Tongue

The tongue (Latin: lingua s. glossa) is a pink-colored, mobile muscular organ of the digestive system. It occupies the oral cavity proper, and from it, the tongue also extends into the oropharynx. The tongue helps to support the floor of the oral cavity, and it helps in food chewing and swallowing and provides tactile, taste and speech formation functions.

Tongue anatomy

The average length of the tongue in men is around 8,5 centimeters, while it is about 8 cm long for women. Various muscles attach the tongue to the hyoid bone, styloid process of the temporal bone, mandible, palate and pharynx. The tip of the tongue and anterior aspects of lateral sides face the teeth and gingiva. Posterior aspects of lateral sides meet the palatoglossal and palatopharyngeal arches and palatine tonsils between them.

Superior to the tongue are the soft and hard palates, while inferior is the floor of the oral cavity and sublingual salivary glands. Posterior to the tongue is situated the epiglottis and posterior opening of the oral cavity - oropharyngeal isthmus. The tongue participates in the formation of the oropharyngeal isthmus, and its base and lingual tonsil mark the inferior boundary of the isthmus. Overall, the tongue is made of various tongue muscles covered by the lingual papillae, and it has three parts, two surfaces and two edges.

Parts

The tongue consists of three parts - apex (also known as the tip), body and root. The apex is the most mobile and narrowest part, and it is located most forward. The body is the largest part. It extends from the end of the apex to the terminal sulcus located on the dorsal surface of the tongue, approximately two-thirds from the apex. Together with the tip, the body forms the anterior part. It is also known as the oral or presulcal part.

The posterior part or posterior one-third is the root, and it is also known as the pharyngeal or postsulcal part. The root goes from the terminal sulcus to the epiglottis. It is the broadest and the least mobile part of the tongue, and it houses lymphoid tissues and lingual tonsil.

Surfaces and edges

The tongue has two surfaces - superior and inferior. The superior surface is called the dorsal surface, while the inferior is the ventral surface. The tongue also has two lateral edges or sides located between both surfaces.

Ventral surface

The ventral surface is a relatively smooth inferior surface that faces the floor of the oral cavity proper. In the midline of the ventral surface, mucosa forms a sagittal-oriented fold called the lingual frenulum, and it anchors the tongue to the floor of the oral cavity. Lateral on both sides of the lingual frenulum is another slight mucosal fold called the fimbriated fold or plica. Sometimes the free edge of the fimbriated fold has small fringe-like extensions or processes. Between the lingual frenulum and fimbriated fold lies the noticeable deep lingual vein.

On either side of the lingual frenulum at the site where it terminates is a small eminence - sublingual caruncle. It is a papilla at which the main ducts of the sublingual and submandibular salivary glands open. From the lateral side of the sublingual caruncle goes the sublingual fold or plica overlying the sublingual gland. Along the sides of the sublingual plica and towards the sublingual caruncle, the minor ducts of the sublingual salivary gland open.

Dorsal surface

The dorsal surface is a convex and curved superior surface that faces the palate. The dorsal surface is rough to touch because it contains numerous small projections called the lingual papillae. The lingual papillae cover only the anterior two-thirds of the tongue, and usually, they are absent posterior to the terminal sulcus.

The terminal sulcus is a groove on the dorsal surface between the body and root. It separates both parts and has an inverse letter "V" shape. In the middle of the terminal sulcus is a small pit called the foramen cecum. In embryonal development, the foramen cecum is the origin site of the thyroglossal duct that forms during the descent of the thyroid gland.

The median lingual sulcus is a sagittal-oriented groove that goes in the midline of the surface from the apex to the foramen cecum. It divides the tongue into two parts. The median lingual sulcus signifies the fusion of two lateral lingual swellings during embryological development, resulting in tongue formation.

Knowing the location of the median lingual sulcus makes it easier to determine the location of the median lingual septum as it goes in the same direction as the sulcus. It is a sagittal-oriented connective tissue septum in the midline of the tongue. The median lingual septum divides the tongue into two symmetric parts - the right and left one.

