Introduction & Content
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Taste is one of our special senses, and it begins the moment you place food on your tongue.
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The chemicals in that food interact with clusters of sensory cells called taste buds,
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and these taste buds sit inside something called papillae spread across the tongue.
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Each taste bud contains several types of cells that detect sweet, sour, salty, bitter, and umami.
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Once they’re activated, they send signals through three different cranial nerves before the brain
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interprets them — and the brain combines that information with smell and even the trigeminal
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system to form what we recognise as flavor. So how does the tongue actually detect taste?
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In this video, we’re going to break the entire system down step-by-step:
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We’re first going to talk about the Macro Anatomy of the Tongue, take a visual look,
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and understand the different papillae on the tongue, where they’re found,
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and which ones actually contain taste buds. Then we’ll zoom into the Microanatomy of Taste
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Buds, we’ll go inside a taste bud and see the specialised cells that detect chemicals from food.
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And while doing that, we’ll understand the Five Taste Modalities, sweet, sour,
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salty, bitter and umami, and what each one actually responds to in detail.
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And we’re going to see How Taste Is Detected, the exact mechanism
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of how molecules dissolve in saliva, reach the receptors, and activate taste cells.
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And of course understand the Nerve Supply and Pathway to the Brain, which cranial nerves
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carry taste from different parts of the tongue, and how the signal reaches the gustatory cortex.
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In the end we’ll see How Taste Combines with Smell and Trigeminal Input — to create the
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full experience we recognise as flavor, and why food tastes different when you’re congested.
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What’s up everyone, my name is Taim. I’m a medical doctor, and I make animated medical
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lectures to make different topics in medicine visually easier to understand. If you’d like a
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PDF version or a quiz of this presentation, you can find it on my website, along with organized
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video lectures to help with your studies. Alright, let’s get started.
Macro Anatomy of the Tongue
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Let’s start by looking at the tongue from an upper view. Just for orientation: here we can
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see the Epiglottis, Lingual Tonsil, the Palatine tonsil between the arches, and we can see the
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terminal sulcus, which divides the tongue into an anterior two-thirds, and a posterior one-third.
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Now, when you look at the surface of the tongue, it’s not smooth at all.
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It’s covered in small bumps called papillae, and these papillae give the tongue its rough
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texture. Some of them help you grip and move food around, while others contain the taste buds that
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detect chemicals from what you eat. To understand how taste works, it’s important to get familiar
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with these different papillae, because each type sits in a specific region and has its own role.
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There are four types. The vast majority of the bumps you see when you stick out your tongue are
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located across the anterior two-thirds, and these are the filiform papillae. Near the tip and sides,
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you find the fungiform papillae scattered between them. Along the posterolateral borders are the
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foliate papillae arranged as vertical folds. And right at the back, just in front of the
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terminal sulcus, you have the large circumvallate papillae lined up in a characteristic V-shape.
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Let’s now mark one section of the tongue, and perform a little surgery,
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by cutting this portion out, and then zoom in. We can now clearly see the different papillae
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and how densely they cover the surface. As you see, the vast majority are the
Filiform Papillae
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filiform papillae. These are thin, cone-shaped projections that cover most of the anterior
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two-thirds of the tongue. Even though they’re the most common, they don’t contain any taste
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buds. Their job is purely mechanical: they help you manipulate food, they increase friction,
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and they contribute to the rough feel of the tongue. In many animals, especially cats,
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these filiform papillae are much more pronounced and even keratinised, allowing them to groom,
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clean, and strip meat from bone. In humans, they are far smaller and purely supportive
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compared to those specialised functions. Alright next, mixed in between the filiform
Fungiform Papillae
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papillae, especially near the tip and the sides of the tongue, are the fungiform papillae. These ones
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are slightly larger and rounder, and they really do resemble tiny mushrooms sitting on the surface.
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Each fungiform papilla contains a few taste buds on its upper surface. They are responsible for a
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lot of the taste sensation in the anterior part of the tongue. If you’ve ever noticed slightly larger
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reddish dots near the front of your tongue, that’s essentially what you’re looking at.
Foliate Papillae
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Moving further back along the sides of the tongue, you’ll find the foliate papillae.
