Taste and the tongue

Definitions & Key takeaways

The tongue is a muscular organ located in the mouth that is involved in a variety of functions, including speech, swallowing, and taste. Taste is believed to be one of the most pleasurable human special senses, and is possible thanks to taste receptor cells in various types of papillae that covers the tongue. Circumvallate, fungiform, and foliate papillae are where the majority of taste buds are located. After chemical tastants excite specific taste receptor epithelial cells, neurotransmitters are released to transduce the signal to afferent sensory nerves. Ultimately, the stimulus terminates in the insula, hypothalamus, and limbic system to determine and appreciate the gustatory sensation.

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Introduction0:00–0:43

The tongue is a muscular organ in the mouth, and it’s used for many things like speech, chewing and swallowing food, and most important of all, tasting delicious foods, which is called gustation.
The surface of the entire tongue is covered by a mucus membrane called the mucosa, and below that there’s a combination of intrinsic and extrinsic muscles which are all innervated by the hypoglossal nerve which is cranial nerve 12.
Intrinsic muscles start and end within the tongue, and help change its shape, whereas extrinsic muscles attach to structures outside the tongue and help guide its movement.

Lingual Papillae0:43–2:19

Now, on the tongue, there’s a V shaped groove called the sulcus terminalis that runs across the posterior portion of the tongue dividing it into an posterior ⅓ and anterior ⅔.
The posterior ⅓ of the tongue is covered in bumps made of lymphoid tissue called lingual papillae which contains B and T cells that help fight off pathogen that enters the mouth.
The anterior ⅔ which is covered in smaller lumps called papillae that help increase the surface area and give it a rough texture that helps food particles stick to the tongue.
There are four different types of papillae and they’re found in different regions of the tongue. The most numerous type are the thread-like, filiform papillae which are scattered all over the anterior ⅔ of the dorsal surface of the tongue.
The filiform papillae are in charge of the sensation of touch on the tongue but not taste. Next are the mushroom-shaped fungiform papillae which are most common at the tip of the tongue.
Then there are the leaf-like foliate papillae, which are most common on the sides of the tongue. And finally there are 8-12 very large round circumvallate papillae, which are located at the back of the anterior ⅔ of the tongue, just in front of the sulcus terminalis.
The fungiform, foliate, and circumvallate papillae contain multiple taste buds, and each taste bud contains specialized epithelial cells called taste receptor cells which detect taste.

Taste buds2:19–3:57

Now although taste buds are typically found on the tongue, some are also found on the soft palate, pharynx, epiglottis, larynx, and upper esophagus.
Taste buds are oval-shaped and looks a bit like an orange. On the inside are specialized epithelial cells called taste receptor cells which do the incredible work of detecting tastes.
These taste receptor cells are arranged like orange wedges with supporting cells in between. There are also basal cells at the bottom of the taste buds which can differentiate into new taste receptor cells to replace the ones that die - typically that happens about every two weeks.
Taste receptor cells are chemoreceptors that respond to tastants, which are any molecules that are found in foods and drinks.
The top of a taste receptor cell has a thin, hair-like microvilli called a gustatory hair that sticks out of a small opening on the surface of the papillae called the taste pore.
These hairs comes into contact with tastants. Next are the cell bodies, which are the orange wedges that lie within the capsule, and underneath the cell bodies are axons from one of the three nerves that transmits the taste sensation to the brain; the facial nerve that innervates taste buds from the anterior ⅔ of the tongue, the glossopharyngeal nerve that innervates taste buds from the posterior ⅓ of the tongue and the rest of the oral cavity, and the vagus nerve innervates some taste buds at the back of the throat and esophagus.

Primary tastes3:57–4:44

Now we have 5 primary tastes; bitter, salty, sour, sweet, and umami, which is a Japanese word that describes the savory taste of meat.
Each taste receptor cell can detect all 5 of these tastes but responds most strongly to one of them. Taste receptor cells inside fungiform papillae at the tip of the tongue are more sensitive to sweet and umami, while salty and sour taste receptors located in the foliate papillae are more common on the sides of the tongue.
The back of the tongue has circumvallate papillae which contain lots of bitter taste receptor cells. More complex tastes, like chocolate or coffee are a combination of taste receptors activating together.

