Taste & Smell: Crash Course A&P #16


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 say “unfortunate” because, 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. They’re actually sharpest right at birth, and they’re 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 living…like 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, they’re 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,
meaning “ball 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 you’ll 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 you’re 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! You’re 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. They’re 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
you’re 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 This episode was filmed in the Doctor Cheryl
C. Kinney Crash Course Studio. It was written by Kathleen Yale, edited by Blake de Pastino,
and our consultant, is Dr. Brandon Jackson. Our director is Nicholas Jenkins, the script
supervisor and editor is Nicole Sweeney, our sound designer is Michael Aranda, and the
graphics team is Thought Café.

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