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These appear as a series of vertical folds. In humans, they’re most active during childhood
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and gradually become less prominent with age, but they still contain taste buds located deep within
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the clefts between the folds. Even though they aren’t as visually obvious as the other types,
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they contribute to taste detection along the posterolateral regions of the tongue.
Circumvallate Papillae
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And finally, at the very back of the tongue, just in front of the terminal sulcus,
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are the circumvallate papillae. These are the largest papillae and form a V-shaped line pointing
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toward the throat. Each one looks like a dome surrounded by a circular pit, and the walls of
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that pit are packed with hundreds of taste buds. Now, even though most taste buds are within
Other Taste Buds
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papillae on the tongue, you can also find small numbers in the soft palate, the epiglottis,
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and parts of the pharynx and larynx. These don’t sit inside papillae, they lie directly
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in the epithelium and they primarily help detect bitter substances before swallowing. This is
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part of the body’s protective system, because it allows potentially harmful or unpleasant-tasting
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chemicals to be detected even moments before they enter the esophagus and then down to your stomach.
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So when we put all of this together, the surface of the tongue is made up of filiform papillae,
Papillae Summary
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which provide texture but no taste; fungiform papillae near the front, which contain a small
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number of taste buds; foliate papillae along the sides, which contain taste buds within their
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folds; and circumvallate papillae at the back, which contain large concentrations of taste buds.
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And most importantly, all regions that contain taste buds are capable of detecting all five
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taste modalities. For many years it was thought that the surface of the tongue had special areas
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for each of these sensations, but it is now known that all tastants are sensed from all
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parts of the tongue and adjacent structures. Alright, we’ve now seen the papillae from
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the outside, and we said that three of these papillae types—fungiform, foliate,
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and circumvallate—contain taste buds. Those taste buds are structurally the same no matter where
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they sit, but their density varies depending on the region. That’s why the back of the tongue,
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especially around the circumvallate papillae, has a much higher concentration compared to the
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tip. Makes sense so far? Good. Let’s zoom into one of these papillae so we can finally see the
Taste Buds
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actual organ of taste: the taste bud. A taste bud is essentially a small,
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oval collection of 50-100 special epithelial cells responsible for detecting taste,
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called taste receptor cells or gustatory cells. And they are tightly packed together with a tiny
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opening at the surface called the taste pore. This pore is simply where dissolved chemicals
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from food come into contact with the taste cells inside. And the “dissolved” part really matters,
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because taste only works when molecules mix with saliva. If the tongue is dry,
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taste detection drops dramatically. Once something dissolves, it can diffuse through the taste pore
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and reach the upper parts of the taste cells. Inside each taste bud, there are four main cell
Cell Types in Taste Buds
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types: Type I, Type II, and Type III, plus a group of basal cells at the bottom. Type I cells are
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glial-like support cells—they help with structural organization and clean up neurotransmitters after
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signaling. Type II cells detect sweet, bitter, and umami tastes using G protein-coupled receptors.
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Type III cells detect sour stimuli and form direct synapses with sensory neurons. At the base of the
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taste bud we have basal cells, which act as stem cells. These continuously divide and replace older
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taste cells that wear out. Because taste cells actually live for only about one to two weeks,
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this rapid turnover is essential to maintain normal taste perception, because the receptors are
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constantly exposed to mechanical stress, heat, and chemical irritation from what we eat and drink.
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In addition to this, recent research as of 2023, has identified a specialized population
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of sodium taste cells that detect salty taste. We’ll talk about those in detail in a moment
Taste Modalities
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Now, we said earlier that there are five taste modalities: sour,
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salty, sweet, bitter, and “umami.”. The sour taste is caused by acids,
Sour
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meaning by the hydrogen ion concentration, and the intensity of this taste sensation is approximately
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proportional to the hydrogen ion concentration. This means that, the more acidic the food,
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the stronger the sour sensation becomes. The salty taste is formed by ionized salts,
Salty
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mainly by the sodium ion concentration. Different salts taste slightly different from one another
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because they can trigger additional taste sensations beyond just saltiness. Let’s look
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at table salt as an example. Table salt, or sodium chloride, has the chemical formula NaCl. When it
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dissolves, it separates into two types of ions: cations, which are positively charged, and anions,
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which are negatively charged. In table salt, sodium becomes the cation Na⁺, while chloride
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becomes the anion Cl⁻. The cations, especially sodium, are the primary source of salty taste.
