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.
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.
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.
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 tastes receptors activating together.
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.