In the mucosa of the root is positioned the lingual tonsil - lymphoid tissue aggregation. Mucosa from the root continues to the epiglottis and forms three mucosal folds. The first one goes from the midline of the root to the epiglottis and is called the median glossoepiglottic fold. Two lateral glossoepiglottic folds extend from the lateral sides of the tongue to the sides of the epiglottis. Between the lateral and median glossoepiglottic folds are paired depressions - epiglottic valleculae.

The tongue connects with the posterior aspect of the soft palate. The soft palate ends with a free edge that contains the uvula in the middle. The lateral sides of the uvula extend to the lateral sides of the root of the tongue. This connection creates an arch, and it is called the palatoglossal arch or anterior palatine arch.

Lingual papillae

One of the tongue functions is to taste and analyze the food, and this function is provided mainly by the lingual papillae - wart-like projections that increase the size of the tongue surface. The taste sensation is associated with the irritation of the taste receptors. These taste receptors are situated in unique epithelial structures that resemble buds in shape, called taste buds. The taste buds of the tongue are located within the epithelium of the lingual papillae.

The dorsal surface of the apex and body contains four groups of lingual papillae - filiform, fungiform, foliate and circumvallate papillae. They are subdivided based on their functions and structure. The tongue has the most significant amount of taste buds. However, the pharynx, epiglottis, palate and upper part of the esophagus also can contain several taste buds. Young children have taste buds on the hard palate, cheeks and even lips. Overall, every person has around 2000 to 4000 taste buds. Afferent nerves further transfer impulses from the taste buds to the central nervous system.

Filiform papillae

The filiform papillae appear thread-like and cone-shaped, and they are thin and only around 1 millimeter long. The filiform papillae are the only papillae covering almost all anterior two-thirds of the dorsal surface of the tongue. Usually, they form rows that go parallel to the terminal sulcus in the anterolateral direction, but in the apex, the rows appear to go more in a transverse direction.

The filiform papillae are the most common type making the dorsal surface of the tongue rough to touch. These papillae appear white or pinkish-grey, and they sometimes look scaly. Besides the taste function, the filiform papillae guide the bolus (chewed food mixed with saliva) from the oral cavity proper further to the oropharynx. Also, they help to clean the teeth and mucosa of the tongue from the food particles. And finally, they help to perceive irritation coming from the external environment.

Fungiform papillae

The fungiform papillae are broader and larger than the filiform papillae. They have a relatively thin stalk with a raised termination part that contains taste receptors. Because of the shape that resembles a mushroom, these papillae are called the fungiform papillae. The fungiform papillae are located mainly in the apex and between the filiform papillae on the body part. They are very vascular, and therefore the fungiform papillae are visible as red dots on the dorsal surface of the tongue.

The tongue contains around 200 - 400 fungiform papillae. One fungiform papilla contains approximately 3 - 6 taste buds, with a maximum of 9 to 18 taste buds. Their height is about 1 - 2 millimeters, but the width is around 1 mm, and the distance between two fungiform papillae usually is approximately 2 mm. The fungiform papillae are not only responsible for the taste sensation, but they also contain a small number of sensory cells for temperature and touches.

Foliate papillae

The foliate papillae are 3 - 5 millimeters long mucosal folds lying parallel and very close to each other on the lateral sides of the tongue close to the terminal sulcus. They are separated by grooves. Every lateral side has around 3 - 11 foliate papillae. They are very well visible on newborn babies, but the foliate papillae become less visible with aging. The mucous membrane covering them contains taste buds.

Circumvallate papillae

The circumvallate papillae are also known as the vallate papillae. They are positioned next to the terminal sulcus and go along it in the presulcal area. Like the terminal sulcus, the row of the circumvallate papillae resembles the inverted letter “V” in shape. This type is more significant than others. The height of the circumvallate papillae is around 1 - 3 millimeters, but the width varies from 0,5 to 2 mm.

The circumvallate papillae also contain taste receptors, and their count can range from 6 to 15, but usually up to 12. The circumvallate papillae do not rise above the mucosa of the tongue, and therefore, they can not get mechanical injuries during food swallowing. Each papilla contains around 150 - 300 taste buds on its sides.

The central part of the papilla reminds a fungiform papilla as it also has a short stalk ending with a broader termination that contains taste buds. However, around the central part is a mucosal wall, and it looks like the papilla is positioned in a pit. The mucosal wall is higher than the central region, and it contains smooth muscle cells that help to hold the food particles within the pit, and later saliva helps to wash them out of the pit.