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However, the anions also play a role in shaping the overall flavor, though their contribution is
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smaller. This explains why different salts produce distinct taste experiences—sodium chloride tastes
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purely salty, while potassium chloride has a salty-bitter quality, and other salt combinations
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create their own unique flavor profiles. Sweet taste responds to sugars like glucose,
Sweet
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fructose, sucrose, but so do a lot of completely different chemical groups – things like glycols,
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alcohols, aldehydes, ketones, esters, some amino acids, and even certain small proteins. Many
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artificial sweeteners also fall into this category despite being structurally unrelated to natural
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sugars. What’s interesting is that even a tiny change in the chemical structure of one of these
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molecules can flip its taste response entirely, sometimes turning something from sweet to bitter.
Bitter
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The bitter taste, like the sweet taste, is not caused by any single type of chemical agent.
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Here again, the substances that give the bitter taste are almost entirely organic substances. Two
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particular classes of substances are especially likely to cause bitter taste sensations: There’s
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the (1) long-chain organic substances that contain nitrogen and (2) alkaloids. The alkaloids include
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many of the drugs used in medicines, such as quinine, caffeine, and nicotine. The bitter taste,
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when it occurs in high intensity, usually causes the person or animal to reject the food. This is
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undoubtedly an important function of the bitter taste sensation because many deadly toxins found
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in poisonous plants are alkaloids, and virtually all of these cause intensely bitter taste,
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usually followed by rejection of the food. Umami is a Japanese word (meaning “delicious”)
Umami
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designating a pleasant taste sensation that is qualitatively different from sour, salty,
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sweet, or bitter. Umami responds to amino acids like glutamate,
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which is why foods such as tomatoes, soy sauce, cheese, and meat have a strong umami profile.
How Taste Buds Detect Taste
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So now the question is: how do these taste cells actually detect them?
Type II Cells
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Sweet, bitter, and umami are all detected by G-protein-coupled receptors located on the
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Type II cells. When any sweet, bitter, or umami substance binds to its receptor on a Type II cell,
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it activates a G-protein called gustducin. This triggers the enzyme phospholipase C beta 2,
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which splits a membrane lipid (PIP2) into DAG and IP3. IP3 then opens calcium channels on
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the endoplasmic reticulum, releasing calcium into the cell. That calcium activates a channel called
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TRPM5, causing influx of sodium, and all that eventually leads the cell to depolarise. Type
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II cells don’t use a typical neuronal synapse, instead, they release ATP through special
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ion channels, and that ATP directly stimulates the sensory nerve fibres at the base.
Type III Cells
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Sour taste is detected by Type III taste cells, and the key trigger is the hydrogen
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ion—H⁺ from acidic foods. When acids enter the mouth, hydrogen
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ions activate OTOP1 channels on the taste cell membrane. These specialized proton channels allow
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H⁺ to flow directly into the cell. This proton entry does two things simultaneously. First,
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the positive charge carried by the hydrogen ions begins to depolarize the membrane. Second,
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the influx of protons acidifies the cell’s interior, and this drop in intracellular pH
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blocks potassium channels called KIR2.1. When these potassium channels close,
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potassium can no longer leave the cell as it normally would, which further shifts the membrane
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potential and causes the cell to depolarize. Once depolarization reaches a threshold,
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voltage-gated sodium channels open, allowing a rapid influx of sodium ions that generates action
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potentials. These action potentials then trigger voltage-gated calcium channels to open. The
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resulting calcium influx is so important, because the increased intracellular calcium concentration
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triggers synaptic vesicles to fuse with the cell membrane through a process called exocytosis. This
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releases neurotransmitters, primarily serotonin and GABA, into the synapse where they stimulate
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the afferent nerve fibers. That signal then travels to the brain and is perceived as sour.
