or in a gaseous state to get

  • sucked up into your nostrils.

  • And yes, that means when you smell poop there are actual poo particles up in your nose.

  • The harder and deeper you sniff, the more molecules you vacuum up, and the more you

  • can smell it.

  • Most of these molecules are filtered out on the way up your nasal cavity, as they get

  • caught by your protective nose hairs, but a few make it all the way to the back of the

  • nose and hit your olfactory epithelium.

  • This is your olfactory system’s main organ -- a small yellowish patch of tissue on the

  • roof of the nasal cavity. The olfactory epithelium contains millions of bowling pin-shaped olfactory

  • sensory neurons surrounded by insulating columnar supporting cells.

  • So these airborne pizza molecules -- many of which are just broken-off parts of fats

  • and proteins -- land on your olfactory epithelium and dissolve in the mucus that coats it.

  • Once in the mucus, theyre able to bind to receptors on your olfactory sensory neurons,

  • which, assuming they hit their necessary threshold, fire action potentials up their long axons

  • and through your ethmoid bone into the olfactory bulb in the brain.

  • But here’s the wonder of specialization for you: Each olfactory neuron has receptors

  • for just one kind of smell.

  • And any given odorant, like this pizza, is made up of hundreds of different chemicals

  • that you can smell, like the thymol of the oregano, the butyric acid of the cheese, and

  • the acetylpyrazine of the crust.

  • So, after each smell-specific neuron is triggered, the signal travels down its axon where it

  • converges with other cells in a structure called a glomerulus.

  • This takes its name from the Latin word glomus, meaningball of yarn” -- which is what

  • it looks like, a tangle of fibers that serves as a kind of a transfer station, where the

  • nose information turns into brain information.

  • Inside the glomerulus, the olfactory axons meet up with the dendrites of another kind

  • of nerve cell, called a mitral cell, which relays the signal to the brain.

  • So for each mitral cell, there are any number of olfactory axons synapsing with it, each

  • representing and identifying a single volatile chemical.

  • As a result, every combination of an olfactory neuron and a mitral cell is like a single

  • note, and the smell coming off of this pizza triggers countless of those combinations,

  • forming a delicious musical chord of smells.

  • Now just imagine a piano with thousands of keys able to produce millions of unique chords,

  • and youll get an idea of how amazing our noses are.

  • Scientists estimate that our 40 million different olfactory receptor neurons help us identify

  • about 10,000 different smells, maybe even more.

  • So, once a mitral cell picks up its signal from an olfactory neuron, it sends it along

  • the olfactory tract to the olfactory cortex of the brain. From there the pizza-smell hits

  • the brain through two avenues:

  • One brings the data to the frontal lobe where they can be consciously identified, like oh,

  • melted mozzarella; while the other pathway heads straight for your emotional ground control

  • -- the hypothalamus, amygdala, and other parts of the limbic system.

  • This emotional pathway is fast, intense, and quick to trigger memories. If the odor is associated

  • with danger, like the smell of smoke, it quickly activates your sympathetic system’s fight or flight response.

  • That’s a big reason that Olivia’s anosmia was so problematic -- without being able to

  • smell, she couldn’t access emotional memories wrapped up in particular scents, or sniff

  • out dangers in her environment.

  • And these same intellectual and emotional dynamics apply to taste, as well. Because

  • after all, taste is 80 percent smell.

  • As you chew your food, air is forced up your nasal passages, so your olfactory receptor

  • cells are registering information at the same time as your taste receptors are, so youre

  • both smelling and tasting simultaneously.

  • So, it’s true that if you have a bad cold, or if you just hold your nose, your sense

  • of taste is impaired. But it’s not like you can’t taste anything -- it’s just that

  • more subtle flavors involve more volatile compounds that are picked up by your olfactory receptors.

  • So you can hold your nose and taste that something is sweet, but you wouldn’t be able to pinpoint

  • it as being carmelized sugar. Likewise, you can taste that something’s generally sour,

  • but you can’t tell the difference between a lemon and a lime.

  • When I read this script I didn’t think it was going to be so difficult to do this, but

  • it is very hard and I am getting very hungry and I would like to get to the part where

  • I get to eat the pizza!