Tongue muscles

The tongue contains many skeletal muscles, and all muscles are paired. The tongue muscles can be subdivided into two large groups - intrinsic and extrinsic tongue muscles. The fibers of the tongue muscles have enormous energy reserves, which is why it does not get tired.

Intrinsic tongue muscles

The intrinsic tongue muscles are four pairs of muscles with origin and insertion sites within the tongue. During intrinsic muscle contractions, the tongue can change its shape and size. All four pairs of muscles go in mutually perpendicular directions, and these muscles are named after the directions they go. The intrinsic tongue muscles mainly help in speech formation and digestion.

• Superior longitudinal muscles of the tongue are located close to the dorsal surface. They originate from the tongue root and insert into its apex. The superior longitudinal muscles shorten the tongue and help to curl the apex and sides of the tongue upwards. These muscles elevate the apex.
• Inferior longitudinal muscles of the tongue are better developed than the superior longitudinal muscles. They are located deeper and closer to the ventral surface of the tongue. They also originate from the root of the tongue and insert into its tip. The inferior longitudinal muscles shorten the tongue and curl it downward. They pull the apex of the tongue down.
• Transverse muscles of the tongue originate from the median lingual septum and insert into the lateral sides of the tongue. They narrow the tongue by thickening it. The transverse muscles increase the height of the tongue and protrude it.
• Fibers of the vertical muscles of the tongue go in the vertical direction, and they connect both tongue surfaces. These muscles flatten and broaden the tongue.

Extrinsic tongue muscles

The extrinsic tongue muscles originate from the nearest bones and insert into the tongue. These muscles allow the tongue to move. The genioglossus and hyoglossus muscles extend below the tongue, while the styloglossus and palatoglossus stretch above it. The extrinsic tongue muscles primarily provide mechanical food procession and food maneuvering functions.

• The genioglossus pulls the tongue forward and down. Therefore, it protrudes and depresses the tongue. It originates from the mental spine of the mandible and inserts on the dorsal surface and root of the tongue and the body of the hyoid bone.
• The styloglossus pulls the tongue back and up. Therefore, it retracts and elevates the tongue. It arises from the styloid process of the temporal bone and blends with the fibers of the inferior longitudinal and hyoglossus muscles. It inserts into the lateral sides of the tongue and its root.
• The hyoglossus pulls the tongue back and down. Therefore, it depresses and retracts the tongue. It originates from the body and greater horn of the hyoid bone and inserts into the root and lateral sides of the tongue.
• The palatoglossus arises from palatine aponeurosis. Fibers of this muscle extend across the tongue and blend with the fibers of the intrinsic tongue muscles. The palatoglossus inserts into the lateral sides of the tongue. This muscle elevates the posterior part of the tongue and closes the oropharyngeal isthmus during swallowing.

Neurovascular supply of tongue

Arterial supply

The tongue receives arterial blood supply primarily from the lingual artery and its branches - dorsal lingual, sublingual and deep lingual arteries. The lingual artery is a branch of the external carotid artery. The tonsillar and ascending palatine arteries (of the facial artery) and ascending pharyngeal artery (of the external carotid artery) also supply the tongue.

Venous drainage

The deep lingual veins drain the blood from the ventral surface, while the dorsal lingual veins provide venous drainage from the dorsal surface and lateral sides of the tongue. Together with the sublingual vein, the deep lingual vein forms a vena comitans of the hypoglossal nerve, and it drains into the lingual vein. Blood from all mentioned veins is carried to the internal jugular vein.

Lymphatic drainage

The lymph vessels draining the tongue can be subdivided into three groups - marginal, ventral and dorsal lymph vessels. The anterior parts of the tongue are drained by the marginal and ventral lymph vessels, while the posterior parts by the dorsal vessels.

The marginal lymph vessels carry the lymph next to the submandibular lymph nodes of the same side (ipsilateral). Two-thirds of the anterior and more central part of the tongue is drained to the deep cervical lymph nodes of the opposite side (contralateral), mainly to the jugulo-digastric and jugulo-omohyoid lymph nodes.

