Taste & Smell: Crash Course Anatomy & Physiology #16

Summary

This 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

00:00

🚴‍♀️ 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.

05:03

👃 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.

10:04

🍕 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.

Keywords

💡Anosmia

Anosmia is a medical condition characterized by the partial or complete loss of the sense of smell. It is a significant aspect of the video's theme as it affects not only the ability to smell but also the sense of taste, which is largely dependent on the sense of smell. In the script, Olivia's experience with anosmia after a bike accident illustrates the profound impact this condition can have on a person's life, making everyday activities like eating less enjoyable and leading to depression.

💡Transduction

Transduction in the context of the video refers to the process by which sensory cells convert chemical, electromagnetic, and mechanical stimuli into action potentials that the nervous system can interpret. This concept is central to understanding how our senses work. For example, the script explains that sensory cells translate various stimuli into signals that the brain can process, which is fundamental to experiencing the world around us.

💡Olfactory epithelium

The olfactory epithelium is the main organ of the olfactory system, located at the back of the nasal cavity. It contains millions of olfactory sensory neurons that are crucial for the sense of smell. In the script, the olfactory epithelium is described as a yellowish patch of tissue where airborne molecules from scents like pizza or lilacs dissolve in mucus and bind to receptors on sensory neurons, initiating the process of smell.

💡Chemoreceptors

Chemoreceptors are specialized cells that detect chemical changes in the environment. In the context of the video, they are found in our taste buds and nasal passages and are responsible for detecting molecules in food and air, which are essential for the senses of taste and smell. The script emphasizes that these chemical senses are our most primitive and fundamental, highlighting their importance in our sensory experiences.

💡Glomerulus

A glomerulus is a structure in the olfactory bulb where olfactory axons meet with the dendrites of mitral cells. It serves as a transfer station, converting nose information into brain information. The script uses the metaphor of a 'ball of yarn' to describe its appearance and function, illustrating how signals from various olfactory neurons converge in the glomerulus before being relayed to the brain.

💡Mitral cell

Mitral cells are a type of nerve cell in the olfactory system that receive signals from olfactory neurons and relay them to the brain. Each mitral cell can be connected to multiple olfactory axons, each representing a single volatile chemical. The script explains that the combination of an olfactory neuron and a mitral cell is akin to a single note in a musical chord, with the overall smell being a complex combination of these notes.

💡Taste buds

Taste buds are sensory structures that allow us to perceive taste. Contrary to popular misconceptions, they are not located on the surface of the tongue but are tucked into tiny pockets. Each taste bud contains taste receptor cells that respond to different molecules in food. The script debunks the myth of the tongue's taste map and explains that taste buds are responsible for detecting tastes like sweet, salty, sour, bitter, and umami.

💡Gustatory cells

Gustatory cells are specialized epithelial cells within taste buds that are responsible for the actual tasting process. They have gustatory hairs that extend to taste pores, where they can interact with dissolved food chemicals. The script describes how these cells are different from nervous tissue and must synapse with sensory neurons to transmit taste information to the brain.

💡Tastants

Tastants are chemical compounds in food that stimulate the taste receptors on gustatory cells, leading to the perception of taste. The script explains that in order to be tasted, these compounds must dissolve in saliva and bind to receptors on gustatory cells, triggering action potentials that the brain interprets as different tastes.

💡Cranial nerves

Cranial nerves are part of the peripheral nervous system and are responsible for various sensory and motor functions in the head. In the context of the video, the seventh, ninth, and tenth cranial nerves are mentioned as the pathways through which taste information is relayed from the taste buds to the brain. This is an essential part of how we perceive and process taste.

💡Sensory neurons

Sensory neurons are specialized nerve cells that convert sensory stimuli into electrical signals that can be interpreted by the brain. In the script, olfactory sensory neurons are highlighted for their role in detecting specific scent signals and transmitting them to the brain via the olfactory bulb. These neurons are crucial for the sense of smell and, by extension, the sense of taste.

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.