Salty Taste Detection
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Now, salt taste is particularly interesting because the research on this has evolved
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dramatically. For years, scientists thought Type I cells were responsible for detecting salt,
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but a major breakthrough in 2020, confirmed in comprehensive reviews published in 2023,
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completely changed our understanding. Salt taste is actually detected by a
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specialized population of cells that express both epithelial sodium channels called ENaC and calcium
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homeostasis modulator channels called CALHM1/3. These sodium taste cells are their own distinct
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sensory cell type—they don’t really fit into the Type I, II, or III categories we just discussed.
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They’re electrically excitable, which means they can fire action potentials just like neurons.
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Here’s how it works: When sodium from food dissolves in saliva and enters through the taste
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pore, it passes directly through ENaC channels into these specialized salt-detecting cells.
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That inward rush of sodium depolarizes the cell membrane. Once depolarization reaches a threshold,
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it triggers voltage-gated sodium channels to open, and these generate full action
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potentials—just like in neurons. These action potentials then activate the CALHM1/3 channels,
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which release ATP directly onto the sensory nerve fiber below through what’s called a
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‘channel synapse.’ This unique mechanism doesn’t use traditional synaptic vesicles,
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and the entire process is incredibly fast because it runs purely on electrical signaling.
Taste Bud Summary
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Even though all these mechanisms differ, the basic arrangement is the same: the upper part of each
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taste cell samples the dissolved chemicals through the taste pore, and the lower part communicates
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with the sensory neurons. These neurons belong to the facial nerve, the glossopharyngeal nerve,
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or the vagus nerve depending on the region of the tongue and pharynx. We’ll break down
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those territories in the next section. Taste buds also show variations depending
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on location. Circumvallate taste buds tend to respond strongly to bitter substances,
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which fits with their role as a final checkpoint before swallowing. Fungiform papillae near the
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tip often respond more strongly to sweet or salty substances. But even though these
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tendencies exist, each taste bud contains a mix of cell types capable of detecting
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multiple taste modalities. There isn’t a “sweet-only” or “bitter-only” taste bud.
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Alright, now. Food has entered the mouth, saliva had broken it down into chemicals. The chemicals
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have entered the taste pore and have stimulated their respective taste cells, and that will now
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activate the nerve endings underneath the cell. Now we need to figure out how that information
Pathway from Tongue to Brain
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actually leaves the tongue and reaches the brain. Now taste is not carried by one single nerve.
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The gustatory system uses three different cranial nerves and each one is responsible for a specific
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region of the tongue and surrounding structures. The anterior two-thirds of the tongue—this whole
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front portion—is supplied by the facial nerve. More specifically, it’s carried
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through a branch called the chorda tympani. These nerve fibres enter the tongue alongside
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the lingual nerve from the mandibular division of the trigeminal nerve, but they separate once they
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reach the taste buds. The trigeminal part gives general sensation like touch and temperature,
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while the chorda tympani fibres carry only taste. So all taste coming from the tip of the tongue and
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most of the anterior surface travels through the chorda tympani and then joins the facial nerve.
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The posterior one-third of the tongue, all the taste information here is carried by the
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glossopharyngeal nerve, cranial nerve IX. This includes the foliate papillae on the
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posterolateral sides and especially the circumvallate papillae at the back. Even
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though circumvallate papillae lie technically in the “anterior two-thirds” by surface landmarks,
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they are innervated by the glossopharyngeal nerve. And because they house such a large number of
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taste buds, a major portion of our overall taste input actually arrives through cranial nerve IX.
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The epiglottis, the upper part of the esophagus, and parts of the pharynx and larynx also contain
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scattered taste buds, mostly tuned towards bitter stimuli. And these areas send their taste fibres
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through the vagus nerve, cranial nerve X. This provides a final protective checkpoint
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before swallowing—if something tastes intensely bitter or unpleasant at this stage, the body
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can still trigger gagging or reflexive avoidance. Alright, now I’ll test you. The anterior ⅔ of the
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tongue is innervated by cranial nerve number… 7, called the Facial nerve. Specifically which branch
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of the facial nerve? The chorda tympani. Good. The Posterior ⅓ and the circumvallate papillae
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is innervated by cranial nerve number … 9, the glossopharyngeal nerve. The rest, the larynx,
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pharynx, epiglottis, are all innervated by cranial nerve number… 10. Vagus nerve. Awesome.