The dorsal vessels carry lymph next to the deep cervical lymph nodes, primarily to the jugulo-digastric and jugulo-omohyoid nodes. The apex of the tongue is drained to the submental lymph nodes of the same or opposite side.

Innervation

The tongue receives motor, general sensory and special sensory innervation. The motor fibers supply the tongue muscles, the general sensory fibers innervate the mucosa, while the special sensory fibers provide the taste sensation.

Motor innervation

The motor innervation of the tongue is primarily provided by the hypoglossal nerve (CN XII). It innervates all the tongue muscles except the palatoglossus muscle. The palatoglossus is supplied by the vagus nerve (CN X).

General sensory innervation

The mucosa covering the anterior two-thirds of the ventral and dorsal surfaces (from the apex to the terminal sulcus) is innervated by the lingual nerve - a branch of the mandibular nerve (CN V3). The mandibular nerve is a third division of the trigeminal nerve (CN V).

The posterior two-thirds of the dorsal surface is supplied by the glossopharyngeal nerve (CN IX). Also, a small portion of the posterior tongue is innervated by the internal laryngeal nerve - a branch of the vagus nerve (CN X). As previously described, touching the area that is supplied by the vagus nerve can cause vomiting.

Special sensory innervation

The taste sensation is provided by three nerves. The fungiform and foliate papillae are innervated by the chorda tympani from the intermediate nerve (part of the facial nerve (CN VII)). The filiform papillae are supplied by the lingual nerve from the mandibular nerve (CN V3). And finally, the circumvallate papillae receive innervation from the glossopharyngeal nerve (CN IX).

Tongue histology

The tongue is covered by the mucosa that is lined by the stratified squamous epithelium. The mucosal layer does not have glands, and it is fused with the connective tissue located between the tongue muscle fibers. The ventral surface of the tongue is smooth and does not have keratinization signs. In contrast, the dorsal surface contains lingual papillae that raise above the mucosa except for the circumvallate papillae. Filiform papillae have partially or fully keratinized epithelium, while the epithelium of the fungiform papillae rarely has keratinization signs. Foliate and circumvallate papillae usually have partially keratinized epithelium.

Between the muscle fibers within the connective tissue are located small salivary glands with their ducts opening close to the bases of the circumvallate and foliate papillae. The apex of the tongue contains mixed salivary glands, but next to the circumvallate and foliate papillae glands are serous type. In contrast, the root of the tongue has mucous salivary glands. Ducts of the mixed glands open on the ventral surface of the tongue close to the lingual frenulum, while the ducts of the mucous salivary glands open in the base of the lingual tonsil.

Tongue functions

The tongue has four main functions - mechanical food procession and food maneuvering in the oral cavity, taste and speech formation functions. Besides major functions, the tongue has several minor functions. It provides breathing during sleep, and its veins ensure fast medication absorption when drugs are applied under the tongue. For example, nitroglycerin dilates the blood vessels of the heart, and a person can put it under the tongue when chest pain is present. The tongue also has a protective function as touching it can cause vomiting. The posterior one-third of the tongue contains nerve fibers from the vagus nerve (CN X), and if a person touches the root of the tongue, the vagus nerve receives stimuli to empty the stomach.

Mechanical food procession and food maneuvering

The tongue participates in food positioning between the teeth, and it mixes food particles with saliva. It presses food against the hard palate and helps in bolus (chewed food that is mixed with saliva) formation. Therefore, the tongue assists in the mastication process.

Also, it moves the food within the oral cavity and from the oral cavity further to the oropharynx. The movements of the tongue enable the swallowing process, and it assists during breastfeeding and sucking. Besides all the mentioned functions, it also cleans the teeth and grooves of the oral cavity from the leftover food particles.

Speech formation

The contractions of the intrinsic tongue muscles change the shape and size of the tongue so it can provide sounds and form a speech. Together with the lips and teeth, the tongue assists in various sound formations. For example, to make the sound of the letter "t," the apex of the tongue should touch the alveolar arch of the maxilla, but when making the sounds of the letter “g,” the dorsal surface of the tongue touches the soft palate.