Transduction4:44–7:24

Now, when you eat, the chewed up particles mix in with saliva and travel to the papillae where they make contact with the gustatory hairs of the taste receptor cells.
There are two mechanisms for converting a chemical signal from the food into a neuron impulse. The first relates to salty or sour tastes, whilst the second mechanism relates to sweet, bitter or savory tastes.
Let’s look at the first mechanism: let’s say you eat something salty and sour, like french fries with ketchup. Sodium ions (Na+) from the salty fries and hydrogen ions (H+) from the sour ketchup get into contact with the gustatory hair.
The taste receptor cells contain ion channels that allow both Na+ and H+ ions into the cell, and the flow of ions causes the taste receptor cells’ membrane to depolarize.
The local depolarization raises the cells resting membrane potential and that triggers voltage-gated channels to open up and allow extracellular Ca++ to flow inside.
The influx of calcium causes vesicles full of neurotransmitters like serotonin, acetylcholine, norepinephrine, and GABA to fuse with the cell membrane.
When that happens, these neurotransmitters get released into the synaptic cleft near the neuron that serves that region.
The facial nerve innervates the anterior ⅔ of the tongue, the glossopharyngeal nerve innervates the posterior ⅓ of the tongue, and the vagus nerve innervates some taste buds at the back of the throat and esophagus.
So these nerves then transmit the sensory information for taste to the brain. Now the second mechanism for converting a chemical signal into a neuron impulse relates to sweet, bitter, or savory tastes.
Imagine that you’re eating honey glazed ham while drinking coffee. The gustatory hairs of the taste receptor cells that respond most strongly to those tastes have specific receptors called G-protein coupled receptors.
When tastants reach the gustatory hairs, they bind to G-protein receptors and that triggers a set of reactions within the taste receptor cell, called a G-protein coupled receptor pathway.
The G-protein pathway causes calcium channels on the endoplasmic reticulum to open. The endoplasmic reticulum stores calcium, so when these gates open, intracellular calcium ions are released into the cell.
As before, the increase in calcium ions causes vesicles full of neurotransmitters to fuse with the cell membrane, releasing neurotransmitters into the synaptic cleft.
The taste information is then transmitted to the brain just like with the salty and sour tastes. So taste receptor cells, respond to tastants by transmitting taste signals through nerves to the brain.

Taste perception7:24–8:51

One interesting feature of taste is that over time, when exposed to the same levels of tastants, they start sending fewer and fewer signals, until they eventually lose the ability to respond altogether.
This is called adaptation, and it can start within just a few minutes of eating. This is one reason why the first bite of food is often the most delicious, and why some people have to keep adding salt or other seasoning to their food throughout a meal.
Other factors that affect taste include hunger, because it makes the taste receptor cells more sensitive to sweet and salty food.
In addition, people often can’t taste foods when they have a viral infection or allergies, and that’s because smell affects taste.
The smell of food travels through the air in the form of small molecules called odorants, which stimulate smell receptors in the nose.
The olfactory nerve or cranial nerve 1 carries sensory information about smell to the brain where it’s processed along the sensation of taste - to give us an experience we call “flavor.” Finally age also affects taste: older people tend to have a poorer ability to taste because they don’t replace taste receptors cells as quickly, and so there are fewer and fewer of them over time.
Okay - as a recap - The tongue is covered by various types of papillae (circumvallate, foliate, filiform and fungiform papillae) which contain different types of taste receptor cells.

Review8:51–9:38

Each taste receptor can recognize all 5 of the major tastes; sweet, sour, salty, bitter, and umami, but is most sensitive to one of them.
This sensory information is carried to the brain by the vagus, glossopharyngeal, and facial nerves. The sense of smell also plays a role in taste and the smell of the food is carried to the brain through the olfactory nerve where it is processed together with the taste information from the mouth and we recognize the result as flavor.