  • We are at the point, everyone where I get to--

  • So, as soon as I take a bite, all of the sensory information in there is quickly sorted by

  • the ten thousand or so taste buds covering my tongue, mouth, and upper throat.

  • Most taste buds are packed deep down between your fungiform papillae -- those little projections

  • that make your tongue kinda rough. You can actually see them if you look in the mirror.

  • Those papillae are not your taste buds.

  • Speaking of what and where your taste buds really are, you know what I could go for right about now?

  • A DEBUNKING!

  • Youre probably familiar with those taste maps of your tongue from elementary school?

  • Well un-familiarize yourself, because they are bogus.

  • Those tongue diagrams date back to the early 1900s, when German scientist D.P. Hanig tried

  • to measure the sensitivity of different areas for salty, sweet, sour, and bitter. The resulting

  • map was very subjective -- pretty much just relfecting what his volunteers felt like they were sensing.

  • While it’s true that our taste sensations can be grouped into sweet, salty, sour, bitter,

  • and the more recently recognized umami, the notion that our tongues detect these tastes

  • only in certain areas is just wrong.

  • By the 1970s research showed that any variations in sensitivity around the tongue were insignificant,

  • and that all tastes register in all parts.

  • You can test this for yourself: put salt on the tip of your tongue and you can still taste

  • it, even though Hanig’s map says you shouldn’t be able to.

  • Now, back to your taste buds.

  • Theyre actually tucked into tiny pockets hidden behind the stratified squamous epithelial

  • cells on your tongue.

  • Each bud has 50 to 100 taste receptor epithelial cells which register and respond to different

  • molecules in your food.

  • Notice that these are specialized epithelial cells, not nervous tissue, so they still have

  • to synapse to sensory neurons that carry information about the type and amount of taste back to your brain.

  • These epithelial receptor cells come in two major types -- gustatory -- or the kind that

  • actually do the tasting, and basal -- the stem cells that replace the gustatory cells

  • after you burn them on a lava-hot melty cheesy Hot Pocket.

  • Basal epithelial cells are extremely dynamic and replace the gustatory cells every week

  • or so, which is why even a terribly burned tongue will feel better in a couple of days.

  • Every gustatory cell projects a thread-like protrusion of the cellular membrane called

  • a gustatory hair, which runs down to a taste pore, a small hole in the stratified squamous

  • epithelium covering the taste bud and the rest of the tongue.

  • In order to taste a bite of pizza, those food chemicals, or tastants, must dissolve in saliva

  • so they can diffuse through those taste pores, and bind to receptors on those gustatory cells,

  • and then trigger an action potential.

  • And each tastant is sensed differently.

  • For example, salty things are full of positively-charged sodium ions that cause sodium channels in

  • the gustatory cells to open, which generate a graded potential, and spark an action potential.

  • Meanwhile, sour-tasting acidic foods are high in hydrogen ions and take a different route

  • by activating proton channels.

  • So taste, like all our senses, is all about how action potentials get triggered.

  • Once an action potential is activated, that taste message is relayed through neurons via

  • the seventh, ninth, and tenth cranial nerves to the taste area of the cerebral cortex,

  • at which point your brain makes sense of it all, and begins releasing digestive enzymes

  • in your saliva and gastric juices in your stomach to help you break that food down so you can use it.

  • So. You know what I learned today?

  • I learned that it is incredibly hard to spend ten minutes with a piece of pizza in your

  • hand, and only be able to take one bite because youre talking all the time.

  • Incredibly hard. So I earned this.

  • But you learned the anatomy and physiology of smell, starting with the olfactory sensory

  • neurons, each of which contains a receptor for a particular scent signal. After leading

  • to a glomerulus, these neurons synapse with mitral cells, which go on to send signals to the brain.

  • Taste begins with taste receptor epithelial cells, rather than nervous cells, where tastants

  • bind to receptors that trigger action potentials to four different cranial nerves that tell you: PIZZA.

  • Thanks for joining me for this tasty episode. And Big thanks to our Headmaster of Learning,

  • Thomas Frank, whose generous contribution on Patreon helps keep Crash Course alive and

  • well for everyone. Thank you, Thomas. If you want to help us keep making great videos like

  • this one, check out Patreon.com/CrashCourse