Taste function

As previously described, the tongue contains various papillae with many taste buds and gustatory cell receptors providing the taste sensation. Overall, the tongue has five different taste areas, and each is more sensitive to a specific taste, although all areas can detect all five tastes. The sides of the tongue are more sensitive than its central part.

The salty taste area is a narrow band of the apex and the anterior aspect of the lateral sides. The sweet taste area overlays the previous one in the apex and extends into the body. The sour taste is detected by the posterior aspects of the lateral sides, while the bitter taste by the posterior part of the body next to the terminal sulcus. The central part of the tongue is the umami taste area. Umami is the savory taste of glutamic acid and glutamate.

Tongue disorders

The tongue is usually inspected during general health checkups as changes in color and visual appearance can indicate an underlying disease. For example, the geographic tongue is characterized as discolored and painless patches with greyish white borders covering the dorsal surface and sides of the tongue. Its appearance resembles a map, and patches usually migrate. Geographic tongue is caused by inflammation, and it can be a sign of intestinal disorders. In contrast, the strawberry tongue appears very red, swollen and bumpy. Together with other symptoms, it can be a sign of Kawasaki disease or scarlet fever.

Changes in color

As mentioned before, a healthy tongue has a pink color, it is mobile, and its dorsal surface has a rough texture with a thin white coating because of the many papillae. The pink can vary from lighter to darker shade. The most common signs of an underlying disorder include changes in tongue color.

• White tongue is an indicator of dehydration, fungal infections, common cold, influenza and other infectious diseases. A thick white coating can also be a sign of intoxication. Candida albicans cause yeast infection (also known as the thrush), appearing as white cottage cheese-like plaques on the mucosa of the oral cavity, including the mucosa of the tongue. Also, leukoplakia can be visible as a white tongue coating.
• Pale tongue color indicates a weak immune system, malnutrition and vitamin deficiencies.
• Black color can be caused by excessive tobacco usage and particular medications (antibiotics and chemical bismuth), inappropriate oral hygiene and radiation therapy. HIV and diabetes patients very rarely but can have a black-colored tongue.
• Purple tongue presents in heart disorders and poor blood circulation, also in Kawasaki disease.
• The red tongue usually is a sign of scarlet fever, glossitis, vitamin B12 deficiency and allergic reaction.
• Gray color can be a sign of gastrointestinal tract diseases. Patients with eczema sometimes also present with it.
• Blue tongue color can be visible in heart and blood circulation disorders and kidney problems.
• Yellow tongue most commonly is seen in liver and stomach diseases, poor oral hygiene, smoking and oral cavity infections.
• Orange tongue is also an indicator of poor oral hygiene. Patients with mouth dryness can have an orange tongue. Also, particular food (carrots high in beta carotene) and antibiotics can cause it.
• Green tongue color indicates poor oral hygiene and fungal infections.

Hairy tongue

Hairy tongue most commonly appears as a furry black coating mainly because of the keratin build-up and overgrowth of the filiform papillae on the dorsal surface of the tongue. Hairy tongue happens due to lack of abrasion, leading to longer filiform papillae. Food, tobacco, bacteria and yeasts accumulate between adjacent papillae and give the typical hairy tongue appearance. Color can vary, but most often, it is black or brown. However, it can also be white, yellow or green.

Hairy tongue is more commonly seen in smokers and people with mouth dryness, poor oral hygiene or drug addictions. People who often use antibiotics or certain medications are at higher risk because medication usage can lead to changes in healthy bacteria and yeast numbers. People who do not have teeth and drink excessive amounts of coffee, tea or alcohol also have a higher risk of hairy tongue development. Hairy tongue most often affects older people and males.

Hairy tongue is not a life-threatening condition, and overall it is not harmful. It usually does not have any other symptoms. However, a burning sensation or a tickling feeling (usually during swallowing as the long papillae touch the palate) can be present. A hairy tongue can cause bad breath and changes in taste (typically, people complain about metallic taste) because of the accumulation of bacteria and debris.

The best possible way to get rid of hairy tongue is a good oral hygiene, including tongue brushing and scraping with toothbrushes or scrapers. If it does not help, certain medications and surgical treatments can remove it. The best prevention is good oral hygiene with regular tongue brushing, as this condition tends to come back.