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Once these signals leave the tongue and upper airway, all three nerves converge in the same
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place deep in the brainstem. They enter the medulla and synapse in a nucleus called the
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nucleus of the solitary tract. This is the very first processing station for taste.
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From there, second-order neurons from the NTS project upwards to a region of the thalamus,
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specifically the ventral posterior medial nucleus. This is the same thalamic relay that
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processes facial sensation, and taste has its own dedicated area within it.
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From the VPM, third-order neurons travel to the primary gustatory cortex. This region is located
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in the insula and the frontal operculum, tucked deep within the lateral sulcus. This is where
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taste becomes a conscious perception. It’s where you identify something as sweet, sour, salty,
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bitter, or umami, and where the brain begins to combine that information with smell, texture,
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temperature, and even emotional associations. A portion of the taste pathway also branches
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off at the brainstem level to control autonomic reflexes. For example, signals from sour or sweet
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food can stimulate salivary nuclei, increasing salivation before you even swallow. Bitter or
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irritating tastes can activate gag reflex pathways. These reflex arcs run through the
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superior and inferior salivatory nuclei and are essential for digestion and protective responses.
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So the gustatory pathway is essentially a three-step chain. First Taste buds,
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Then cranial nerves VII, IX, and X, then to the nucleus of the solitary tract,
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then thalamus, and then the gustatory cortex. Alright, so now we’ve seen how taste buds detect
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chemicals, how the nerves carry that information, and how the brain processes it. But here’s
What is Flavor?
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the part that surprises most people: what we commonly call “taste” is mostly not taste at all.
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A huge portion of what you experience as flavor actually comes from smell.
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When you chew food, tiny tiny molecules travel from your mouth up into the nasal cavity. These
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molecules stimulate the olfactory receptors, and the olfactory cortex then passes that information
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to the gustatory areas. So if you think something tastes “sweet,” “fruity,” “smoky,” or “spicy,” a
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lot of that comes from olfactory input layered on top of the basic taste signals from the tongue.
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That’s why when you’re congested, food suddenly becomes dull and flat. The taste buds are still
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detecting sweet, salty, sour, bitter, and umami normally, but the smell component is missing,
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so the brain only receives the most basic part of the information. Remember during the COVID-19
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pandemic? That many people reported losing both taste and smell? In most cases of COVID-19,
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the taste buds themselves were not damaged at all. What actually happened was inside the nose.
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The virus affected the supporting cells around the olfactory receptors, the sustentacular cells,
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not the neurons directly. When those supporting cells became inflamed, the olfactory receptors
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temporarily stopped functioning. And because around 80 to 90 percent of flavor comes from
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smell, losing your olfactory input almost completely wipes out the flavor experience.
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The tongue can still detect sweet, salty, sour, bitter, and umami perfectly well, but without
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the smell component the brain only receives the simplest part of the signal. That’s why everything
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tastes strangely muted, flat, or even unpleasant despite your taste buds working normally.
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There’s also the trigeminal system, which is carried by the trigeminal nerve. This system
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doesn’t detect taste or smell, but it senses things like heat from chili peppers, cooling from
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mint, tingling from carbonation, and the sharpness of wasabi or mustard. These sensations add another
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layer to what we think of as “taste,” even though they’re technically separate from gustation.
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So when you put everything together, flavor is really a combination of three systems working
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at the same time. Taste from the tongue and epiglottis. Smell from the olfactory epithelium,
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and sensations from the trigeminal nerve The brain integrates all of this instantly,
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and that’s why something as simple as a piece of chocolate can taste smooth,
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sweet, aromatic, and warm all at once. And with that, we’ve covered the next
Ending
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special sense in our list. I really hope this was helpful. In the next video, we’ll cover
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the next special sense, which is how smell works I’ve made free courses for other topics here on
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YouTube if you wanna keep learning, otherwise if you want a handmade PDF version of this
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lecture or take a quiz to test your knowledge, or access an organized list of all my videos,
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you can find everything on my website. Thanks for watching! See you in the next one.