Taste & Smell: Crash Course Anatomy & Physiology #16
Summary
TLDRThis script narrates the story of Olivia, who lost her sense of smell and taste due to a bike accident, leading to anosmia. It delves into the science of sensory perception, explaining how our chemical senses are crucial for memory, emotion, and danger detection. The script explores the anatomy of the olfactory system, the role of olfactory neurons and mitral cells, and how our sense of taste is intricately linked to our ability to smell, debunking the myth of the tongue's taste map and detailing the process of taste bud activation.
Takeaways
- 🚑 Olivia's accident: A 35-year-old woman named Olivia lost her sense of smell after a bike accident, leading to anosmia and affecting her daily life and emotions.
- 🌸 Anosmia: The condition of losing the sense of smell, which can be caused by various factors including head trauma, respiratory infections, and aging.
- 🔬 Sensory transduction: The process by which sensory cells convert different types of stimuli into action potentials for the nervous system to interpret.
- 👃 Olfaction: The sense of smell, which involves chemoreceptors detecting molecules in the air and is vital for memory, emotion, and safety.
- 🍕 The pizza example: Describes the process of smelling by sniffing pizza molecules, illustrating how volatile substances are necessary for the sense of smell.
- 🧠 Olfactory system: The journey of smell from the olfactory epithelium to the olfactory bulb and then to the brain, where signals are processed and identified.
- 🍰 Specialization of neurons: Each olfactory neuron is specialized to detect one type of smell, contributing to the complex identification of numerous odors.
- 🎹 Smell as a chord: The comparison of the sense of smell to playing a piano, with each combination of olfactory neuron and mitral cell representing a unique note.
- 🍏 Gustation and olfaction: The interplay between taste and smell, where taste is largely influenced by the sense of smell, especially when chewing food.
- 🗺️ Debunking the taste map: The myth of the tongue map that assigns specific tastes to different areas of the tongue is debunked, as all tastes can be sensed across the entire tongue.
- 🔍 Taste bud anatomy: The location and function of taste buds, which contain receptor cells that respond to different molecules in food and send signals to the brain.
Q & A
What condition did Olivia develop after her bike accident?
-Olivia developed anosmia, a partial or complete loss of the sense of smell.
How does anosmia affect a person's ability to taste?
-Anosmia affects a person's ability to taste because taste is 80 percent smell. Without the sense of smell, subtle flavors that rely on volatile compounds are not detected.
What are the two main chemical senses in humans?
-The two main chemical senses in humans are taste (gustation) and smell (olfaction).
What are chemoreceptors and where are they found?
-Chemoreceptors are sensory cells that detect molecules in food and air. They are found in the taste buds and nasal passages.
Describe the process of transduction in sensory cells.
-Transduction is the process by which sensory cells translate chemical, electromagnetic, and mechanical stimuli into action potentials that the nervous system can interpret.
How do olfactory sensory neurons function?
-Olfactory sensory neurons have receptors for specific smells. When odorant molecules bind to these receptors, the neurons fire action potentials that travel to the olfactory bulb in the brain.
What role do mitral cells play in the sense of smell?
-Mitral cells relay the signal from olfactory neurons to the brain. They receive signals at the glomerulus and send them along the olfactory tract to the olfactory cortex.
How do taste buds detect different tastes?
-Taste buds contain gustatory cells that detect molecules in food. These cells have receptors that bind to tastants, triggering action potentials that send taste information to the brain.
What is the significance of gustatory hairs in taste buds?
-Gustatory hairs are thread-like projections from gustatory cells that extend to taste pores. They detect dissolved food chemicals and trigger the sensory response for taste.
What was debunked about the traditional taste map of the tongue?
-The traditional taste map of the tongue, which suggested that different areas of the tongue detect specific tastes, was debunked. Research showed that all tastes register in all parts of the tongue.
Outlines
🚴♀️ Case Study: Olivia's Anosmia Journey
Olivia, a 35-year-old woman, suffered head trauma from a bike accident, resulting in anosmia (loss of smell). She couldn't smell or taste, impacting her life significantly. This case illustrates how sensory cells translate stimuli into action potentials, a process known as transduction, and how it affects our perception of the world.
👃 The Olfactory Process
The script describes how the sense of smell works. Odorant molecules, like those from pizza, enter the nose, bind to receptors on olfactory sensory neurons, and send signals to the brain through a complex pathway involving the olfactory epithelium, glomeruli, and mitral cells. This process allows us to detect thousands of different smells, which are crucial for both conscious identification and emotional responses.
🍕 The Anatomy and Physiology of Taste
The taste process involves taste receptor epithelial cells, which register molecules in food. Different taste sensations (sweet, salty, sour, bitter, umami) are detected by these cells and sent to the brain via cranial nerves. The summary also debunks the myth of the tongue map, explaining that all taste sensations are detected across the tongue. This detailed explanation underscores how taste and smell work together to enhance our perception of flavors.
🎬 Credits and Acknowledgments
The final paragraph lists the contributors to the script and video production, including the writer, editor, consultant, director, script supervisor, sound designer, and graphics team. This segment acknowledges their roles in creating the educational content.
Mindmap
Keywords
💡Anosmia
💡Transduction
💡Olfactory epithelium
💡Chemoreceptors
💡Glomerulus
💡Mitral cell
💡Taste buds
💡Gustatory cells
💡Tastants
💡Cranial nerves
💡Sensory neurons
Highlights
Olivia, a 35-year-old woman, suffered from anosmia, a loss of the sense of smell, after a bike accident.
Anosmia can be caused by head trauma, respiratory infections, or aging.
Olivia experienced a diminished sense of taste along with her loss of smell, as taste is 80% smell.
The loss of smell affected Olivia's quality of life, leading to depression and a less interesting world.
Our senses are transduced by sensory cells into action potentials for the nervous system to interpret.
Chemical senses like taste and smell use chemoreceptors to detect molecules in food and air.
Newborns have sharp chemical senses, using scent to orient themselves and distinguish their mother's milk.
Tastes and smells are powerful in activating memories, emotions, and alerting to danger.
The process of smelling involves sniffing volatile molecules into the nose and up to the olfactory epithelium.
Olfactory sensory neurons in the epithelium have receptors for specific smells, leading to action potentials.
Smell signals are processed in the brain through the olfactory bulb, cortex, and emotional pathways.
Anosmia prevents access to emotional memories associated with scents and the ability to detect environmental dangers.
Taste buds, not visible on the tongue's surface, are hidden in pockets and contain receptor cells for different tastes.
Tongue maps indicating specific taste zones are outdated and incorrect.
Taste receptor cells are replaced regularly by basal cells, allowing for quick recovery from damage.
Tastants must dissolve in saliva to bind to taste receptors and trigger neural signals to the brain.
Different tastes activate taste cells through specific channels, like sodium for salty and proton channels for sour.
The brain interprets taste signals through the cerebral cortex, initiating the digestive process.
Transcripts
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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